text: /*
  
  Copyright (c) 2003-2016, Arvid Norberg
  All rights reserved.
  
  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions
  are met:
  
      * Redistributions of source code must retain the above copyright
        notice, this list of conditions and the following disclaimer.
      * Redistributions in binary form must reproduce the above copyright
        notice, this list of conditions and the following disclaimer in
        the documentation and/or other materials provided with the distribution.
      * Neither the name of the author nor the names of its
        contributors may be used to endorse or promote products derived
        from this software without specific prior written permission.
  
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  POSSIBILITY OF SUCH DAMAGE.
  
  */
  
  #include <vector>
  #include <cmath>
  #include <algorithm>
  #include <numeric>
  #include <limits>
  #include <functional>
  #include <tuple>
  
  #include "libtorrent/piece_picker.hpp"
  #include "libtorrent/bitfield.hpp"
  #include "libtorrent/random.hpp"
  #include "libtorrent/alloca.hpp"
  #include "libtorrent/performance_counters.hpp" // for counters
  #include "libtorrent/alert_types.hpp" // for picker_log_alert
  
  #if TORRENT_USE_ASSERTS
  #include "libtorrent/peer_connection.hpp"
  #include "libtorrent/torrent.hpp"
  #include "libtorrent/torrent_peer.hpp"
  #endif
  
  #include "libtorrent/invariant_check.hpp"
  
  // this is really only useful for debugging unit tests
  //#define TORRENT_PICKER_LOG
  
  using namespace std::placeholders;
  
  namespace libtorrent
  {
  	// TODO: find a better place for this
  	const piece_block piece_block::invalid(
  		std::numeric_limits<piece_index_t>::max()
  		, std::numeric_limits<int>::max());
  
  	constexpr prio_index_t piece_picker::piece_pos::we_have_index;
  
  	piece_picker::piece_picker()
  		: m_priority_boundaries(1, m_pieces.end_index())
  	{
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "new piece_picker" << std::endl;
  #endif
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_invariant();
  #endif
  	}
  
  	void piece_picker::init(int const blocks_per_piece, int const blocks_in_last_piece, int const total_num_pieces)
  	{
  		TORRENT_ASSERT(blocks_per_piece > 0);
  		TORRENT_ASSERT(total_num_pieces > 0);
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "piece_picker::init()" << std::endl;
  #endif
  		// allocate the piece_map to cover all pieces
  		// and make them invalid (as if we don't have a single piece)
  		m_piece_map.resize(total_num_pieces, piece_pos(0, 0));
  		m_reverse_cursor = m_piece_map.end_index();
  		m_cursor = piece_index_t(0);
  
  		for (int i = 0; i < piece_pos::num_download_categories; ++i)
  			m_downloads[i].clear();
  		m_block_info.clear();
  		m_free_block_infos.clear();
  
  		m_num_filtered += m_num_have_filtered;
  		m_num_have_filtered = 0;
  		m_num_have = 0;
  		m_num_passed = 0;
  		m_dirty = true;
  		for (std::vector<piece_pos>::iterator i = m_piece_map.begin()
  			, end(m_piece_map.end()); i != end; ++i)
  		{
  			i->peer_count = 0;
  			i->download_state = piece_pos::piece_open;
  			i->index = prio_index_t(0);
  #ifdef TORRENT_DEBUG_REFCOUNTS
  			i->have_peers.clear();
  #endif
  		}
  
  		for (auto i = m_piece_map.begin() + static_cast<int>(m_cursor)
  			, end(m_piece_map.end()); i != end && (i->have() || i->filtered());
  			++i, ++m_cursor);
  
  		for (auto i = m_piece_map.rend() - static_cast<int>(m_reverse_cursor);
  			m_reverse_cursor > piece_index_t(0) && (i->have() || i->filtered());
  			++i, --m_reverse_cursor);
  
  		m_blocks_per_piece = std::uint16_t(blocks_per_piece);
  		m_blocks_in_last_piece = std::uint16_t(blocks_in_last_piece);
  		if (m_blocks_in_last_piece == 0) m_blocks_in_last_piece = std::uint16_t(blocks_per_piece);
  
  		TORRENT_ASSERT(m_blocks_in_last_piece <= m_blocks_per_piece);
  	}
  
  	void piece_picker::piece_info(piece_index_t const index, piece_picker::downloading_piece& st) const
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  		int state = m_piece_map[index].download_queue();
  		if (state != piece_pos::piece_open)
  		{
  			std::vector<downloading_piece>::const_iterator piece = find_dl_piece(state, index);
  			TORRENT_ASSERT(piece != m_downloads[state].end());
  			st = *piece;
  			return;
  		}
  		st.info_idx = 0;
  		st.index = index;
  		st.writing = 0;
  		st.requested = 0;
  		if (m_piece_map[index].have())
  		{
  			st.finished = std::uint16_t(blocks_in_piece(index));
  			return;
  		}
  		st.finished = 0;
  	}
  
  	piece_picker::piece_stats_t piece_picker::piece_stats(piece_index_t const index) const
  	{
  		piece_pos const& pp = m_piece_map[index];
  		piece_stats_t ret = {
  			int(pp.peer_count + m_seeds),
  			pp.priority(this),
  			pp.have(),
  			pp.downloading()
  		};
  		return ret;
  	}
  
  	std::vector<piece_picker::downloading_piece>::iterator
  	piece_picker::add_download_piece(piece_index_t const piece)
  	{
  		TORRENT_ASSERT(piece >= piece_index_t(0));
  		TORRENT_ASSERT(piece < m_piece_map.end_index());
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  		int block_index;
  
  		if (m_free_block_infos.empty())
  		{
  			// we need to allocate more space in m_block_info
  			block_index = int(m_block_info.size() / m_blocks_per_piece);
  			TORRENT_ASSERT((m_block_info.size() % m_blocks_per_piece) == 0);
  			m_block_info.resize(m_block_info.size() + m_blocks_per_piece);
  		}
  		else
  		{
  			// there is already free space in m_block_info, grab one range
  			block_index = int(m_free_block_infos.back());
  			m_free_block_infos.pop_back();
  		}
  
  		// always insert into bucket 0 (piece_downloading)
  		downloading_piece ret;
  		ret.index = piece;
  		int download_state = piece_pos::piece_downloading;
  		std::vector<downloading_piece>::iterator downloading_iter
  			= std::lower_bound(m_downloads[download_state].begin()
  			, m_downloads[download_state].end(), ret);
  		TORRENT_ASSERT(downloading_iter == m_downloads[download_state].end()
  			|| downloading_iter->index != piece);
  		TORRENT_ASSERT(block_index >= 0);
  		TORRENT_ASSERT(block_index < (std::numeric_limits<std::uint16_t>::max)());
  		ret.info_idx = std::uint16_t(block_index);
  		TORRENT_ASSERT(int(ret.info_idx) * m_blocks_per_piece
  			+ m_blocks_per_piece <= int(m_block_info.size()));
  
  		block_info* info = blocks_for_piece(ret);
  		for (int i = 0; i < m_blocks_per_piece; ++i)
  		{
  			info[i].num_peers = 0;
  			info[i].state = block_info::state_none;
  			info[i].peer = nullptr;
  #if TORRENT_USE_ASSERTS
  			info[i].piece_index = piece;
  			info[i].peers.clear();
  #endif
  		}
  		downloading_iter = m_downloads[download_state].insert(downloading_iter, ret);
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  		return downloading_iter;
  	}
  
  	void piece_picker::erase_download_piece(std::vector<downloading_piece>::iterator i)
  	{
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  		int download_state = m_piece_map[i->index].download_queue();
  		TORRENT_ASSERT(download_state != piece_pos::piece_open);
  		TORRENT_ASSERT(find_dl_piece(download_state, i->index) == i);
  #if TORRENT_USE_ASSERTS
  		int prev_size = int(m_downloads[download_state].size());
  #endif
  
  		// since we're removing a downloading_piece, we also need to free its
  		// blocks that are allocated from the m_block_info array.
  		m_free_block_infos.push_back(i->info_idx);
  
  		TORRENT_ASSERT(find_dl_piece(download_state, i->index) == i);
  		m_piece_map[i->index].download_state = piece_pos::piece_open;
  		m_downloads[download_state].erase(i);
  
  		TORRENT_ASSERT(prev_size == int(m_downloads[download_state].size()) + 1);
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  	}
  
  	std::vector<piece_picker::downloading_piece> piece_picker::get_download_queue() const
  	{
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  		std::vector<downloading_piece> ret;
  		for (int k = 0; k < piece_pos::num_download_categories; ++k)
  			ret.insert(ret.end(), m_downloads[k].begin(), m_downloads[k].end());
  		return ret;
  	}
  
  	int piece_picker::get_download_queue_size() const
  	{
  		int ret = 0;
  		for (int k = 0; k < piece_pos::num_download_categories; ++k)
  			ret += int(m_downloads[k].size());
  		return ret;
  	}
  
  	void piece_picker::get_download_queue_sizes(int* partial
  		, int* full, int* finished, int* zero_prio) const
  	{
  		*partial = int(m_downloads[piece_pos::piece_downloading].size());
  		*full = int(m_downloads[piece_pos::piece_full].size());
  		*finished = int(m_downloads[piece_pos::piece_finished].size());
  		*zero_prio = int(m_downloads[piece_pos::piece_zero_prio].size());
  	}
  
  	piece_picker::block_info* piece_picker::blocks_for_piece(
  		downloading_piece const& dp)
  	{
  		int idx = int(dp.info_idx) * m_blocks_per_piece;
  		TORRENT_ASSERT(idx + m_blocks_per_piece <= int(m_block_info.size()));
  		return &m_block_info[idx];
  	}
  
  	piece_picker::block_info const* piece_picker::blocks_for_piece(
  		downloading_piece const& dp) const
  	{
  		return const_cast<piece_picker*>(this)->blocks_for_piece(dp);
  	}
  
  #if TORRENT_USE_INVARIANT_CHECKS
  
  	void piece_picker::check_piece_state() const
  	{
  		for (int k = 0; k < piece_pos::num_download_categories; ++k)
  		{
  			if (!m_downloads[k].empty())
  			{
  				for (std::vector<downloading_piece>::const_iterator i = m_downloads[k].begin();
  						i != m_downloads[k].end() - 1; ++i)
  				{
  					downloading_piece const& dp = *i;
  					downloading_piece const& next = *(i + 1);
  					TORRENT_ASSERT(dp.index < next.index);
  					TORRENT_ASSERT(int(dp.info_idx) * m_blocks_per_piece
  						+ m_blocks_per_piece <= int(m_block_info.size()));
  					block_info const* info = blocks_for_piece(dp);
  					for (int j = 0; j < m_blocks_per_piece; ++j)
  					{
  						if (info[j].peer)
  						{
  							torrent_peer* p = info[j].peer;
  							TORRENT_ASSERT(p->in_use);
  							TORRENT_ASSERT(p->connection == nullptr
  								|| static_cast<peer_connection*>(p->connection)->m_in_use);
  						}
  					}
  				}
  			}
  		}
  	}
  
  	void piece_picker::verify_pick(std::vector<piece_block> const& picked
  		, typed_bitfield<piece_index_t> const& bits) const
  	{
  		TORRENT_ASSERT(bits.size() == int(m_piece_map.size()));
  		for (piece_block const& pb : picked)
  		{
  			TORRENT_ASSERT(bits[pb.piece_index]);
  			TORRENT_ASSERT(!m_piece_map[pb.piece_index].have());
  			TORRENT_ASSERT(!m_piece_map[pb.piece_index].filtered());
  		}
  	}
  
  	void piece_picker::verify_priority(prio_index_t const range_start
  		, prio_index_t const range_end
  		, int const prio) const
  	{
  		TORRENT_ASSERT(range_start <= range_end);
  		TORRENT_ASSERT(range_end <= m_pieces.end_index());
  		for (auto index : range(m_pieces, range_start, range_end))
  		{
  			int p = m_piece_map[index].priority(this);
  			TORRENT_ASSERT(p == prio);
  		}
  	}
  
  #if defined TORRENT_PICKER_LOG
  	void piece_picker::print_pieces() const
  	{
  		int limit = 20;
  		std::cerr << "[" << this << "] ";
  		if (m_dirty)
  		{
  			std::cerr << " === dirty ===" << std::endl;
  			return;
  		}
  
  		for (prio_index_t b : m_priority_boundaries)
  			std::cerr << b << " ";
  
  		std::cerr << std::endl;
  		prio_index_t index(0);
  		std::cerr << "[" << this << "] ";
  		auto j = m_priority_boundaries.begin();
  		for (auto i = m_pieces.begin(), end(m_pieces.end()); i != end; ++i, ++index)
  		{
  			if (limit == 0)
  			{
  				std::cerr << " ...";
  				break;
  			}
  			if (*i == -1) break;
  			while (j != m_priority_boundaries.end() && *j <= index)
  			{
  				std::cerr << "| ";
  				++j;
  			}
  			std::cerr << *i << "(" << m_piece_map[*i].index << ") ";
  			--limit;
  		}
  		std::cerr << std::endl;
  	}
  #endif // TORRENT_PICKER_LOG
  #endif // TORRENT_USE_INVARIANT_CHECKS
  
  #if TORRENT_USE_INVARIANT_CHECKS
  	void piece_picker::check_peer_invariant(typed_bitfield<piece_index_t> const& have
  		, torrent_peer const* p) const
  	{
  #ifdef TORRENT_DEBUG_REFCOUNTS
  		for (piece_index_t i(0); i < have.end_index(); ++i)
  		{
  			bool const h = have[i];
  			TORRENT_ASSERT(int(m_piece_map[i].have_peers.count(p)) == (h ? 1 : 0));
  		}
  #else
  		TORRENT_UNUSED(have);
  		TORRENT_UNUSED(p);
  #endif
  	}
  
  	void piece_picker::check_invariant(torrent const* t) const
  	{
  		TORRENT_ASSERT(m_num_have >= 0);
  		TORRENT_ASSERT(m_num_have_filtered >= 0);
  		TORRENT_ASSERT(m_num_filtered >= 0);
  		TORRENT_ASSERT(m_seeds >= 0);
  
  		// make sure the priority boundaries are monotonically increasing. The
  		// difference between two cursors cannot be negative, but ranges are
  		// allowed to be empty.
  		prio_index_t last(0);
  		for (prio_index_t b : m_priority_boundaries)
  		{
  			TORRENT_ASSERT(b >= last);
  			last = b;
  		}
  
  		for (int k = 0; k < piece_pos::num_download_categories; ++k)
  		{
  			if (!m_downloads[k].empty())
  			{
  				for (std::vector<downloading_piece>::const_iterator i = m_downloads[k].begin();
  						i != m_downloads[k].end() - 1; ++i)
  				{
  					downloading_piece const& dp = *i;
  					downloading_piece const& next = *(i + 1);
  //					TORRENT_ASSERT(dp.finished + dp.writing >= next.finished + next.writing);
  					TORRENT_ASSERT(dp.index < next.index);
  					TORRENT_ASSERT(int(dp.info_idx) * m_blocks_per_piece
  						+ m_blocks_per_piece <= int(m_block_info.size()));
  #if TORRENT_USE_ASSERTS
  					block_info const* info = blocks_for_piece(dp);
  					for (int j = 0; j < m_blocks_per_piece; ++j)
  					{
  						if (!info[j].peer) continue;
  						torrent_peer* p = info[j].peer;
  						TORRENT_ASSERT(p->in_use);
  						TORRENT_ASSERT(p->connection == nullptr
  							|| static_cast<peer_connection*>(p->connection)->m_in_use);
  					}
  #endif
  				}
  			}
  		}
  
  		if (t != nullptr)
  			TORRENT_ASSERT(int(m_piece_map.size()) == t->torrent_file().num_pieces());
  
  		for (int j = 0; j < piece_pos::num_download_categories; ++j)
  		{
  			for (auto const& dp : m_downloads[j])
  			{
  				TORRENT_ASSERT(m_piece_map[dp.index].download_queue() == j);
  				const int num_blocks = blocks_in_piece(dp.index);
  				int num_requested = 0;
  				int num_finished = 0;
  				int num_writing = 0;
  				int num_open = 0;
  				block_info const* info = blocks_for_piece(dp);
  				for (int k = 0; k < num_blocks; ++k)
  				{
  					TORRENT_ASSERT(info[k].piece_index == dp.index);
  					TORRENT_ASSERT(info[k].peer == nullptr
  						|| info[k].peer->in_use);
  
  					if (info[k].state == block_info::state_finished)
  					{
  						++num_finished;
  						TORRENT_ASSERT(info[k].num_peers == 0);
  					}
  					else if (info[k].state == block_info::state_requested)
  					{
  						++num_requested;
  						TORRENT_ASSERT(info[k].num_peers > 0);
  					}
  					else if (info[k].state == block_info::state_writing)
  					{
  						++num_writing;
  						TORRENT_ASSERT(info[k].num_peers == 0);
  					}
  					else if (info[k].state == block_info::state_none)
  					{
  						++num_open;
  						TORRENT_ASSERT(info[k].num_peers == 0);
  					}
  				}
  
  				switch(j)
  				{
  					case piece_pos::piece_downloading:
  						TORRENT_ASSERT(!m_piece_map[dp.index].filtered());
  						TORRENT_ASSERT(num_open > 0);
  					break;
  					case piece_pos::piece_full:
  						TORRENT_ASSERT(!m_piece_map[dp.index].filtered());
  						TORRENT_ASSERT(num_open == 0);
  						// if requested == 0, the piece should be in the finished state
  						TORRENT_ASSERT(num_requested > 0);
  					break;
  					case piece_pos::piece_finished:
  						TORRENT_ASSERT(!m_piece_map[dp.index].filtered());
  						TORRENT_ASSERT(num_open == 0);
  						TORRENT_ASSERT(num_requested == 0);
  						TORRENT_ASSERT(num_finished + num_writing == num_blocks);
  					break;
  					case piece_pos::piece_zero_prio:
  						TORRENT_ASSERT(m_piece_map[dp.index].filtered());
  					break;
  				}
  
  				TORRENT_ASSERT(num_requested == dp.requested);
  				TORRENT_ASSERT(num_writing == dp.writing);
  				TORRENT_ASSERT(num_finished == dp.finished);
  
  				if (m_piece_map[dp.index].download_queue() == piece_pos::piece_full
  					|| m_piece_map[dp.index].download_queue() == piece_pos::piece_finished)
  					TORRENT_ASSERT(num_finished + num_writing + num_requested == num_blocks);
  			}
  		}
  		TORRENT_ASSERT(m_cursor >= piece_index_t(0));
  		TORRENT_ASSERT(m_cursor <= m_piece_map.end_index());
  		TORRENT_ASSERT(m_reverse_cursor >= piece_index_t(0));
  		TORRENT_ASSERT(m_reverse_cursor <= m_piece_map.end_index());
  		TORRENT_ASSERT(m_reverse_cursor > m_cursor
  			|| (m_cursor == m_piece_map.end_index()
  				&& m_reverse_cursor == piece_index_t(0)));
  
  		if (!m_dirty)
  		{
  			TORRENT_ASSERT(!m_priority_boundaries.empty());
  			int prio = 0;
  			prio_index_t start(0);
  			for (prio_index_t b : m_priority_boundaries)
  			{
  				verify_priority(start, b, prio);
  				++prio;
  				start = b;
  			}
  			TORRENT_ASSERT(m_priority_boundaries.back() == m_pieces.end_index());
  		}
  
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		{
  			piece_index_t index(0);
  			for (auto i = m_piece_map.begin()
  				, end(m_piece_map.end()); i != end && (i->have() || i->filtered());
  				++i, ++index);
  			TORRENT_ASSERT(m_cursor == index);
  			int const num_pieces = int(m_piece_map.size());
  			index = m_piece_map.end_index();
  			if (num_pieces > 0)
  			{
  				for (auto i = m_piece_map.rend() - static_cast<int>(index); index > piece_index_t(0)
  					&& (i->have() || i->filtered()); ++i, --index);
  				TORRENT_ASSERT(index == m_piece_map.end_index()
  					|| m_piece_map[index].have()
  					|| m_piece_map[index].filtered());
  				TORRENT_ASSERT(m_reverse_cursor == index);
  			}
  			else
  			{
  				TORRENT_ASSERT(m_reverse_cursor == piece_index_t(0));
  			}
  		}
  
  		int num_filtered = 0;
  		int num_have_filtered = 0;
  		int num_have = 0;
  		piece_index_t piece(0);
  		for (auto i = m_piece_map.begin(); i != m_piece_map.end(); ++i, ++piece)
  		{
  			piece_pos const& p = *i;
  
  			if (p.filtered())
  			{
  				if (p.index != piece_pos::we_have_index)
  					++num_filtered;
  				else
  					++num_have_filtered;
  			}
  
  #ifdef TORRENT_DEBUG_REFCOUNTS
  			TORRENT_ASSERT(int(p.have_peers.size()) == p.peer_count + m_seeds);
  #endif
  			if (p.index == piece_pos::we_have_index)
  				++num_have;
  
  			if (p.index == piece_pos::we_have_index)
  			{
  				TORRENT_ASSERT(t == nullptr || t->have_piece(piece));
  				TORRENT_ASSERT(p.downloading() == false);
  			}
  
  			if (t != nullptr)
  				TORRENT_ASSERT(!t->have_piece(piece));
  
  			int const prio = p.priority(this);
  
  #if TORRENT_USE_ASSERTS
  			if (p.downloading())
  			{
  				if (p.reverse())
  					TORRENT_ASSERT(prio == -1 || (prio % piece_picker::prio_factor == 2));
  				else
  					TORRENT_ASSERT(prio == -1 || (prio % piece_picker::prio_factor == 0));
  			}
  			else
  			{
  				TORRENT_ASSERT(prio == -1 || (prio % piece_picker::prio_factor == 1));
  			}
  #endif
  
  			if (!m_dirty)
  			{
  				TORRENT_ASSERT(prio < int(m_priority_boundaries.size()));
  				if (prio >= 0)
  				{
  					TORRENT_ASSERT(p.index < m_pieces.end_index());
  					TORRENT_ASSERT(m_pieces[p.index] == piece);
  				}
  				else
  				{
  					TORRENT_ASSERT(prio == -1);
  					// make sure there's no entry
  					// with this index. (there shouldn't
  					// be since the priority is -1)
  					TORRENT_ASSERT(std::count(m_pieces.begin(), m_pieces.end(), piece) == 0);
  				}
  			}
  
  			int const count_downloading = int(std::count_if(
  				m_downloads[piece_pos::piece_downloading].begin()
  				, m_downloads[piece_pos::piece_downloading].end()
  				, has_index(piece)));
  
  			int const count_full = int(std::count_if(
  				m_downloads[piece_pos::piece_full].begin()
  				, m_downloads[piece_pos::piece_full].end()
  				, has_index(piece)));
  
  			int const count_finished = int(std::count_if(
  				m_downloads[piece_pos::piece_finished].begin()
  				, m_downloads[piece_pos::piece_finished].end()
  				, has_index(piece)));
  
  			int const count_zero = int(std::count_if(
  				m_downloads[piece_pos::piece_zero_prio].begin()
  				, m_downloads[piece_pos::piece_zero_prio].end()
  				, has_index(piece)));
  
  			TORRENT_ASSERT(i->download_queue() == piece_pos::piece_open
  				|| count_zero + count_downloading + count_full
  					+ count_finished == 1);
  
  			switch(i->download_queue())
  			{
  				case piece_pos::piece_open:
  					TORRENT_ASSERT(count_downloading
  						+ count_full + count_finished + count_zero == 0);
  					break;
  				case piece_pos::piece_downloading:
  					TORRENT_ASSERT(count_downloading == 1);
  					break;
  				case piece_pos::piece_full:
  					TORRENT_ASSERT(count_full == 1);
  					break;
  				case piece_pos::piece_finished:
  					TORRENT_ASSERT(count_finished == 1);
  					break;
  				case piece_pos::piece_zero_prio:
  					TORRENT_ASSERT(count_zero == 1);
  					break;
  			};
  		}
  		TORRENT_ASSERT(num_have == m_num_have);
  		TORRENT_ASSERT(num_filtered == m_num_filtered);
  		TORRENT_ASSERT(num_have_filtered == m_num_have_filtered);
  
  		if (!m_dirty)
  		{
  			for (piece_index_t i : m_pieces)
  			{
  				TORRENT_ASSERT(m_piece_map[i].priority(this) >= 0);
  			}
  		}
  #endif // TORRENT_EXPENSIVE_INVARIANT_CHECKS
  	}
  #endif
  
  	std::pair<int, int> piece_picker::distributed_copies() const
  	{
  		TORRENT_ASSERT(m_seeds >= 0);
  		const int num_pieces = int(m_piece_map.size());
  
  		if (num_pieces == 0) return std::make_pair(1, 0);
  		int min_availability = piece_pos::max_peer_count;
  		// find the lowest availability count
  		// count the number of pieces that have that availability
  		// and also the number of pieces that have more than that.
  		int integer_part = 0;
  		int fraction_part = 0;
  		for (std::vector<piece_pos>::const_iterator i = m_piece_map.begin()
  			, end(m_piece_map.end()); i != end; ++i)
  		{
  			int peer_count = int(i->peer_count);
  			// take ourself into account
  			if (i->have()) ++peer_count;
  			if (min_availability > peer_count)
  			{
  				min_availability = peer_count;
  				fraction_part += integer_part;
  				integer_part = 1;
  			}
  			else if (peer_count == min_availability)
  			{
  				++integer_part;
  			}
  			else
  			{
  				TORRENT_ASSERT(peer_count > min_availability);
  				++fraction_part;
  			}
  		}
  		TORRENT_ASSERT(integer_part + fraction_part == num_pieces);
  		return std::make_pair(min_availability + m_seeds, fraction_part * 1000 / num_pieces);
  	}
  
  	prio_index_t piece_picker::priority_begin(int const prio) const
  	{
  		TORRENT_ASSERT(prio >= 0);
  		TORRENT_ASSERT(prio < int(m_priority_boundaries.size()));
  		return prio == 0 ? prio_index_t(0) : m_priority_boundaries[prio-1];
  	}
  
  	prio_index_t piece_picker::priority_end(int const prio) const
  	{
  		TORRENT_ASSERT(prio >= 0);
  		TORRENT_ASSERT(prio < int(m_priority_boundaries.size()));
  		return m_priority_boundaries[prio];
  	}
  
  	std::pair<prio_index_t, prio_index_t> piece_picker::priority_range(int const prio) const
  	{
  		TORRENT_ASSERT(prio >= 0);
  		TORRENT_ASSERT(prio < int(m_priority_boundaries.size()));
  		return { priority_begin(prio), priority_end(prio) };
  	}
  
  	void piece_picker::add(piece_index_t index)
  	{
  		TORRENT_ASSERT(!m_dirty);
  		piece_pos const& p = m_piece_map[index];
  		TORRENT_ASSERT(!p.filtered());
  		TORRENT_ASSERT(!p.have());
  
  		int priority = p.priority(this);
  		TORRENT_ASSERT(priority >= 0);
  		if (priority < 0) return;
  
  		if (int(m_priority_boundaries.size()) <= priority)
  			m_priority_boundaries.resize(priority + 1, m_pieces.end_index());
  
  		TORRENT_ASSERT(int(m_priority_boundaries.size()) >= priority);
  
  		auto const range = priority_range(priority);
  		prio_index_t new_index = (range.second == range.first)
  			? range.first
  			: prio_index_t(random(static_cast<int>(range.second - range.first)) + static_cast<int>(range.first));
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "add " << index << " (" << priority << ")" << std::endl;
  		std::cerr << "[" << this << "] " << "  p: state: " << p.download_state
  			<< " peer_count: " << p.peer_count
  			<< " prio: " << p.piece_priority
  			<< " index: " << p.index << std::endl;
  		print_pieces();
  #endif
  		m_pieces.push_back(piece_index_t(-1));
  
  		for (;;)
  		{
  			TORRENT_ASSERT(new_index < m_pieces.end_index());
  			{
  				piece_index_t temp = m_pieces[new_index];
  				m_pieces[new_index] = index;
  				m_piece_map[index].index = new_index;
  				index = temp;
  			}
  			prio_index_t temp(-1);
  			do
  			{
  				temp = m_priority_boundaries[priority]++;
  				++priority;
  			} while (temp == new_index && priority < int(m_priority_boundaries.size()));
  			new_index = temp;
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  			std::cerr << "[" << this << "] " << " index: " << index
  				<< " prio: " << priority
  				<< " new_index: " << new_index
  				<< std::endl;
  #endif
  			if (priority >= int(m_priority_boundaries.size())) break;
  			TORRENT_ASSERT(temp >= prio_index_t(0));
  		}
  		if (index != piece_index_t(-1))
  		{
  			TORRENT_ASSERT(new_index == prev(m_pieces.end_index()));
  			m_pieces[new_index] = index;
  			m_piece_map[index].index = new_index;
  
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  		}
  	}
  
  	void piece_picker::remove(int priority, prio_index_t elem_index)
  	{
  		TORRENT_ASSERT(!m_dirty);
  		TORRENT_ASSERT(priority >= 0);
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "remove " << m_pieces[elem_index] << " (" << priority << ")" << std::endl;
  #endif
  		prio_index_t next_index = elem_index;
  		TORRENT_ASSERT(m_piece_map[m_pieces[elem_index]].priority(this) == -1);
  		for (;;)
  		{
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			TORRENT_ASSERT(elem_index < m_pieces.end_index());
  			prio_index_t temp;
  			do
  			{
  				temp = --m_priority_boundaries[priority];
  				++priority;
  			} while (next_index == temp && priority < int(m_priority_boundaries.size()));
  			if (next_index == temp) break;
  			next_index = temp;
  
  			piece_index_t const piece = m_pieces[next_index];
  			m_pieces[elem_index] = piece;
  			m_piece_map[piece].index = elem_index;
  			TORRENT_ASSERT(m_piece_map[piece].priority(this) == priority - 1);
  			TORRENT_ASSERT(elem_index < prev(m_pieces.end_index()));
  			elem_index = next_index;
  
  			if (priority == int(m_priority_boundaries.size()))
  				break;
  		}
  		m_pieces.pop_back();
  		TORRENT_ASSERT(next_index == m_pieces.end_index());
  #ifdef TORRENT_PICKER_LOG
  		print_pieces();
  #endif
  	}
  
  	// will update the piece with the given properties (priority, elem_index)
  	// to place it at the correct position
  	void piece_picker::update(int priority, prio_index_t elem_index)
  	{
  		TORRENT_ASSERT(!m_dirty);
  		TORRENT_ASSERT(priority >= 0);
  		TORRENT_ASSERT(int(m_priority_boundaries.size()) > priority);
  
  		// make sure the passed in elem_index actually lives in the specified
  		// priority bucket. If it doesn't, it means this piece changed
  		// state without updating the corresponding entry in the pieces list
  		TORRENT_ASSERT(m_priority_boundaries[priority] >= elem_index);
  		TORRENT_ASSERT(elem_index >= priority_begin(priority));
  		TORRENT_ASSERT(elem_index < priority_end(priority));
  
  		piece_index_t const index = m_pieces[elem_index];
  		// update the piece_map
  		piece_pos& p = m_piece_map[index];
  		TORRENT_ASSERT(p.index == elem_index || p.have());
  
  		int const new_priority = p.priority(this);
  
  		if (new_priority == priority) return;
  
  		if (new_priority == -1)
  		{
  			remove(priority, elem_index);
  			return;
  		}
  
  		if (int(m_priority_boundaries.size()) <= new_priority)
  			m_priority_boundaries.resize(new_priority + 1, m_pieces.end_index());
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "update " << index << " (" << priority << "->" << new_priority << ")" << std::endl;
  #endif
  		if (priority > new_priority)
  		{
  			prio_index_t new_index;
  			piece_index_t temp = index;
  			for (;;)
  			{
  #ifdef TORRENT_PICKER_LOG
  				print_pieces();
  #endif
  				TORRENT_ASSERT(priority > 0);
  				--priority;
  				new_index = m_priority_boundaries[priority]++;
  				if (temp != m_pieces[new_index])
  				{
  					temp = m_pieces[new_index];
  					m_pieces[elem_index] = temp;
  					m_piece_map[temp].index = elem_index;
  					TORRENT_ASSERT(elem_index < m_pieces.end_index());
  				}
  				elem_index = new_index;
  				if (priority == new_priority) break;
  			}
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			m_pieces[elem_index] = index;
  			m_piece_map[index].index = elem_index;
  			TORRENT_ASSERT(elem_index < m_pieces.end_index());
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			shuffle(priority, elem_index);
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			TORRENT_ASSERT(m_piece_map[index].priority(this) == priority);
  		}
  		else
  		{
  			prio_index_t new_index;
  			piece_index_t temp = index;
  			for (;;)
  			{
  #ifdef TORRENT_PICKER_LOG
  				print_pieces();
  #endif
  				TORRENT_ASSERT(priority >= 0);
  				TORRENT_ASSERT(priority < int(m_priority_boundaries.size()));
  				new_index = --m_priority_boundaries[priority];
  				if (temp != m_pieces[new_index])
  				{
  					temp = m_pieces[new_index];
  					m_pieces[elem_index] = temp;
  					m_piece_map[temp].index = elem_index;
  					TORRENT_ASSERT(elem_index < m_pieces.end_index());
  				}
  				elem_index = new_index;
  				++priority;
  				if (priority == new_priority) break;
  			}
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			m_pieces[elem_index] = index;
  			m_piece_map[index].index = elem_index;
  			TORRENT_ASSERT(elem_index < m_pieces.end_index());
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			shuffle(priority, elem_index);
  #ifdef TORRENT_PICKER_LOG
  			print_pieces();
  #endif
  			TORRENT_ASSERT(m_piece_map[index].priority(this) == priority);
  		}
  	}
  
  	void piece_picker::shuffle(int priority, prio_index_t elem_index)
  	{
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "shuffle()" << std::endl;
  #endif
  
  		TORRENT_ASSERT(!m_dirty);
  		TORRENT_ASSERT(priority >= 0);
  		TORRENT_ASSERT(elem_index >= prio_index_t(0));
  		TORRENT_ASSERT(elem_index < m_pieces.end_index());
  		TORRENT_ASSERT(m_piece_map[m_pieces[elem_index]].priority(this) == priority);
  
  		auto const range = priority_range(priority);
  		prio_index_t const other_index(random(static_cast<int>(range.second - range.first) - 1) + static_cast<int>(range.first));
  
  		if (other_index == elem_index) return;
  
  		// swap other_index with elem_index
  		piece_pos& p1 = m_piece_map[m_pieces[other_index]];
  		piece_pos& p2 = m_piece_map[m_pieces[elem_index]];
  
  		std::swap(p1.index, p2.index);
  		std::swap(m_pieces[other_index], m_pieces[elem_index]);
  	}
  
  	void piece_picker::restore_piece(piece_index_t index)
  	{
  		INVARIANT_CHECK;
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "restore_piece(" << index << ")" << std::endl;
  #endif
  		int download_state = m_piece_map[index].download_queue();
  		TORRENT_ASSERT(download_state != piece_pos::piece_open);
  		if (download_state == piece_pos::piece_open) return;
  
  		std::vector<downloading_piece>::iterator i = find_dl_piece(download_state, index);
  
  		TORRENT_ASSERT(i != m_downloads[download_state].end());
  		TORRENT_ASSERT(int(i->info_idx) * m_blocks_per_piece
  			+ m_blocks_per_piece <= int(m_block_info.size()));
  
  		i->locked = false;
  
  		piece_pos& p = m_piece_map[index];
  		int const prev_priority = p.priority(this);
  		erase_download_piece(i);
  		int const new_priority = p.priority(this);
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  		if (new_priority == prev_priority) return;
  		if (m_dirty) return;
  		if (prev_priority == -1) add(index);
  		else update(prev_priority, p.index);
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  	}
  
  	void piece_picker::inc_refcount_all(const torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  		++m_seeds;
  		if (m_seeds == 1)
  		{
  			// when m_seeds is increased from 0 to 1
  			// we may have to add pieces that previously
  			// didn't have any peers
  			m_dirty = true;
  		}
  #ifdef TORRENT_DEBUG_REFCOUNTS
  		for (std::vector<piece_pos>::iterator i = m_piece_map.begin()
  			, end(m_piece_map.end()); i != end; ++i)
  		{
  			TORRENT_ASSERT(i->have_peers.count(peer) == 0);
  			i->have_peers.insert(peer);
  		}
  #else
  		TORRENT_UNUSED(peer);
  #endif
  	}
  
  	void piece_picker::dec_refcount_all(const torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  		if (m_seeds > 0)
  		{
  			--m_seeds;
  			if (m_seeds == 0)
  			{
  				// when m_seeds is decreased from 1 to 0
  				// we may have to remove pieces that previously
  				// didn't have any peers
  				m_dirty = true;
  			}
  #ifdef TORRENT_DEBUG_REFCOUNTS
  			for (std::vector<piece_pos>::iterator i = m_piece_map.begin()
  				, end(m_piece_map.end()); i != end; ++i)
  			{
  				TORRENT_ASSERT(i->have_peers.count(peer) == 1);
  				i->have_peers.erase(peer);
  			}
  #else
  			TORRENT_UNUSED(peer);
  #endif
  			return;
  		}
  		TORRENT_ASSERT(m_seeds == 0);
  
  		for (auto& i : m_piece_map)
  		{
  #ifdef TORRENT_DEBUG_REFCOUNTS
  			TORRENT_ASSERT(i.have_peers.count(peer) == 1);
  			i.have_peers.erase(peer);
  #else
  			TORRENT_UNUSED(peer);
  #endif
  
  			TORRENT_ASSERT(i.peer_count > 0);
  			--i.peer_count;
  		}
  
  		m_dirty = true;
  	}
  
  	void piece_picker::inc_refcount(piece_index_t const index
  		, const torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "inc_refcount(" << index << ")" << std::endl;
  #endif
  		piece_pos& p = m_piece_map[index];
  
  #ifdef TORRENT_DEBUG_REFCOUNTS
  		TORRENT_ASSERT(p.have_peers.count(peer) == 0);
  		p.have_peers.insert(peer);
  #else
  		TORRENT_UNUSED(peer);
  #endif
  
  		int prev_priority = p.priority(this);
  		++p.peer_count;
  		if (m_dirty) return;
  		int new_priority = p.priority(this);
  		if (prev_priority == new_priority) return;
  		if (prev_priority == -1)
  			add(index);
  		else
  			update(prev_priority, p.index);
  	}
  
  	// this function decrements the m_seeds counter
  	// and increments the peer counter on every piece
  	// instead. Sometimes of we connect to a seed that
  	// later sends us a dont-have message, we'll need to
  	// turn that m_seed into counts on the pieces since
  	// they can't be negative
  	void piece_picker::break_one_seed()
  	{
  		INVARIANT_CHECK;
  
  		TORRENT_ASSERT(m_seeds > 0);
  		--m_seeds;
  
  		for (std::vector<piece_pos>::iterator i = m_piece_map.begin()
  			, end(m_piece_map.end()); i != end; ++i)
  		{
  			++i->peer_count;
  		}
  
  		m_dirty = true;
  	}
  
  	void piece_picker::dec_refcount(piece_index_t const index
  		, const torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "dec_refcount(" << index << ")" << std::endl;
  #endif
  
  		piece_pos& p = m_piece_map[index];
  
  		if (p.peer_count == 0)
  		{
  			TORRENT_ASSERT(m_seeds > 0);
  			// this is the case where we have one or more
  			// seeds, and one of them saying: I don't have this
  			// piece anymore. we need to break up one of the seed
  			// counters into actual peer counters on the pieces
  			break_one_seed();
  		}
  
  		int prev_priority = p.priority(this);
  
  #ifdef TORRENT_DEBUG_REFCOUNTS
  		TORRENT_ASSERT(p.have_peers.count(peer) == 1);
  		p.have_peers.erase(peer);
  #else
  		TORRENT_UNUSED(peer);
  #endif
  
  		TORRENT_ASSERT(p.peer_count > 0);
  		--p.peer_count;
  		if (m_dirty) return;
  		if (prev_priority >= 0) update(prev_priority, p.index);
  	}
  
  	void piece_picker::inc_refcount(typed_bitfield<piece_index_t> const& bitmask
  		, const torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "inc_refcount(bitfield)" << std::endl;
  #endif
  
  		// nothing set, nothing to do here
  		if (bitmask.none_set()) return;
  
  		if (bitmask.all_set() && bitmask.size() == int(m_piece_map.size()))
  		{
  			inc_refcount_all(peer);
  			return;
  		}
  
  		int const size = std::min(50, bitmask.size() / 2);
  
  		// this is an optimization where if just a few
  		// pieces end up changing, instead of making
  		// the piece list dirty, just update those pieces
  		// instead
  		TORRENT_ALLOCA(incremented, piece_index_t, size);
  		int num_inc = 0;
  
  		if (!m_dirty)
  		{
  			// first count how many pieces we're updating. If it's few (less than half)
  			// we'll just update them one at a time. Otherwise we'll just update the counters
  			// and mark the picker as dirty, so we'll rebuild it next time we need it.
  			// this only matters if we're not already dirty, in which case the fasted
  			// thing to do is to just update the counters and be done
  			piece_index_t index = piece_index_t(0);
  			for (auto i = bitmask.begin(), end(bitmask.end()); i != end; ++i, ++index)
  			{
  				if (!*i) continue;
  				if (num_inc < size) incremented[num_inc] = index;
  				++num_inc;
  				if (num_inc >= size) break;
  			}
  
  			if (num_inc < size)
  			{
  				// not that many pieces were updated
  				// just update those individually instead of
  				// rebuilding the whole piece list
  				for (int i = 0; i < num_inc; ++i)
  				{
  					piece_index_t const piece = incremented[i];
  					piece_pos& p = m_piece_map[piece];
  					int prev_priority = p.priority(this);
  					++p.peer_count;
  #ifdef TORRENT_DEBUG_REFCOUNTS
  					TORRENT_ASSERT(p.have_peers.count(peer) == 0);
  					p.have_peers.insert(peer);
  #else
  					TORRENT_UNUSED(peer);
  #endif
  					int new_priority = p.priority(this);
  					if (prev_priority == new_priority) continue;
  					else if (prev_priority >= 0) update(prev_priority, p.index);
  					else add(piece);
  				}
  				return;
  			}
  		}
  
  		piece_index_t index = piece_index_t(0);
  		bool updated = false;
  		for (auto i = bitmask.begin(), end(bitmask.end()); i != end; ++i, ++index)
  		{
  			if (*i)
  			{
  #ifdef TORRENT_DEBUG_REFCOUNTS
  				TORRENT_ASSERT(m_piece_map[index].have_peers.count(peer) == 0);
  				m_piece_map[index].have_peers.insert(peer);
  #else
  				TORRENT_UNUSED(peer);
  #endif
  
  				++m_piece_map[index].peer_count;
  				updated = true;
  			}
  		}
  
  		// if we're already dirty, no point in doing anything more
  		if (m_dirty) return;
  
  		if (updated) m_dirty = true;
  	}
  
  	void piece_picker::dec_refcount(typed_bitfield<piece_index_t> const& bitmask
  		, const torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  		TORRENT_ASSERT(bitmask.size() <= int(m_piece_map.size()));
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "dec_refcount(bitfield)" << std::endl;
  #endif
  
  		// nothing set, nothing to do here
  		if (bitmask.none_set()) return;
  
  		if (bitmask.all_set() && bitmask.size() == int(m_piece_map.size()))
  		{
  			dec_refcount_all(peer);
  			return;
  		}
  
  		int const size = std::min(50, bitmask.size() / 2);
  
  		// this is an optimization where if just a few
  		// pieces end up changing, instead of making
  		// the piece list dirty, just update those pieces
  		// instead
  		TORRENT_ALLOCA(decremented, piece_index_t, size);
  		int num_dec = 0;
  
  		if (!m_dirty)
  		{
  			// first count how many pieces we're updating. If it's few (less than half)
  			// we'll just update them one at a time. Otherwise we'll just update the counters
  			// and mark the picker as dirty, so we'll rebuild it next time we need it.
  			// this only matters if we're not already dirty, in which case the fasted
  			// thing to do is to just update the counters and be done
  			piece_index_t index = piece_index_t(0);
  			for (auto i = bitmask.begin(), end(bitmask.end()); i != end; ++i, ++index)
  			{
  				if (!*i) continue;
  				if (num_dec < size) decremented[num_dec] = index;
  				++num_dec;
  				if (num_dec >= size) break;
  			}
  
  			if (num_dec < size)
  			{
  				// not that many pieces were updated
  				// just update those individually instead of
  				// rebuilding the whole piece list
  				for (int i = 0; i < num_dec; ++i)
  				{
  					piece_index_t const piece = decremented[i];
  					piece_pos& p = m_piece_map[piece];
  					int prev_priority = p.priority(this);
  
  					if (p.peer_count == 0)
  					{
  						TORRENT_ASSERT(m_seeds > 0);
  						// this is the case where we have one or more
  						// seeds, and one of them saying: I don't have this
  						// piece anymore. we need to break up one of the seed
  						// counters into actual peer counters on the pieces
  						break_one_seed();
  					}
  
  #ifdef TORRENT_DEBUG_REFCOUNTS
  					TORRENT_ASSERT(p.have_peers.count(peer) == 1);
  					p.have_peers.erase(peer);
  #else
  					TORRENT_UNUSED(peer);
  #endif
  					TORRENT_ASSERT(p.peer_count > 0);
  					--p.peer_count;
  					if (!m_dirty && prev_priority >= 0) update(prev_priority, p.index);
  				}
  				return;
  			}
  		}
  
  		piece_index_t index = piece_index_t(0);
  		bool updated = false;
  		for (auto i = bitmask.begin(), end(bitmask.end()); i != end; ++i, ++index)
  		{
  			if (*i)
  			{
  				piece_pos& p = m_piece_map[index];
  				if (p.peer_count == 0)
  				{
  					TORRENT_ASSERT(m_seeds > 0);
  					// this is the case where we have one or more
  					// seeds, and one of them saying: I don't have this
  					// piece anymore. we need to break up one of the seed
  					// counters into actual peer counters on the pieces
  					break_one_seed();
  				}
  
  #ifdef TORRENT_DEBUG_REFCOUNTS
  				TORRENT_ASSERT(p.have_peers.count(peer) == 1);
  				p.have_peers.erase(peer);
  #else
  				TORRENT_UNUSED(peer);
  #endif
  
  				TORRENT_ASSERT(p.peer_count > 0);
  				--p.peer_count;
  				updated = true;
  			}
  		}
  
  		// if we're already dirty, no point in doing anything more
  		if (m_dirty) return;
  
  		if (updated) m_dirty = true;
  	}
  
  	void piece_picker::update_pieces() const
  	{
  		TORRENT_ASSERT(m_dirty);
  		if (m_priority_boundaries.empty()) m_priority_boundaries.resize(1, prio_index_t(0));
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "update_pieces" << std::endl;
  #endif
  
  		// This code is unfortunately not very straight-forward. What we do here
  		// is to count the number of pieces at every priority level. After this
  		// first step, m_priority_boundaries will contain *deltas* rather than
  		// absolute indices. This is fixed up in a second pass below
  		std::fill(m_priority_boundaries.begin(), m_priority_boundaries.end(), prio_index_t(0));
  		for (auto& pos : m_piece_map)
  		{
  			int prio = pos.priority(this);
  			if (prio == -1) continue;
  			if (prio >= int(m_priority_boundaries.size()))
  				m_priority_boundaries.resize(prio + 1, prio_index_t(0));
  			pos.index = m_priority_boundaries[prio];
  			++m_priority_boundaries[prio];
  		}
  
  #ifdef TORRENT_PICKER_LOG
  		print_pieces();
  #endif
  
  		// m_priority_boundaries just contain counters of
  		// each priority level at this point. Now, make the m_priority_boundaries
  		// be cumulative indices into m_pieces (but m_pieces hasn't been set up
  		// yet)
  		int new_size = 0;
  		for (prio_index_t& b : m_priority_boundaries)
  		{
  			new_size += static_cast<int>(b);
  			b = prio_index_t(new_size);
  		}
  		m_pieces.resize(new_size, piece_index_t(0));
  
  #ifdef TORRENT_PICKER_LOG
  		print_pieces();
  #endif
  
  		// set up m_pieces to contain valid piece indices, based on piece
  		// priority. m_piece_map[].index is still just an index relative to the
  		// respective priority range.
  		piece_index_t piece = piece_index_t(0);
  		for (auto i = m_piece_map.begin(), end(m_piece_map.end()); i != end; ++i, ++piece)
  		{
  			piece_pos& p = *i;
  			int const prio = p.priority(this);
  			if (prio == -1) continue;
  			prio_index_t const new_index(priority_begin(prio)
  				+ prio_index_t::diff_type(static_cast<int>(p.index)));
  			m_pieces[new_index] = piece;
  		}
  
  		prio_index_t start(0);
  		for (auto b : m_priority_boundaries)
  		{
  			if (start == b) continue;
  			auto r = range(m_pieces, start, b);
  			aux::random_shuffle(r.begin(), r.end());
  			start = b;
  		}
  
  		// this is where we set fix up the m_piece_map[].index to actually map
  		// back to the piece list ordered by priority (m_pieces)
  		prio_index_t index(0);
  		for (auto p : m_pieces)
  		{
  			m_piece_map[p].index = index;
  			++index;
  		}
  
  		m_dirty = false;
  #ifdef TORRENT_PICKER_LOG
  		print_pieces();
  #endif
  	}
  
  	void piece_picker::piece_passed(piece_index_t const index)
  	{
  		piece_pos& p = m_piece_map[index];
  		int download_state = p.download_queue();
  
  		// this is kind of odd. Could this happen?
  		TORRENT_ASSERT(download_state != piece_pos::piece_open);
  		if (download_state == piece_pos::piece_open) return;
  
  		auto const i = find_dl_piece(download_state, index);
  		TORRENT_ASSERT(i != m_downloads[download_state].end());
  
  		TORRENT_ASSERT(i->locked == false);
  		if (i->locked) return;
  
  		TORRENT_ASSERT(!i->passed_hash_check);
  		i->passed_hash_check = true;
  		++m_num_passed;
  
  		if (i->finished < blocks_in_piece(index)) return;
  
  		we_have(index);
  	}
  
  	void piece_picker::we_dont_have(piece_index_t const index)
  	{
  		INVARIANT_CHECK;
  		piece_pos& p = m_piece_map[index];
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "piece_picker::we_dont_have("
  			<< index << ")" << std::endl;
  #endif
  
  		if (!p.have())
  		{
  			// even though we don't have the piece, it
  			// might still have passed hash check
  			int download_state = p.download_queue();
  			if (download_state == piece_pos::piece_open) return;
  
  			auto const i = find_dl_piece(download_state, index);
  			if (i->passed_hash_check)
  			{
  				i->passed_hash_check = false;
  				TORRENT_ASSERT(m_num_passed > 0);
  				--m_num_passed;
  			}
  			erase_download_piece(i);
  			return;
  		}
  
  		TORRENT_ASSERT(m_num_passed > 0);
  		--m_num_passed;
  		if (p.filtered())
  		{
  			++m_num_filtered;
  			--m_num_have_filtered;
  		}
  		else
  		{
  			// update cursors
  			if (index < m_cursor) m_cursor = index;
  			if (index >= m_reverse_cursor) m_reverse_cursor = next(index);
  			if (m_reverse_cursor == m_cursor)
  			{
  				m_reverse_cursor = piece_index_t(0);
  				m_cursor = m_piece_map.end_index();
  			}
  		}
  
  		--m_num_have;
  		p.set_not_have();
  
  		if (m_dirty) return;
  		if (p.priority(this) >= 0) add(index);
  	}
  
  	// this is used to indicate that we successfully have
  	// downloaded a piece, and that no further attempts
  	// to pick that piece should be made. The piece will
  	// be removed from the available piece list.
  	void piece_picker::we_have(piece_index_t const index)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "piece_picker::we_have("
  			<< index << ")" << std::endl;
  #endif
  		piece_pos& p = m_piece_map[index];
  		prio_index_t const info_index = p.index;
  		int const priority = p.priority(this);
  		TORRENT_ASSERT(priority < int(m_priority_boundaries.size()) || m_dirty);
  
  		if (p.have()) return;
  
  		int state = p.download_queue();
  		if (state != piece_pos::piece_open)
  		{
  			auto const i = find_dl_piece(state, index);
  			TORRENT_ASSERT(i != m_downloads[state].end());
  			// decrement num_passed here to compensate
  			// for the unconditional increment further down
  			if (i->passed_hash_check) --m_num_passed;
  			erase_download_piece(i);
  		}
  
  		if (p.filtered())
  		{
  			--m_num_filtered;
  			++m_num_have_filtered;
  		}
  		++m_num_have;
  		++m_num_passed;
  		p.set_have();
  		if (m_cursor == prev(m_reverse_cursor)
  			&& m_cursor == index)
  		{
  			m_cursor = m_piece_map.end_index();
  			m_reverse_cursor = piece_index_t(0);
  			TORRENT_ASSERT(num_pieces() > 0);
  		}
  		else if (m_cursor == index)
  		{
  			++m_cursor;
  			for (auto i = m_piece_map.begin() + static_cast<int>(m_cursor)
  				, end(m_piece_map.end()); i != end && (i->have() || i->filtered());
  				++i, ++m_cursor);
  		}
  		else if (prev(m_reverse_cursor) == index)
  		{
  			--m_reverse_cursor;
  			TORRENT_ASSERT(m_piece_map[m_reverse_cursor].have()
  				|| m_piece_map[m_reverse_cursor].filtered());
  			for (auto i = m_piece_map.begin() + static_cast<int>(m_reverse_cursor) - 1;
  				m_reverse_cursor > piece_index_t(0) && (i->have() || i->filtered());
  				--i, --m_reverse_cursor);
  			TORRENT_ASSERT(m_piece_map[m_reverse_cursor].have()
  				|| m_piece_map[m_reverse_cursor].filtered());
  		}
  		TORRENT_ASSERT(m_reverse_cursor > m_cursor
  			|| (m_cursor == m_piece_map.end_index() && m_reverse_cursor == piece_index_t(0)));
  		if (priority == -1) return;
  		if (m_dirty) return;
  		remove(priority, info_index);
  		TORRENT_ASSERT(p.priority(this) == -1);
  	}
  
  	bool piece_picker::set_piece_priority(piece_index_t const index, int const new_piece_priority)
  	{
  		INVARIANT_CHECK;
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "set_piece_priority(" << index
  			<< ", " << new_piece_priority << ")" << std::endl;
  #endif
  
  		TORRENT_ASSERT(new_piece_priority >= 0);
  		TORRENT_ASSERT(new_piece_priority < priority_levels);
  
  		piece_pos& p = m_piece_map[index];
  
  		// if the priority isn't changed, don't do anything
  		if (new_piece_priority == int(p.piece_priority)) return false;
  
  		int prev_priority = p.priority(this);
  		TORRENT_ASSERT(m_dirty || prev_priority < int(m_priority_boundaries.size()));
  
  		bool ret = false;
  		if (new_piece_priority == piece_pos::filter_priority
  			&& p.piece_priority != piece_pos::filter_priority)
  		{
  			// the piece just got filtered
  			if (p.have())
  			{
  				++m_num_have_filtered;
  			}
  			else
  			{
  				++m_num_filtered;
  
  				// update m_cursor
  				if (m_cursor == prev(m_reverse_cursor) && m_cursor == index)
  				{
  					m_cursor = m_piece_map.end_index();
  					m_reverse_cursor = piece_index_t(0);
  				}
  				else if (m_cursor == index)
  				{
  					++m_cursor;
  					while (m_cursor < m_piece_map.end_index()
  						&& (m_piece_map[m_cursor].have()
  						|| m_piece_map[m_cursor].filtered()))
  						++m_cursor;
  				}
  				else if (m_reverse_cursor == next(index))
  				{
  					--m_reverse_cursor;
  					while (m_reverse_cursor > piece_index_t(0)
  						&& (m_piece_map[prev(m_reverse_cursor)].have()
  						|| m_piece_map[prev(m_reverse_cursor)].filtered()))
  						--m_reverse_cursor;
  				}
  			}
  			ret = true;
  		}
  		else if (new_piece_priority != piece_pos::filter_priority
  			&& p.piece_priority == piece_pos::filter_priority)
  		{
  			// the piece just got unfiltered
  			if (p.have())
  			{
  				--m_num_have_filtered;
  			}
  			else
  			{
  				--m_num_filtered;
  				// update cursors
  				if (index < m_cursor) m_cursor = index;
  				if (index >= m_reverse_cursor) m_reverse_cursor = next(index);
  				if (m_reverse_cursor == m_cursor)
  				{
  					m_reverse_cursor = piece_index_t(0);
  					m_cursor = m_piece_map.end_index();
  				}
  			}
  			ret = true;
  		}
  		TORRENT_ASSERT(m_num_filtered >= 0);
  		TORRENT_ASSERT(m_num_have_filtered >= 0);
  
  		p.piece_priority = static_cast<std::uint16_t>(new_piece_priority);
  		int const new_priority = p.priority(this);
  
  		if (prev_priority != new_priority && !m_dirty)
  		{
  			if (prev_priority == -1) add(index);
  			else update(prev_priority, p.index);
  		}
  
  		if (p.downloading())
  		{
  			std::vector<downloading_piece>::iterator i = find_dl_piece(
  				p.download_queue(), index);
  			if (i != m_downloads[p.download_queue()].end())
  				update_piece_state(i);
  		}
  
  		return ret;
  	}
  
  	int piece_picker::piece_priority(piece_index_t const index) const
  	{
  		return m_piece_map[index].piece_priority;
  	}
  
  	void piece_picker::piece_priorities(std::vector<int>& pieces) const
  	{
  		pieces.resize(m_piece_map.size());
  		auto j = pieces.begin();
  		for (auto i = m_piece_map.begin(),
  			end(m_piece_map.end()); i != end; ++i, ++j)
  		{
  			*j = i->piece_priority;
  		}
  	}
  
  	namespace
  	{
  		int append_blocks(std::vector<piece_block>& dst, std::vector<piece_block>& src
  			, int const num_blocks)
  		{
  			if (src.empty()) return num_blocks;
  			int const to_copy = (std::min)(int(src.size()), num_blocks);
  
  			dst.insert(dst.end(), src.begin(), src.begin() + to_copy);
  			src.erase(src.begin(), src.begin() + to_copy);
  			return num_blocks - to_copy;
  		}
  	}
  
  	// lower availability comes first. This is a less-than comparison, it returns
  	// true if lhs has lower availability than rhs
  	bool piece_picker::partial_compare_rarest_first(downloading_piece const* lhs
  		, downloading_piece const* rhs) const
  	{
  		int lhs_availability = m_piece_map[lhs->index].peer_count;
  		int rhs_availability = m_piece_map[rhs->index].peer_count;
  		if (lhs_availability != rhs_availability)
  			return lhs_availability < rhs_availability;
  
  		// if the availability is the same, prefer the piece that's closest to
  		// being complete.
  		int lhs_blocks_left = m_blocks_per_piece - lhs->finished - lhs->writing
  			- lhs->requested;
  		TORRENT_ASSERT(lhs_blocks_left > 0);
  		int rhs_blocks_left = m_blocks_per_piece - rhs->finished - rhs->writing
  			- rhs->requested;
  		TORRENT_ASSERT(rhs_blocks_left > 0);
  		return lhs_blocks_left < rhs_blocks_left;
  	}
  
  	// pieces describes which pieces the peer we're requesting from has.
  	// interesting_blocks is an out parameter, and will be filled with (up to)
  	// num_blocks of interesting blocks that the peer has.
  	// prefer_contiguous_blocks can be set if this peer should download whole
  	// pieces rather than trying to download blocks from the same piece as other
  	// peers. the peer argument is the torrent_peer of the peer we're
  	// picking pieces from. This is used when downloading whole pieces, to only
  	// pick from the same piece the same peer is downloading from.
  
  	// options are:
  	// * rarest_first
  	//     pick the rarest pieces first
  	// * reverse
  	//     reverse the piece picking. Pick the most common
  	//     pieces first or the last pieces (if picking sequential)
  	// * sequential
  	//     download pieces in-order
  	// * on_parole
  	//     the peer is on parole, only pick whole pieces which
  	//     has only been downloaded and requested from the same
  	//     peer
  	// * prioritize_partials
  	//     pick blocks from downloading pieces first
  
  	// only one of rarest_first or sequential can be set
  
  	// the return value is a combination of picker_log_alert::picker_flags_t,
  	// indicating which path thought the picker we took to arrive at the
  	// returned block picks.
  	std::uint32_t piece_picker::pick_pieces(typed_bitfield<piece_index_t> const& pieces
  		, std::vector<piece_block>& interesting_blocks, int num_blocks
  		, int prefer_contiguous_blocks, torrent_peer* peer
  		, int options, std::vector<piece_index_t> const& suggested_pieces
  		, int num_peers
  		, counters& pc
  		) const
  	{
  		TORRENT_ASSERT(peer == nullptr || peer->in_use);
  		std::uint32_t ret = 0;
  
  		// prevent the number of partial pieces to grow indefinitely
  		// make this scale by the number of peers we have. For large
  		// scale clients, we would have more peers, and allow a higher
  		// threshold for the number of partials
  		// deduct pad files because they case partial pieces which are OK
  		// the second condition is to make sure we cap the number of partial
  		// _bytes_. The larger the pieces are, the fewer partial pieces we want.
  		// 2048 corresponds to 32 MiB
  		// TODO: 2 make the 2048 limit configurable
  		const int num_partials = int(m_downloads[piece_pos::piece_downloading].size())
  			- m_num_pad_files;
  		if (num_partials > num_peers * 3 / 2
  			|| num_partials * m_blocks_per_piece > 2048)
  		{
  			// if we have too many partial pieces, prioritize completing
  			// them. In order for this to have an affect, also disable
  			// prefer whole pieces (otherwise partial pieces would be de-prioritized)
  			options |= prioritize_partials;
  			prefer_contiguous_blocks = 0;
  
  			ret |= picker_log_alert::partial_ratio;
  		}
  
  		if (prefer_contiguous_blocks) ret |= picker_log_alert::prefer_contiguous;
  
  		// only one of rarest_first and sequential can be set.
  		TORRENT_ASSERT(((options & rarest_first) ? 1 : 0)
  			+ ((options & sequential) ? 1 : 0) <= 1);
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  		TORRENT_ASSERT(num_blocks > 0);
  		TORRENT_ASSERT(pieces.size() == int(m_piece_map.size()));
  
  		TORRENT_ASSERT(!m_priority_boundaries.empty() || m_dirty);
  
  		// this will be filled with blocks that we should not request
  		// unless we can't find num_blocks among the other ones.
  		std::vector<piece_block> backup_blocks;
  		std::vector<piece_block> backup_blocks2;
  		static const std::vector<piece_index_t> empty_vector;
  
  		// When prefer_contiguous_blocks is set (usually set when downloading from
  		// fast peers) the partial pieces will not be prioritized, but actually
  		// ignored as long as possible. All blocks found in downloading
  		// pieces are regarded as backup blocks
  
  		if (options & prioritize_partials)
  		{
  			// first, allocate a small array on the stack of all the partial
  			// pieces (downloading_piece). We'll then sort this list by
  			// availability or by some other condition. The list of partial pieces
  			// in m_downloads is ordered by piece index, this is to have O(log n)
  			// lookups when finding a downloading_piece for a specific piece index.
  			// this is important and needs to stay sorted that way, that's why
  			// we're copying it here
  			TORRENT_ALLOCA(ordered_partials, downloading_piece const*
  				, m_downloads[piece_pos::piece_downloading].size());
  			int num_ordered_partials = 0;
  
  			// now, copy over the pointers. We also apply a filter here to not
  			// include ineligible pieces in certain modes. For instance, a piece
  			// that the current peer doesn't have is not included.
  			for (std::vector<downloading_piece>::const_iterator i
  				= m_downloads[piece_pos::piece_downloading].begin()
  				, end(m_downloads[piece_pos::piece_downloading].end()); i != end; ++i)
  			{
  				pc.inc_stats_counter(counters::piece_picker_partial_loops);
  
  				// in time critical mode, only pick high priority pieces
  				if ((options & time_critical_mode)
  					&& piece_priority(i->index) != priority_levels - 1)
  					continue;
  
  				if (!is_piece_free(i->index, pieces)) continue;
  
  				TORRENT_ASSERT(m_piece_map[i->index].download_queue()
  					== piece_pos::piece_downloading);
  
  				ordered_partials[num_ordered_partials++] = &*i;
  			}
  
  			// now, sort the list.
  			if (options & rarest_first)
  			{
  				ret |= picker_log_alert::rarest_first_partials;
  
  				// TODO: this could probably be optimized by incrementally
  				// calling partial_sort to sort one more element in the list. Because
  				// chances are that we'll just need a single piece, and once we've
  				// picked from it we're done. Sorting the rest of the list in that
  				// case is a waste of time.
  				std::sort(ordered_partials.begin(), ordered_partials.begin() + num_ordered_partials
  					, std::bind(&piece_picker::partial_compare_rarest_first, this
  						, _1, _2));
  			}
  
  			for (int i = 0; i < num_ordered_partials; ++i)
  			{
  				ret |= picker_log_alert::prioritize_partials;
  
  				num_blocks = add_blocks_downloading(*ordered_partials[i], pieces
  					, interesting_blocks, backup_blocks, backup_blocks2
  					, num_blocks, prefer_contiguous_blocks, peer, options);
  				if (num_blocks <= 0) return ret;
  				if (int(backup_blocks.size()) >= num_blocks
  					&& int(backup_blocks2.size()) >= num_blocks)
  					break;
  			}
  
  			num_blocks = append_blocks(interesting_blocks, backup_blocks
  				, num_blocks);
  			if (num_blocks <= 0) return ret;
  
  			num_blocks = append_blocks(interesting_blocks, backup_blocks2
  				, num_blocks);
  			if (num_blocks <= 0) return ret;
  		}
  
  		if (!suggested_pieces.empty())
  		{
  			for (piece_index_t i : suggested_pieces)
  			{
  				// in time critical mode, only pick high priority pieces
  				if ((options & time_critical_mode)
  					&& piece_priority(i) != priority_levels - 1)
  					continue;
  
  				pc.inc_stats_counter(counters::piece_picker_suggest_loops);
  				if (!is_piece_free(i, pieces)) continue;
  
  				ret |= picker_log_alert::suggested_pieces;
  
  				num_blocks = add_blocks(i, pieces
  					, interesting_blocks, backup_blocks
  					, backup_blocks2, num_blocks
  					, prefer_contiguous_blocks, peer, empty_vector
  					, options);
  				if (num_blocks <= 0) return ret;
  			}
  		}
  
  		if (options & sequential)
  		{
  			if (m_dirty) update_pieces();
  			TORRENT_ASSERT(!m_dirty);
  
  			for (auto i = m_pieces.begin();
  				i != m_pieces.end() && piece_priority(*i) == priority_levels - 1; ++i)
  			{
  				if (!is_piece_free(*i, pieces)) continue;
  
  				ret |= picker_log_alert::prio_sequential_pieces;
  
  				num_blocks = add_blocks(*i, pieces
  					, interesting_blocks, backup_blocks
  					, backup_blocks2, num_blocks
  					, prefer_contiguous_blocks, peer, suggested_pieces
  					, options);
  				if (num_blocks <= 0) return ret;
  			}
  
  			// in time critical mode, only pick high priority pieces
  			if ((options & time_critical_mode) == 0)
  			{
  				if (options & reverse)
  				{
  					for (piece_index_t i = prev(m_reverse_cursor); i >= m_cursor; --i)
  					{
  						if (!is_piece_free(i, pieces)) continue;
  						// we've already added high priority pieces
  						if (piece_priority(i) == priority_levels - 1) continue;
  
  						ret |= picker_log_alert::reverse_sequential;
  
  						num_blocks = add_blocks(i, pieces
  							, interesting_blocks, backup_blocks
  							, backup_blocks2, num_blocks
  							, prefer_contiguous_blocks, peer, suggested_pieces
  							, options);
  						if (num_blocks <= 0) return ret;
  					}
  				}
  				else
  				{
  					for (piece_index_t i = m_cursor; i < m_reverse_cursor; ++i)
  					{
  						if (!is_piece_free(i, pieces)) continue;
  						// we've already added high priority pieces
  						if (piece_priority(i) == priority_levels - 1) continue;
  
  						ret |= picker_log_alert::sequential_pieces;
  
  						num_blocks = add_blocks(i, pieces
  							, interesting_blocks, backup_blocks
  							, backup_blocks2, num_blocks
  							, prefer_contiguous_blocks, peer, suggested_pieces
  							, options);
  						if (num_blocks <= 0) return ret;
  					}
  				}
  			}
  		}
  		else if (options & rarest_first)
  		{
  			if (m_dirty) update_pieces();
  			TORRENT_ASSERT(!m_dirty);
  
  			// in time critical mode, we're only allowed to pick high priority
  			// pieces. This is why reverse mode is disabled when we're in
  			// time-critical mode, because all high priority pieces are at the
  			// front of the list
  			if ((options & reverse) && (options & time_critical_mode) == 0)
  			{
  				for (int i = int(m_priority_boundaries.size()) - 1; i >= 0; --i)
  				{
  					prio_index_t const start = priority_begin(i);
  					prio_index_t const end = priority_end(i);
  					for (prio_index_t p = prev(end); p >= start; --p)
  					{
  						pc.inc_stats_counter(counters::piece_picker_reverse_rare_loops);
  
  						if (!is_piece_free(m_pieces[p], pieces)) continue;
  
  						ret |= picker_log_alert::reverse_rarest_first;
  
  						num_blocks = add_blocks(m_pieces[p], pieces
  							, interesting_blocks, backup_blocks
  							, backup_blocks2, num_blocks
  							, prefer_contiguous_blocks, peer, suggested_pieces
  							, options);
  						if (num_blocks <= 0) return ret;
  					}
  				}
  			}
  			else
  			{
  				for (piece_index_t i : m_pieces)
  				{
  					pc.inc_stats_counter(counters::piece_picker_rare_loops);
  
  					// in time critical mode, only pick high priority pieces
  					// it's safe to break here because in this mode we
  					// pick pieces in priority order. Once we hit a lower priority
  					// piece, we won't encounter any more high priority ones
  					if ((options & time_critical_mode)
  						&& piece_priority(i) != priority_levels - 1)
  						break;
  
  					if (!is_piece_free(i, pieces)) continue;
  
  					ret |= picker_log_alert::rarest_first;
  
  					num_blocks = add_blocks(i, pieces
  						, interesting_blocks, backup_blocks
  						, backup_blocks2, num_blocks
  						, prefer_contiguous_blocks, peer, suggested_pieces
  						, options);
  					if (num_blocks <= 0) return ret;
  				}
  			}
  		}
  		else if (options & time_critical_mode)
  		{
  			// if we're in time-critical mode, we are only allowed to pick
  			// high priority pieces.
  			for (auto i = m_pieces.begin();
  				i != m_pieces.end() && piece_priority(*i) == priority_levels - 1; ++i)
  			{
  				if (!is_piece_free(*i, pieces)) continue;
  
  				ret |= picker_log_alert::time_critical;
  
  				num_blocks = add_blocks(*i, pieces
  					, interesting_blocks, backup_blocks
  					, backup_blocks2, num_blocks
  					, prefer_contiguous_blocks, peer, suggested_pieces
  					, options);
  				if (num_blocks <= 0) return ret;
  			}
  		}
  		else
  		{
  			// we're not using rarest first (only for the first
  			// bucket, since that's where the currently downloading
  			// pieces are)
  			piece_index_t const start_piece = piece_index_t(random(std::uint32_t(m_piece_map.size()-1)));
  
  			piece_index_t piece = start_piece;
  			while (num_blocks > 0)
  			{
  				// skip pieces we can't pick, and suggested pieces
  				// since we've already picked those
  				while (!is_piece_free(piece, pieces)
  					|| std::find(suggested_pieces.begin()
  						, suggested_pieces.end(), piece)
  						!= suggested_pieces.end())
  				{
  					pc.inc_stats_counter(counters::piece_picker_rand_start_loops);
  					++piece;
  					if (piece == m_piece_map.end_index()) piece = piece_index_t(0);
  					// could not find any more pieces
  					if (piece == start_piece) { goto get_out; }
  				}
  
  				if (prefer_contiguous_blocks > 1 && !m_piece_map[piece].downloading())
  				{
  					TORRENT_ASSERT(can_pick(piece, pieces));
  					TORRENT_ASSERT(m_piece_map[piece].downloading() == false);
  
  					piece_index_t start, end;
  					std::tie(start, end) = expand_piece(piece
  						, prefer_contiguous_blocks, pieces, options);
  					TORRENT_ASSERT(end > start);
  					for (piece_index_t k = start; k < end; ++k)
  					{
  						TORRENT_ASSERT(m_piece_map[k].downloading() == false);
  						TORRENT_ASSERT(m_piece_map[k].priority(this) >= 0);
  						const int num_blocks_in_piece = blocks_in_piece(k);
  
  						ret |= picker_log_alert::random_pieces;
  
  						for (int j = 0; j < num_blocks_in_piece; ++j)
  						{
  							pc.inc_stats_counter(counters::piece_picker_rand_loops);
  							TORRENT_ASSERT(is_piece_free(k, pieces));
  							interesting_blocks.push_back(piece_block(k, j));
  							--num_blocks;
  							--prefer_contiguous_blocks;
  							if (prefer_contiguous_blocks <= 0
  								&& num_blocks <= 0) break;
  						}
  					}
  					piece = end;
  				}
  				else
  				{
  					ret |= picker_log_alert::random_pieces;
  
  					num_blocks = add_blocks(piece, pieces
  						, interesting_blocks, backup_blocks
  						, backup_blocks2, num_blocks
  						, prefer_contiguous_blocks, peer, empty_vector
  						, options);
  					++piece;
  				}
  
  				if (piece == m_piece_map.end_index()) piece = piece_index_t(0);
  				// could not find any more pieces
  				if (piece == start_piece) break;
  			}
  		}
  get_out:
  
  		if (num_blocks <= 0) return ret;
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		verify_pick(interesting_blocks, pieces);
  		verify_pick(backup_blocks, pieces);
  		verify_pick(backup_blocks2, pieces);
  #endif
  
  		ret |= picker_log_alert::backup1;
  		num_blocks = append_blocks(interesting_blocks, backup_blocks, num_blocks);
  		if (num_blocks <= 0) return ret;
  
  		ret |= picker_log_alert::backup2;
  		num_blocks = append_blocks(interesting_blocks, backup_blocks2, num_blocks);
  		if (num_blocks <= 0) return ret;
  
  		// ===== THIS IS FOR END-GAME MODE =====
  
  		// don't double-pick anything if the peer is on parole
  		if (options & on_parole) return ret;
  
  		// in end game mode we pick a single block
  		// that has already been requested from someone
  		// all pieces that are interesting are in
  		// m_downloads[0] and m_download[1]
  		// (i.e. partial and full pieces)
  
  		std::vector<piece_block> temp;
  
  		// pick one random block from one random partial piece.
  		// only pick from non-downloaded blocks.
  		// first, create a temporary array of the partial pieces
  		// this peer has, and can pick from. Cap the stack allocation
  		// at 200 pieces.
  
  		int partials_size = std::min(200, int(
  				m_downloads[piece_pos::piece_downloading].size()
  			+ m_downloads[piece_pos::piece_full].size()));
  		if (partials_size == 0) return ret;
  
  		TORRENT_ALLOCA(partials, downloading_piece const*, partials_size);
  		int c = 0;
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		// if we get here, we're about to pick a busy block. First, make sure
  		// we really exhausted the available blocks
  		for (std::vector<downloading_piece>::const_iterator i
  			= m_downloads[piece_pos::piece_downloading].begin()
  			, end(m_downloads[piece_pos::piece_downloading].end()); i != end; ++i)
  		{
  			downloading_piece const& dp = *i;
  
  			if ((options & time_critical_mode)
  				&& piece_priority(dp.index) != priority_levels - 1)
  				continue;
  
  			// we either don't have this piece, or we've already requested from it
  			if (!pieces[dp.index]) continue;
  
  			// if we already have the piece, obviously we should not have
  			// since this is a partial piece in the piece_downloading state, we
  			// should not already have it
  			TORRENT_ASSERT(!m_piece_map[dp.index].have());
  
  			// if it was filtered, it would be in the prio_zero queue
  			TORRENT_ASSERT(!m_piece_map[dp.index].filtered());
  
  			// we're not allowed to pick from locked pieces
  			if (dp.locked) continue;
  
  			bool found = false;
  			for (std::vector<piece_block>::const_iterator j
  				= interesting_blocks.begin(), end2(interesting_blocks.end());
  				j != end2; ++j)
  			{
  				if (j->piece_index != dp.index) continue;
  				found = true;
  				break;
  			}
  
  			// we expect to find this piece in our interesting_blocks list
  			TORRENT_ASSERT(found);
  		}
  #endif
  
  		for (auto const& dp : m_downloads[piece_pos::piece_full])
  		{
  			if (c == partials_size) break;
  
  			TORRENT_ASSERT(dp.requested > 0);
  			// this peer doesn't have this piece, try again
  			if (!pieces[dp.index]) continue;
  			// don't pick pieces with priority 0
  			TORRENT_ASSERT(piece_priority(dp.index) > 0);
  
  			if ((options & time_critical_mode)
  				&& piece_priority(dp.index) != priority_levels - 1)
  				continue;
  
  			partials[c++] = &dp;
  		}
  
  		partials_size = c;
  		while (partials_size > 0)
  		{
  			pc.inc_stats_counter(counters::piece_picker_busy_loops);
  			int piece = int(random(partials_size - 1));
  			downloading_piece const* dp = partials[piece];
  			TORRENT_ASSERT(pieces[dp->index]);
  			TORRENT_ASSERT(piece_priority(dp->index) > 0);
  			// fill in with blocks requested from other peers
  			// as backups
  			int const num_blocks_in_piece = blocks_in_piece(dp->index);
  			TORRENT_ASSERT(dp->requested > 0);
  			block_info const* binfo = blocks_for_piece(*dp);
  			for (int j = 0; j < num_blocks_in_piece; ++j)
  			{
  				block_info const& info = binfo[j];
  				TORRENT_ASSERT(info.peer == nullptr || info.peer->in_use);
  				TORRENT_ASSERT(info.piece_index == dp->index);
  				if (info.state != block_info::state_requested || info.peer == peer)
  					continue;
  				temp.push_back(piece_block(dp->index, j));
  			}
  			// are we done?
  			if (!temp.empty())
  			{
  				ret |= picker_log_alert::end_game;
  				interesting_blocks.push_back(temp[random(std::uint32_t(temp.size()) - 1)]);
  				--num_blocks;
  				break;
  			}
  
  			// the piece we picked only had blocks outstanding requested
  			// by ourself. Remove it and pick another one.
  			partials[piece] = partials[partials_size - 1];
  			--partials_size;
  		}
  
  #if TORRENT_USE_INVARIANT_CHECKS
  // make sure that we at this point have added requests to all unrequested blocks
  // in all downloading pieces
  
  		for (auto const& i : m_downloads[piece_pos::piece_downloading])
  		{
  			if (!pieces[i.index]) continue;
  			if (piece_priority(i.index) == 0) continue;
  			if (i.locked) continue;
  
  			if ((options & time_critical_mode)
  				&& piece_priority(i.index) != priority_levels - 1)
  				continue;
  
  			const int num_blocks_in_piece = blocks_in_piece(i.index);
  			block_info const* binfo = blocks_for_piece(i);
  			for (int j = 0; j < num_blocks_in_piece; ++j)
  			{
  				block_info const& info = binfo[j];
  				TORRENT_ASSERT(info.piece_index == i.index);
  				if (info.state != block_info::state_none) continue;
  				auto k = std::find(interesting_blocks.begin(), interesting_blocks.end()
  					, piece_block(i.index, j));
  				if (k != interesting_blocks.end()) continue;
  
  				std::fprintf(stderr, "interesting blocks:\n");
  				for (auto const& p : interesting_blocks)
  				{
  					std::fprintf(stderr, "(%d, %d)"
  						, static_cast<int>(p.piece_index), p.block_index);
  				}
  				std::fprintf(stderr, "\nnum_blocks: %d\n", num_blocks);
  
  				for (auto const& l : m_downloads[piece_pos::piece_downloading])
  				{
  					block_info const* binfo2 = blocks_for_piece(l);
  					std::fprintf(stderr, "%d : ", static_cast<int>(l.index));
  					const int cnt = blocks_in_piece(l.index);
  					for (int m = 0; m < cnt; ++m)
  						std::fprintf(stderr, "%d", binfo2[m].state);
  					std::fprintf(stderr, "\n");
  				}
  
  				TORRENT_ASSERT_FAIL();
  			}
  		}
  
  		if (interesting_blocks.empty())
  		{
  			for (piece_index_t i = piece_index_t(0);
  				i != m_piece_map.end_index(); ++i)
  			{
  				if (!pieces[i]) continue;
  				if (m_piece_map[i].priority(this) <= 0) continue;
  				if (have_piece(i)) continue;
  
  				int const download_state = m_piece_map[i].download_queue();
  				if (download_state == piece_pos::piece_open) continue;
  				std::vector<downloading_piece>::const_iterator k
  					= find_dl_piece(download_state, i);
  
  				TORRENT_ASSERT(k != m_downloads[download_state].end());
  				if (k == m_downloads[download_state].end()) continue;
  			}
  		}
  #endif
  		return ret;
  	}
  
  	// have piece means that the piece passed hash check
  	// AND has been successfully written to disk
  	bool piece_picker::have_piece(piece_index_t const index) const
  	{
  		piece_pos const& p = m_piece_map[index];
  		return p.index == piece_pos::we_have_index;
  	}
  
  	int piece_picker::blocks_in_piece(piece_index_t const index) const
  	{
  		TORRENT_ASSERT(index >= piece_index_t(0));
  		TORRENT_ASSERT(index < m_piece_map.end_index());
  		if (next(index) == m_piece_map.end_index())
  			return m_blocks_in_last_piece;
  		else
  			return m_blocks_per_piece;
  	}
  
  	bool piece_picker::is_piece_free(piece_index_t const piece
  		, typed_bitfield<piece_index_t> const& bitmask) const
  	{
  		return bitmask[piece]
  			&& !m_piece_map[piece].have()
  			&& !m_piece_map[piece].filtered();
  	}
  
  	bool piece_picker::can_pick(piece_index_t const piece
  		, typed_bitfield<piece_index_t> const& bitmask) const
  	{
  		return bitmask[piece]
  			&& !m_piece_map[piece].have()
  			// TODO: when expanding pieces for cache stripe reasons,
  			// the !downloading condition doesn't make much sense
  			&& !m_piece_map[piece].downloading()
  			&& !m_piece_map[piece].filtered();
  	}
  
  #if TORRENT_USE_INVARIANT_CHECKS
  	void piece_picker::check_peers()
  	{
  		for (std::vector<block_info>::iterator i = m_block_info.begin()
  			, end(m_block_info.end()); i != end; ++i)
  		{
  			TORRENT_ASSERT(i->peer == nullptr || static_cast<torrent_peer*>(i->peer)->in_use);
  		}
  	}
  #endif
  
  	void piece_picker::clear_peer(torrent_peer* peer)
  	{
  		for (std::vector<block_info>::iterator i = m_block_info.begin()
  			, end(m_block_info.end()); i != end; ++i)
  		{
  			if (i->peer == peer) i->peer = nullptr;
  		}
  	}
  
  	// the first bool is true if this is the only peer that has requested and downloaded
  	// blocks from this piece.
  	// the second bool is true if this is the only active peer that is requesting
  	// and downloading blocks from this piece. Active means having a connection.
  	// TODO: 2 the first_block returned here is the largest free range, not
  	// the first-fit range, which would be better
  	std::tuple<bool, bool, int, int> piece_picker::requested_from(
  		piece_picker::downloading_piece const& p
  		, int const num_blocks_in_piece, torrent_peer* peer) const
  	{
  		bool exclusive = true;
  		bool exclusive_active = true;
  		int contiguous_blocks = 0;
  		int max_contiguous = 0;
  		int first_block = 0;
  		block_info const* binfo = blocks_for_piece(p);
  		for (int j = 0; j < num_blocks_in_piece; ++j)
  		{
  			piece_picker::block_info const& info = binfo[j];
  			TORRENT_ASSERT(info.peer == nullptr || info.peer->in_use);
  			TORRENT_ASSERT(info.piece_index == p.index);
  			if (info.state == piece_picker::block_info::state_none)
  			{
  				++contiguous_blocks;
  				continue;
  			}
  			if (contiguous_blocks > max_contiguous)
  			{
  				max_contiguous = contiguous_blocks;
  				first_block = j - contiguous_blocks;
  			}
  			contiguous_blocks = 0;
  			if (info.peer != peer)
  			{
  				exclusive = false;
  				if (info.state == piece_picker::block_info::state_requested
  					&& info.peer != nullptr)
  				{
  					exclusive_active = false;
  				}
  			}
  		}
  		if (contiguous_blocks > max_contiguous)
  		{
  			max_contiguous = contiguous_blocks;
  			first_block = num_blocks_in_piece - contiguous_blocks;
  		}
  		return std::make_tuple(exclusive, exclusive_active, max_contiguous
  			, first_block);
  	}
  
  	int piece_picker::add_blocks(piece_index_t piece
  		, typed_bitfield<piece_index_t> const& pieces
  		, std::vector<piece_block>& interesting_blocks
  		, std::vector<piece_block>& backup_blocks
  		, std::vector<piece_block>& backup_blocks2
  		, int num_blocks, int prefer_contiguous_blocks
  		, torrent_peer* peer, std::vector<piece_index_t> const& ignore
  		, int options) const
  	{
  		TORRENT_ASSERT(is_piece_free(piece, pieces));
  
  		// ignore pieces found in the ignore list
  		if (std::find(ignore.begin(), ignore.end(), piece) != ignore.end()) return num_blocks;
  
  		if (m_piece_map[piece].download_queue() != piece_pos::piece_open
  			&& m_piece_map[piece].download_queue() != piece_pos::piece_downloading)
  			return num_blocks;
  
  		TORRENT_ASSERT(m_piece_map[piece].priority(this) >= 0);
  		int state = m_piece_map[piece].download_queue();
  		if (state == piece_pos::piece_downloading)
  		{
  			// if we're prioritizing partials, we've already
  			// looked through the downloading pieces
  			if (options & prioritize_partials) return num_blocks;
  
  			std::vector<downloading_piece>::const_iterator i = find_dl_piece(
  				piece_pos::piece_downloading, piece);
  			TORRENT_ASSERT(i != m_downloads[state].end());
  
  			return add_blocks_downloading(*i, pieces
  				, interesting_blocks, backup_blocks, backup_blocks2
  				, num_blocks, prefer_contiguous_blocks, peer, options);
  		}
  
  		int num_blocks_in_piece = blocks_in_piece(piece);
  
  		// pick a new piece
  		if (prefer_contiguous_blocks == 0)
  		{
  			if (num_blocks_in_piece > num_blocks)
  				num_blocks_in_piece = num_blocks;
  			TORRENT_ASSERT(is_piece_free(piece, pieces));
  			for (int j = 0; j < num_blocks_in_piece; ++j)
  				interesting_blocks.push_back(piece_block(piece, j));
  			num_blocks -= num_blocks_in_piece;
  		}
  		else
  		{
  			piece_index_t start, end;
  			std::tie(start, end) = expand_piece(piece, prefer_contiguous_blocks
  				, pieces, options);
  			for (piece_index_t k = start; k < end; ++k)
  			{
  				TORRENT_ASSERT(m_piece_map[k].priority(this) > 0);
  				num_blocks_in_piece = blocks_in_piece(k);
  				TORRENT_ASSERT(is_piece_free(k, pieces));
  				for (int j = 0; j < num_blocks_in_piece; ++j)
  				{
  					interesting_blocks.push_back(piece_block(k, j));
  					--num_blocks;
  					--prefer_contiguous_blocks;
  					if (prefer_contiguous_blocks == 0
  						&& num_blocks <= 0) break;
  				}
  			}
  		}
  #if TORRENT_USE_INVARIANT_CHECKS
  		verify_pick(interesting_blocks, pieces);
  #endif
  		return (std::max)(num_blocks, 0);
  	}
  
  	int piece_picker::add_blocks_downloading(downloading_piece const& dp
  		, typed_bitfield<piece_index_t> const& pieces
  		, std::vector<piece_block>& interesting_blocks
  		, std::vector<piece_block>& backup_blocks
  		, std::vector<piece_block>& backup_blocks2
  		, int num_blocks, int prefer_contiguous_blocks
  		, torrent_peer* peer, int options) const
  	{
  		if (!pieces[dp.index]) return num_blocks;
  		TORRENT_ASSERT(!m_piece_map[dp.index].filtered());
  
  		// this piece failed to write. We're currently restoring
  		// it. It's not OK to send more requests to it right now.
  		if (dp.locked) return num_blocks;
  
  		int num_blocks_in_piece = blocks_in_piece(dp.index);
  
  		// is true if all the other pieces that are currently
  		// requested from this piece are from the same
  		// peer as 'peer'.
  		bool exclusive;
  		bool exclusive_active;
  
  		// used to report back the largest contiguous block run
  		int contiguous_blocks;
  		int first_block;
  		std::tie(exclusive, exclusive_active, contiguous_blocks, first_block)
  			= requested_from(dp, num_blocks_in_piece, peer);
  
  		// no need in picking from the largest contiguous block run unless
  		// we're interested in it. In fact, we really want the opposite.
  		if (prefer_contiguous_blocks == 0) first_block = 0;
  
  		// peers on parole are only allowed to pick blocks from
  		// pieces that only they have downloaded/requested from
  		if ((options & on_parole) && !exclusive) return num_blocks;
  
  		block_info const* binfo = blocks_for_piece(dp);
  
  		// we prefer whole blocks, but there are other peers
  		// downloading from this piece and there aren't enough contiguous blocks
  		// to pick, add it as backups.
  		// if we're on parole, don't let the contiguous blocks stop us, we want
  		// to primarily request from a piece all by ourselves.
  		if (prefer_contiguous_blocks > contiguous_blocks
  			&& !exclusive_active
  			&& (options & on_parole) == 0)
  		{
  			if (int(backup_blocks2.size()) >= num_blocks)
  				return num_blocks;
  
  			for (int j = 0; j < num_blocks_in_piece; ++j)
  			{
  				// ignore completed blocks and already requested blocks
  				int block_idx = (j + first_block) % num_blocks_in_piece;
  				block_info const& info = binfo[block_idx];
  				TORRENT_ASSERT(info.piece_index == dp.index);
  				if (info.state != block_info::state_none) continue;
  				backup_blocks2.push_back(piece_block(dp.index, block_idx));
  			}
  			return num_blocks;
  		}
  
  		for (int j = 0; j < num_blocks_in_piece; ++j)
  		{
  			// ignore completed blocks and already requested blocks
  			int block_idx = (j + first_block) % num_blocks_in_piece;
  			block_info const& info = binfo[block_idx];
  			TORRENT_ASSERT(info.piece_index == dp.index);
  			if (info.state != block_info::state_none) continue;
  
  			// this block is interesting (we don't have it yet).
  			interesting_blocks.push_back(piece_block(dp.index, block_idx));
  			// we have found a block that's free to download
  			--num_blocks;
  			// if we prefer contiguous blocks, continue picking from this
  			// piece even though we have num_blocks
  			if (prefer_contiguous_blocks > 0)
  			{
  				--prefer_contiguous_blocks;
  				continue;
  			}
  			if (num_blocks <= 0) return 0;
  		}
  
  		if (num_blocks <= 0) return 0;
  		if (options & on_parole) return num_blocks;
  
  		if (int(backup_blocks.size()) >= num_blocks) return num_blocks;
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		verify_pick(backup_blocks, pieces);
  #endif
  		return num_blocks;
  	}
  
  	std::pair<piece_index_t, piece_index_t>
  	piece_picker::expand_piece(piece_index_t const piece, int contiguous_blocks
  		, typed_bitfield<piece_index_t> const& have, int options) const
  	{
  		if (contiguous_blocks == 0) return std::make_pair(piece, next(piece));
  
  		// round to even pieces and expand in order to get the number of
  		// contiguous pieces we want
  		int const whole_pieces = (contiguous_blocks + m_blocks_per_piece - 1)
  			/ m_blocks_per_piece;
  
  		piece_index_t start = piece;
  		piece_index_t lower_limit;
  
  		if (options & align_expanded_pieces)
  		{
  			lower_limit = piece_index_t(static_cast<int>(piece) - (static_cast<int>(piece) % whole_pieces));
  		}
  		else
  		{
  			lower_limit = piece_index_t(static_cast<int>(piece) - whole_pieces + 1);
  			if (lower_limit < piece_index_t(0)) lower_limit = piece_index_t(0);
  		}
  
  		while (start > lower_limit && can_pick(prev(start), have))
  			--start;
  
  		TORRENT_ASSERT(start >= piece_index_t(0));
  		piece_index_t end = next(piece);
  		piece_index_t upper_limit;
  		if (options & align_expanded_pieces)
  		{
  			upper_limit = piece_index_t(static_cast<int>(lower_limit) + whole_pieces);
  		}
  		else
  		{
  			upper_limit = piece_index_t(static_cast<int>(start) + whole_pieces);
  		}
  		if (upper_limit > have.end_index()) upper_limit = have.end_index();
  		while (end < upper_limit && can_pick(end, have))
  			++end;
  		return std::make_pair(start, end);
  	}
  
  	bool piece_picker::is_piece_finished(piece_index_t const index) const
  	{
  		piece_pos const& p = m_piece_map[index];
  		if (p.index == piece_pos::we_have_index) return true;
  
  		int state = p.download_queue();
  		if (state == piece_pos::piece_open)
  		{
  			for (int i = 0; i < piece_pos::num_download_categories; ++i)
  				TORRENT_ASSERT(find_dl_piece(i, index) == m_downloads[i].end());
  			return false;
  		}
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(state, index);
  		TORRENT_ASSERT(i != m_downloads[state].end());
  		TORRENT_ASSERT(int(i->finished) <= m_blocks_per_piece);
  		int max_blocks = blocks_in_piece(index);
  		if (int(i->finished) + int(i->writing) < max_blocks) return false;
  		TORRENT_ASSERT(int(i->finished) + int(i->writing) == max_blocks);
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		block_info const* info = blocks_for_piece(*i);
  		for (int k = 0; k < max_blocks; ++k)
  		{
  			TORRENT_ASSERT(info[k].piece_index == index);
  			TORRENT_ASSERT(info[k].state == block_info::state_finished
  				|| info[k].state == block_info::state_writing);
  		}
  #endif
  
  		return true;
  	}
  
  	bool piece_picker::has_piece_passed(piece_index_t index) const
  	{
  		TORRENT_ASSERT(index < m_piece_map.end_index());
  		TORRENT_ASSERT(index >= piece_index_t(0));
  
  		piece_pos const& p = m_piece_map[index];
  		if (p.index == piece_pos::we_have_index) return true;
  
  		int const state = p.download_queue();
  		if (state == piece_pos::piece_open)
  		{
  			for (int i = 0; i < piece_pos::num_download_categories; ++i)
  				TORRENT_ASSERT(find_dl_piece(i, index) == m_downloads[i].end());
  			return false;
  		}
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(state, index);
  		TORRENT_ASSERT(i != m_downloads[state].end());
  		return i->passed_hash_check;
  	}
  
  	std::vector<piece_picker::downloading_piece>::iterator piece_picker::find_dl_piece(
  		const int queue, piece_index_t const index)
  	{
  		TORRENT_ASSERT(queue >= 0 && queue < piece_pos::num_download_categories);
  		downloading_piece cmp;
  		cmp.index = index;
  		std::vector<piece_picker::downloading_piece>::iterator i = std::lower_bound(
  			m_downloads[queue].begin(), m_downloads[queue].end(), cmp);
  		if (i == m_downloads[queue].end()) return i;
  		if (i->index == index) return i;
  		return m_downloads[queue].end();
  	}
  
  	std::vector<piece_picker::downloading_piece>::const_iterator piece_picker::find_dl_piece(
  		int queue, piece_index_t const index) const
  	{
  		return const_cast<piece_picker*>(this)->find_dl_piece(queue, index);
  	}
  
  	std::vector<piece_picker::downloading_piece>::iterator
  	piece_picker::update_piece_state(
  		std::vector<piece_picker::downloading_piece>::iterator dp)
  	{
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "update_piece_state(" << dp->index << ")" << std::endl;
  #endif
  
  		int const num_blocks = blocks_in_piece(dp->index);
  		piece_pos& p = m_piece_map[dp->index];
  		int const current_state = p.download_state;
  		TORRENT_ASSERT(current_state != piece_pos::piece_open);
  		if (current_state == piece_pos::piece_open)
  			return dp;
  
  		// this function is not allowed to create new downloading pieces
  		int new_state = 0;
  		if (p.filtered())
  		{
  			new_state = piece_pos::piece_zero_prio;
  		}
  		else if (dp->requested + dp->finished + dp->writing == 0)
  		{
  			new_state = piece_pos::piece_open;
  		}
  		else if (dp->requested + dp->finished + dp->writing < num_blocks)
  		{
  			new_state = p.reverse()
  				? piece_pos::piece_downloading_reverse
  				: piece_pos::piece_downloading;
  		}
  		else if (dp->requested > 0)
  		{
  			TORRENT_ASSERT(dp->requested + dp->finished + dp->writing == num_blocks);
  			new_state = p.reverse()
  				? piece_pos::piece_full_reverse
  				: piece_pos::piece_full;
  		}
  		else
  		{
  			TORRENT_ASSERT(dp->finished + dp->writing == num_blocks);
  			new_state = piece_pos::piece_finished;
  		}
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << " new_state: " << new_state << " current_state: " << current_state << std::endl;
  #endif
  		if (new_state == current_state) return dp;
  		if (new_state == piece_pos::piece_open) return dp;
  
  		// assert that the iterator that was passed-in in fact lives in
  		// the correct list
  		TORRENT_ASSERT(find_dl_piece(p.download_queue(), dp->index) == dp);
  
  		// remove the downloading_piece from the list corresponding
  		// to the old state
  		downloading_piece dp_info = *dp;
  		m_downloads[p.download_queue()].erase(dp);
  
  		int const prio = p.priority(this);
  		TORRENT_ASSERT(prio < int(m_priority_boundaries.size())
  			|| m_dirty);
  		p.download_state = static_cast<std::uint16_t>(new_state);
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << " " << dp_info.index << " state (" << current_state << " -> " << new_state << ")" << std::endl;
  #endif
  
  		// insert the downloading_piece in the list corresponding to
  		// the new state
  		downloading_piece cmp;
  		cmp.index = dp_info.index;
  		std::vector<downloading_piece>::iterator i = std::lower_bound(
  			m_downloads[p.download_queue()].begin()
  			, m_downloads[p.download_queue()].end(), cmp);
  		TORRENT_ASSERT(i == m_downloads[p.download_queue()].end()
  			|| i->index != dp_info.index);
  		i = m_downloads[p.download_queue()].insert(i, dp_info);
  
  		if (!m_dirty)
  		{
  			if (prio == -1 && p.priority(this) != -1) add(dp_info.index);
  			else if (prio != -1) update(prio, p.index);
  		}
  
  		return i;
  	}
  
  	bool piece_picker::is_requested(piece_block block) const
  	{
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  		TORRENT_ASSERT(block.piece_index < m_piece_map.end_index());
  
  		int state = m_piece_map[block.piece_index].download_queue();
  		if (state == piece_pos::piece_open) return false;
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(state
  			, block.piece_index);
  
  		TORRENT_ASSERT(i != m_downloads[state].end());
  
  		block_info const* info = blocks_for_piece(*i);
  		TORRENT_ASSERT(info[block.block_index].piece_index == block.piece_index);
  		return info[block.block_index].state == block_info::state_requested;
  	}
  
  	bool piece_picker::is_downloaded(piece_block block) const
  	{
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  		TORRENT_ASSERT(block.piece_index < m_piece_map.end_index());
  
  		if (m_piece_map[block.piece_index].index == piece_pos::we_have_index) return true;
  		int state = m_piece_map[block.piece_index].download_queue();
  		if (state == piece_pos::piece_open) return false;
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(state
  			, block.piece_index);
  		TORRENT_ASSERT(i != m_downloads[state].end());
  
  		block_info const* info = blocks_for_piece(*i);
  		TORRENT_ASSERT(info[block.block_index].piece_index == block.piece_index);
  		return info[block.block_index].state == block_info::state_finished
  			|| info[block.block_index].state == block_info::state_writing;
  	}
  
  	bool piece_picker::is_finished(piece_block block) const
  	{
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  		TORRENT_ASSERT(block.piece_index < m_piece_map.end_index());
  
  		piece_pos const& p = m_piece_map[block.piece_index];
  		if (p.index == piece_pos::we_have_index) return true;
  		if (p.download_queue() == piece_pos::piece_open) return false;
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(p.download_queue()
  			, block.piece_index);
  		TORRENT_ASSERT(i != m_downloads[p.download_queue()].end());
  
  		block_info const* info = blocks_for_piece(*i);
  		TORRENT_ASSERT(info[block.block_index].piece_index == block.piece_index);
  		return info[block.block_index].state == block_info::state_finished;
  	}
  
  	// options may be 0 or piece_picker::reverse
  	// returns false if the block could not be marked as downloading
  	bool piece_picker::mark_as_downloading(piece_block block
  		, torrent_peer* peer, int options)
  	{
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "mark_as_downloading( {"
  			<< block.piece_index << ", " << block.block_index << "} )" << std::endl;
  #endif
  
  		TORRENT_ASSERT(peer == nullptr || peer->in_use);
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  		TORRENT_ASSERT(block.piece_index < m_piece_map.end_index());
  		TORRENT_ASSERT(block.block_index < blocks_in_piece(block.piece_index));
  		TORRENT_ASSERT(!m_piece_map[block.piece_index].have());
  
  		piece_pos& p = m_piece_map[block.piece_index];
  		if (p.download_queue() == piece_pos::piece_open)
  		{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  			INVARIANT_CHECK;
  #endif
  			int const prio = p.priority(this);
  			TORRENT_ASSERT(prio < int(m_priority_boundaries.size())
  				|| m_dirty);
  
  			p.download_state = (options & reverse)
  				? piece_pos::piece_downloading_reverse
  				: piece_pos::piece_downloading;
  
  			if (prio >= 0 && !m_dirty) update(prio, p.index);
  
  			auto const dp = add_download_piece(block.piece_index);
  			block_info* binfo = blocks_for_piece(*dp);
  			block_info& info = binfo[block.block_index];
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  			info.state = block_info::state_requested;
  			info.peer = peer;
  			info.num_peers = 1;
  #if TORRENT_USE_ASSERTS
  			TORRENT_ASSERT(info.peers.count(peer) == 0);
  			info.peers.insert(peer);
  #endif
  			++dp->requested;
  			// update_full may move the downloading piece to
  			// a different vector, so 'dp' may be invalid after
  			// this call
  			update_piece_state(dp);
  		}
  		else
  		{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  			INVARIANT_CHECK;
  #endif
  			std::vector<downloading_piece>::iterator i = find_dl_piece(p.download_queue()
  				, block.piece_index);
  			TORRENT_ASSERT(i != m_downloads[p.download_queue()].end());
  			block_info* binfo = blocks_for_piece(*i);
  			block_info& info = binfo[block.block_index];
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  			if (info.state == block_info::state_writing
  				|| info.state == block_info::state_finished)
  			{
  				return false;
  			}
  
  			if ((options & reverse) && !p.reverse() && i->requested == 0)
  			{
  				// this piece isn't reverse, but there's no other peer
  				// downloading from it and we just requested a block from a
  				// reverse peer. Make it reverse
  				int prio = p.priority(this);
  				p.make_reverse();
  				if (prio >= 0 && !m_dirty) update(prio, p.index);
  			}
  
  			TORRENT_ASSERT(info.state == block_info::state_none
  				|| (info.state == block_info::state_requested
  					&& (info.num_peers > 0)));
  			info.peer = peer;
  			if (info.state != block_info::state_requested)
  			{
  				info.state = block_info::state_requested;
  				++i->requested;
  				i = update_piece_state(i);
  			}
  			++info.num_peers;
  
  			// if we make a non-reverse request from a reversed piece,
  			// undo the reverse state
  			if ((options & reverse) == 0 && p.reverse())
  			{
  				int prio = p.priority(this);
  				// make it non-reverse
  				p.unreverse();
  				if (prio >= 0 && !m_dirty) update(prio, p.index);
  			}
  
  #if TORRENT_USE_ASSERTS
  			TORRENT_ASSERT(info.peers.count(peer) == 0);
  			info.peers.insert(peer);
  #endif
  		}
  		return true;
  	}
  
  	int piece_picker::num_peers(piece_block block) const
  	{
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  		TORRENT_ASSERT(block.piece_index < m_piece_map.end_index());
  		TORRENT_ASSERT(block.block_index < blocks_in_piece(block.piece_index));
  
  		piece_pos const& p = m_piece_map[block.piece_index];
  		if (!p.downloading()) return 0;
  
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(p.download_queue()
  			, block.piece_index);
  		TORRENT_ASSERT(i != m_downloads[p.download_queue()].end());
  
  		block_info const* binfo = blocks_for_piece(*i);
  		block_info const& info = binfo[block.block_index];
  		TORRENT_ASSERT(&info >= &m_block_info[0]);
  		TORRENT_ASSERT(&info < &m_block_info[0] + m_block_info.size());
  		TORRENT_ASSERT(info.piece_index == block.piece_index);
  		return info.num_peers;
  	}
  
  	void piece_picker::get_availability(aux::vector<int, piece_index_t>& avail) const
  	{
  		TORRENT_ASSERT(m_seeds >= 0);
  		INVARIANT_CHECK;
  
  		avail.resize(m_piece_map.size());
  		auto j = avail.begin();
  		for (auto i = m_piece_map.begin(), end(m_piece_map.end()); i != end; ++i, ++j)
  			*j = i->peer_count + m_seeds;
  	}
  
  	int piece_picker::get_availability(piece_index_t const piece) const
  	{
  		return m_piece_map[piece].peer_count + m_seeds;
  	}
  
  	bool piece_picker::mark_as_writing(piece_block block, torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "mark_as_writing( {" << block.piece_index << ", " << block.block_index << "} )" << std::endl;
  #endif
  
  		TORRENT_ASSERT(peer == nullptr || static_cast<torrent_peer*>(peer)->in_use);
  
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  		TORRENT_ASSERT(block.piece_index < m_piece_map.end_index());
  		TORRENT_ASSERT(block.block_index < blocks_in_piece(block.piece_index));
  		// this is not valid for web peers
  		// TORRENT_ASSERT(peer != 0);
  
  		piece_pos& p = m_piece_map[block.piece_index];
  		if (p.downloading() == 0)
  		{
  			// if we already have this piece, just ignore this
  			if (have_piece(block.piece_index)) return false;
  
  			int const prio = p.priority(this);
  			TORRENT_ASSERT(prio < int(m_priority_boundaries.size())
  				|| m_dirty);
  			p.download_state = piece_pos::piece_downloading;
  			// prio being -1 can happen if a block is requested before
  			// the piece priority was set to 0
  			if (prio >= 0 && !m_dirty) update(prio, p.index);
  
  			auto const dp = add_download_piece(block.piece_index);
  			block_info* binfo = blocks_for_piece(*dp);
  			block_info& info = binfo[block.block_index];
  			TORRENT_ASSERT(&info >= &m_block_info[0]);
  			TORRENT_ASSERT(&info < &m_block_info[0] + m_block_info.size());
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  			info.state = block_info::state_writing;
  			info.peer = peer;
  			info.num_peers = 0;
  #if TORRENT_USE_ASSERTS
  			info.peers.clear();
  #endif
  			dp->writing = 1;
  
  			update_piece_state(dp);
  		}
  		else
  		{
  			std::vector<downloading_piece>::iterator i = find_dl_piece(p.download_queue()
  				, block.piece_index);
  			TORRENT_ASSERT(i != m_downloads[p.download_queue()].end());
  			block_info* binfo = blocks_for_piece(*i);
  			block_info& info = binfo[block.block_index];
  
  			TORRENT_ASSERT(&info >= &m_block_info[0]);
  			TORRENT_ASSERT(&info < &m_block_info[0] + m_block_info.size());
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  
  			info.peer = peer;
  			if (info.state == block_info::state_requested) --i->requested;
  			if (info.state == block_info::state_writing
  				|| info.state == block_info::state_finished)
  				return false;
  
  			++i->writing;
  			info.state = block_info::state_writing;
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  
  			// all other requests for this block should have been
  			// cancelled now
  			info.num_peers = 0;
  #if TORRENT_USE_ASSERTS
  			info.peers.clear();
  #endif
  
  			update_piece_state(i);
  		}
  		return true;
  	}
  
  	// calling this function prevents this piece from being picked
  	// by the piece picker until the pieces is restored. This allow
  	// the disk thread to synchronize and flush any failed state
  	// (used for disk write failures and piece hash failures).
  	void piece_picker::lock_piece(piece_index_t const piece)
  	{
  		INVARIANT_CHECK;
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "lock_piece(" << piece << ")" << std::endl;
  #endif
  
  		int state = m_piece_map[piece].download_queue();
  		if (state == piece_pos::piece_open) return;
  		std::vector<downloading_piece>::iterator i = find_dl_piece(state, piece);
  		if (i == m_downloads[state].end()) return;
  
  		TORRENT_ASSERT(i->passed_hash_check == false);
  		if (i->passed_hash_check)
  		{
  			// it's not clear why this would happen,
  			// but it seems reasonable to not break the
  			// accounting over it.
  			i->passed_hash_check = false;
  			TORRENT_ASSERT(m_num_passed > 0);
  			--m_num_passed;
  		}
  
  		// prevent this piece from being picked until it's restored
  		i->locked = true;
  	}
  
  	// TODO: 2 it would be nice if this could be folded into lock_piece()
  	// the main distinction is that this also maintains the m_num_passed
  	// counter and the passed_hash_check member
  	// Is there ever a case where we call write filed without also locking
  	// the piece? Perhaps write_failed() should imply locking it.
  	void piece_picker::write_failed(piece_block block)
  	{
  		INVARIANT_CHECK;
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "write_failed( {" << block.piece_index << ", " << block.block_index << "} )" << std::endl;
  #endif
  
  		int state = m_piece_map[block.piece_index].download_queue();
  		if (state == piece_pos::piece_open) return;
  		std::vector<downloading_piece>::iterator i = find_dl_piece(state, block.piece_index);
  		if (i == m_downloads[state].end()) return;
  
  		block_info* binfo = blocks_for_piece(*i);
  		block_info& info = binfo[block.block_index];
  		TORRENT_ASSERT(&info >= &m_block_info[0]);
  		TORRENT_ASSERT(&info < &m_block_info[0] + m_block_info.size());
  		TORRENT_ASSERT(info.piece_index == block.piece_index);
  		TORRENT_ASSERT(info.state == block_info::state_writing);
  		TORRENT_ASSERT(info.num_peers == 0);
  
  		TORRENT_ASSERT(i->writing > 0);
  		TORRENT_ASSERT(info.state == block_info::state_writing);
  
  		if (info.state == block_info::state_finished) return;
  		if (info.state == block_info::state_writing) --i->writing;
  
  		info.peer = nullptr;
  		info.state = block_info::state_none;
  		if (i->passed_hash_check)
  		{
  			// the hash was good, but we failed to write
  			// some of the blocks to disk, which means we
  			// can't consider the piece complete
  			i->passed_hash_check = false;
  			TORRENT_ASSERT(m_num_passed > 0);
  			--m_num_passed;
  		}
  
  		// prevent this hash job from actually completing
  		// this piece, by setting the failure state.
  		// the piece is unlocked in the call to restore_piece()
  		i->locked = true;
  
  		i = update_piece_state(i);
  
  		if (i->finished + i->writing + i->requested == 0)
  		{
  			piece_pos& p = m_piece_map[block.piece_index];
  			int const prev_priority = p.priority(this);
  			erase_download_piece(i);
  			int const new_priority = p.priority(this);
  
  			if (m_dirty) return;
  			if (new_priority == prev_priority) return;
  			if (prev_priority == -1) add(block.piece_index);
  			else update(prev_priority, p.index);
  		}
  	}
  
  	void piece_picker::mark_as_canceled(const piece_block block, torrent_peer* peer)
  	{
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "mark_as_cancelled( {"
  			<< block.piece_index << ", " << block.block_index
  			<< "} )" << std::endl;
  #endif
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  		TORRENT_ASSERT(block.block_index >= 0);
  		piece_pos& p = m_piece_map[block.piece_index];
  
  		if (p.download_queue() == piece_pos::piece_open) return;
  
  		std::vector<downloading_piece>::iterator i = find_dl_piece(p.download_queue()
  			, block.piece_index);
  
  		TORRENT_ASSERT(i != m_downloads[p.download_queue()].end());
  		block_info* binfo = blocks_for_piece(*i);
  		block_info& info = binfo[block.block_index];
  
  		if (info.state == block_info::state_finished) return;
  
  		TORRENT_ASSERT(info.num_peers == 0);
  		info.peer = peer;
  		TORRENT_ASSERT(info.state == block_info::state_writing
  			|| peer == nullptr);
  		if (info.state == block_info::state_writing)
  		{
  			--i->writing;
  			info.state = block_info::state_none;
  			// i may be invalid after this call
  			i = update_piece_state(i);
  
  			if (i->finished + i->writing + i->requested == 0)
  			{
  				int const prev_priority = p.priority(this);
  				erase_download_piece(i);
  				int const new_priority = p.priority(this);
  
  				if (m_dirty) return;
  				if (new_priority == prev_priority) return;
  				if (prev_priority == -1) add(block.piece_index);
  				else update(prev_priority, p.index);
  			}
  		}
  		else
  		{
  			TORRENT_ASSERT(info.state == block_info::state_none);
  		}
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  	}
  
  	void piece_picker::mark_as_finished(piece_block block, torrent_peer* peer)
  	{
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "mark_as_finished( {"
  			<< block.piece_index << ", " << block.block_index << "} )" << std::endl;
  #endif
  
  		TORRENT_ASSERT(peer == nullptr || static_cast<torrent_peer*>(peer)->in_use);
  		TORRENT_ASSERT(block.block_index >= 0);
  
  		piece_pos& p = m_piece_map[block.piece_index];
  
  		if (p.download_queue() == piece_pos::piece_open)
  		{
  			// if we already have this piece, just ignore this
  			if (have_piece(block.piece_index)) return;
  
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  			INVARIANT_CHECK;
  #endif
  
  			int const prio = p.priority(this);
  			TORRENT_ASSERT(prio < int(m_priority_boundaries.size())
  				|| m_dirty);
  			p.download_state = piece_pos::piece_downloading;
  			if (prio >= 0 && !m_dirty) update(prio, p.index);
  
  			auto const dp = add_download_piece(block.piece_index);
  			block_info* binfo = blocks_for_piece(*dp);
  			block_info& info = binfo[block.block_index];
  			TORRENT_ASSERT(&info >= &m_block_info[0]);
  			TORRENT_ASSERT(&info < &m_block_info[0] + m_block_info.size());
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  			info.peer = peer;
  			TORRENT_ASSERT(info.state == block_info::state_none);
  			TORRENT_ASSERT(info.num_peers == 0);
  			++dp->finished;
  			info.state = block_info::state_finished;
  			// dp may be invalid after this call
  			update_piece_state(dp);
  		}
  		else
  		{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  			INVARIANT_CHECK;
  #endif
  
  			std::vector<downloading_piece>::iterator i = find_dl_piece(p.download_queue()
  				, block.piece_index);
  			TORRENT_ASSERT(i != m_downloads[p.download_queue()].end());
  			block_info* binfo = blocks_for_piece(*i);
  			block_info& info = binfo[block.block_index];
  			TORRENT_ASSERT(info.piece_index == block.piece_index);
  
  			if (info.state == block_info::state_finished) return;
  
  			TORRENT_ASSERT(info.num_peers == 0);
  
  			// peers may have been disconnected in between mark_as_writing
  			// and mark_as_finished. When a peer disconnects, its m_peer_info
  			// pointer is set to nullptr. If so, preserve the previous peer
  			// pointer, instead of forgetting who we downloaded this block from
  			if (info.state != block_info::state_writing || peer != nullptr)
  				info.peer = peer;
  
  			++i->finished;
  			if (info.state == block_info::state_writing)
  			{
  				TORRENT_ASSERT(i->writing > 0);
  				--i->writing;
  				info.state = block_info::state_finished;
  			}
  			else
  			{
  				TORRENT_ASSERT(info.state == block_info::state_none);
  				info.state = block_info::state_finished;
  			}
  
  			i = update_piece_state(i);
  
  			if (i->finished < blocks_in_piece(i->index))
  				return;
  
  			if (i->passed_hash_check)
  				we_have(i->index);
  		}
  
  #if TORRENT_USE_INVARIANT_CHECKS
  		check_piece_state();
  #endif
  	}
  
  	void piece_picker::get_downloaders(std::vector<torrent_peer*>& d
  		, piece_index_t const index) const
  	{
  		d.clear();
  		int const state = m_piece_map[index].download_queue();
  		int const num_blocks = blocks_in_piece(index);
  		d.reserve(num_blocks);
  
  		if (state == piece_pos::piece_open)
  		{
  			for (int i = 0; i < num_blocks; ++i) d.push_back(nullptr);
  			return;
  		}
  
  		std::vector<downloading_piece>::const_iterator i
  			= find_dl_piece(state, index);
  		TORRENT_ASSERT(i != m_downloads[state].end());
  		block_info const* binfo = blocks_for_piece(*i);
  		for (int j = 0; j != num_blocks; ++j)
  		{
  			TORRENT_ASSERT(binfo[j].peer == nullptr
  				|| binfo[j].peer->in_use);
  			d.push_back(binfo[j].peer);
  		}
  	}
  
  	torrent_peer* piece_picker::get_downloader(piece_block block) const
  	{
  		int const state = m_piece_map[block.piece_index].download_queue();
  		if (state == piece_pos::piece_open) return nullptr;
  
  		std::vector<downloading_piece>::const_iterator i = find_dl_piece(state
  			, block.piece_index);
  
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		block_info const* binfo = blocks_for_piece(*i);
  		TORRENT_ASSERT(binfo[block.block_index].piece_index == block.piece_index);
  		if (binfo[block.block_index].state == block_info::state_none)
  			return nullptr;
  
  		torrent_peer* peer = binfo[block.block_index].peer;
  		TORRENT_ASSERT(peer == nullptr || static_cast<torrent_peer*>(peer)->in_use);
  		return peer;
  	}
  
  	// this is called when a request is rejected or when
  	// a peer disconnects. The piece might be in any state
  	void piece_picker::abort_download(piece_block block, torrent_peer* peer)
  	{
  #ifdef TORRENT_EXPENSIVE_INVARIANT_CHECKS
  		INVARIANT_CHECK;
  #endif
  
  #ifdef TORRENT_PICKER_LOG
  		std::cerr << "[" << this << "] " << "abort_download( {" << block.piece_index << ", " << block.block_index << "} )" << std::endl;
  #endif
  		TORRENT_ASSERT(peer == nullptr || peer->in_use);
  
  		TORRENT_ASSERT(block.block_index != piece_block::invalid.block_index);
  		TORRENT_ASSERT(block.piece_index != piece_block::invalid.piece_index);
  
  		int state = m_piece_map[block.piece_index].download_queue();
  		if (state == piece_pos::piece_open) return;
  
  		std::vector<downloading_piece>::iterator i = find_dl_piece(state
  			, block.piece_index);
  		TORRENT_ASSERT(i != m_downloads[state].end());
  
  		block_info* binfo = blocks_for_piece(*i);
  		block_info& info = binfo[block.block_index];
  		TORRENT_ASSERT(info.peer == nullptr || info.peer->in_use);
  		TORRENT_ASSERT(info.piece_index == block.piece_index);
  
  		TORRENT_ASSERT(info.state != block_info::state_none);
  
  		if (info.state != block_info::state_requested) return;
  
  		piece_pos& p = m_piece_map[block.piece_index];
  		int prev_prio = p.priority(this);
  
  #if TORRENT_USE_ASSERTS
  		TORRENT_ASSERT(info.peers.count(peer));
  		info.peers.erase(peer);
  #endif
  		TORRENT_ASSERT(info.num_peers > 0);
  		if (info.num_peers > 0) --info.num_peers;
  		if (info.peer == peer) info.peer = nullptr;
  		TORRENT_ASSERT(info.peers.size() == info.num_peers);
  
  		TORRENT_ASSERT(block.block_index < blocks_in_piece(block.piece_index));
  
  		// if there are other peers, leave the block requested
  		if (info.num_peers > 0) return;
  
  		// clear the downloader of this block
  		info.peer = nullptr;
  
  		// clear this block as being downloaded
  		info.state = block_info::state_none;
  		TORRENT_ASSERT(i->requested > 0);
  		--i->requested;
  
  		// if there are no other blocks in this piece
  		// that's being downloaded, remove it from the list
  		if (i->requested + i->finished + i->writing == 0)
  		{
  			TORRENT_ASSERT(prev_prio < int(m_priority_boundaries.size())
  				|| m_dirty);
  			erase_download_piece(i);
  			int const prio = p.priority(this);
  			if (!m_dirty)
  			{
  				if (prev_prio == -1 && prio >= 0) add(block.piece_index);
  				else if (prev_prio >= 0) update(prev_prio, p.index);
  			}
  			return;
  		}
  
  		i = update_piece_state(i);
  	}
  
  	int piece_picker::unverified_blocks() const
  	{
  		int counter = 0;
  		for (int k = 0; k < piece_pos::num_download_categories; ++k)
  		{
  			for (auto const& dp : m_downloads[k])
  			{
  				counter += int(dp.finished);
  			}
  		}
  		return counter;
  	}
  
  }
  