text: // Copyright (c) 2014-2018, The Monero Project
  //
  // All rights reserved.
  //
  // Redistribution and use in source and binary forms, with or without modification, are
  // permitted provided that the following conditions are met:
  //
  // 1. Redistributions of source code must retain the above copyright notice, this list of
  //    conditions and the following disclaimer.
  //
  // 2. 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.
  //
  // 3. Neither the name of the copyright holder 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 HOLDER 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.
  //
  // Parts of this file are originally copyright (c) 2012-2013 The Cryptonote developers
  
  #include <algorithm>
  #include <cassert>
  #include <cstddef>
  #include <cstdint>
  #include <vector>
  
  #include "int-util.h"
  #include "crypto/hash.h"
  #include "cryptonote_config.h"
  #include "difficulty.h"
  
  #undef BELDEX_DEFAULT_LOG_CATEGORY
  #define BELDEX_DEFAULT_LOG_CATEGORY "difficulty"
  
  namespace cryptonote {
  
    using std::size_t;
    using std::uint64_t;
    using std::vector;
  
  #if defined(__x86_64__)
    static inline void mul(uint64_t a, uint64_t b, uint64_t &low, uint64_t &high) {
      low = mul128(a, b, &high);
    }
  
  #else
  
    static inline void mul(uint64_t a, uint64_t b, uint64_t &low, uint64_t &high) {
      // __int128 isn't part of the standard, so the previous function wasn't portable. mul128() in Windows is fine,
      // but this portable function should be used elsewhere. Credit for this function goes to latexi95.
  
      uint64_t aLow = a & 0xFFFFFFFF;
      uint64_t aHigh = a >> 32;
      uint64_t bLow = b & 0xFFFFFFFF;
      uint64_t bHigh = b >> 32;
  
      uint64_t res = aLow * bLow;
      uint64_t lowRes1 = res & 0xFFFFFFFF;
      uint64_t carry = res >> 32;
  
      res = aHigh * bLow + carry;
      uint64_t highResHigh1 = res >> 32;
      uint64_t highResLow1 = res & 0xFFFFFFFF;
  
      res = aLow * bHigh;
      uint64_t lowRes2 = res & 0xFFFFFFFF;
      carry = res >> 32;
  
      res = aHigh * bHigh + carry;
      uint64_t highResHigh2 = res >> 32;
      uint64_t highResLow2 = res & 0xFFFFFFFF;
  
      //Addition
  
      uint64_t r = highResLow1 + lowRes2;
      carry = r >> 32;
      low = (r << 32) | lowRes1;
      r = highResHigh1 + highResLow2 + carry;
      uint64_t d3 = r & 0xFFFFFFFF;
      carry = r >> 32;
      r = highResHigh2 + carry;
      high = d3 | (r << 32);
    }
  
  #endif
  
    static inline bool cadd(uint64_t a, uint64_t b) {
      return a + b < a;
    }
  
    static inline bool cadc(uint64_t a, uint64_t b, bool c) {
      return a + b < a || (c && a + b == (uint64_t) -1);
    }
  
    bool check_hash(const crypto::hash &hash, difficulty_type difficulty) {
      uint64_t low, high, top, cur;
      // First check the highest word, this will most likely fail for a random hash.
      mul(swap64le(((const uint64_t *) &hash)[3]), difficulty, top, high);
      if (high != 0) {
        return false;
      }
      mul(swap64le(((const uint64_t *) &hash)[0]), difficulty, low, cur);
      mul(swap64le(((const uint64_t *) &hash)[1]), difficulty, low, high);
      bool carry = cadd(cur, low);
      cur = high;
      mul(swap64le(((const uint64_t *) &hash)[2]), difficulty, low, high);
      carry = cadc(cur, low, carry);
      carry = cadc(high, top, carry);
      return !carry;
    }
  
    difficulty_type next_difficulty(std::vector<std::uint64_t> timestamps, std::vector<difficulty_type> cumulative_difficulties, size_t target_seconds) {
  
      if(timestamps.size() > DIFFICULTY_WINDOW)
      {
        timestamps.resize(DIFFICULTY_WINDOW);
        cumulative_difficulties.resize(DIFFICULTY_WINDOW);
      }
  
  
      size_t length = timestamps.size();
      assert(length == cumulative_difficulties.size());
      if (length <= 1) {
        return 1;
      }
      static_assert(DIFFICULTY_WINDOW >= 2, "Window is too small");
      assert(length <= DIFFICULTY_WINDOW);
      sort(timestamps.begin(), timestamps.end());
      size_t cut_begin, cut_end;
      static_assert(2 * DIFFICULTY_CUT <= DIFFICULTY_WINDOW - 2, "Cut length is too large");
      if (length <= DIFFICULTY_WINDOW - 2 * DIFFICULTY_CUT) {
        cut_begin = 0;
        cut_end = length;
      } else {
        cut_begin = (length - (DIFFICULTY_WINDOW - 2 * DIFFICULTY_CUT) + 1) / 2;
        cut_end = cut_begin + (DIFFICULTY_WINDOW - 2 * DIFFICULTY_CUT);
      }
      assert(/*cut_begin >= 0 &&*/ cut_begin + 2 <= cut_end && cut_end <= length);
      uint64_t time_span = timestamps[cut_end - 1] - timestamps[cut_begin];
      if (time_span == 0) {
        time_span = 1;
      }
      difficulty_type total_work = cumulative_difficulties[cut_end - 1] - cumulative_difficulties[cut_begin];
      assert(total_work > 0);
      uint64_t low, high;
      mul(total_work, target_seconds, low, high);
      // blockchain errors "difficulty overhead" if this function returns zero.
      // TODO: consider throwing an exception instead
      if (high != 0 || low + time_span - 1 < low) {
        return 0;
      }
      return (low + time_span - 1) / time_span;
    }
  
  }
  