// Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of Open Ethereum.

// Open Ethereum is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Open Ethereum is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Open Ethereum.  If not, see <http://www.gnu.org/licenses/>.

//! Database cache manager

use std::collections::{VecDeque, HashSet};
use std::hash::Hash;

const COLLECTION_QUEUE_SIZE: usize = 8;

/// DB cache manager
pub struct CacheManager<T> {
	pref_cache_size: usize,
	max_cache_size: usize,
	bytes_per_cache_entry: usize,
	cache_usage: VecDeque<HashSet<T>>
}

impl<T> CacheManager<T> where T: Eq + Hash {
	/// Create new cache manager with preferred (heap) sizes.
	pub fn new(pref_cache_size: usize, max_cache_size: usize, bytes_per_cache_entry: usize) -> Self {
		CacheManager {
			pref_cache_size,
			max_cache_size,
			bytes_per_cache_entry,
			cache_usage: (0..COLLECTION_QUEUE_SIZE).into_iter().map(|_| Default::default()).collect(),
		}
	}

	/// Mark element as used.
	pub fn note_used(&mut self, id: T) {
		if !self.cache_usage[0].contains(&id) {
			if let Some(c) = self.cache_usage.iter_mut().skip(1).find(|e| e.contains(&id)) {
				c.remove(&id);
			}
			self.cache_usage[0].insert(id);
		}
	}

	/// Collects unused objects from cache.
	/// First params is the current size of the cache.
	/// Second one is an with objects to remove. It should also return new size of the cache.
	pub fn collect_garbage<F>(&mut self, current_size: usize, mut notify_unused: F) where F: FnMut(HashSet<T>) -> usize {
		if current_size < self.pref_cache_size {
			self.rotate_cache_if_needed();
			return;
		}

		for _ in 0..COLLECTION_QUEUE_SIZE {
			if let Some(back) = self.cache_usage.pop_back() {
				let current_size = notify_unused(back);
				self.cache_usage.push_front(Default::default());
				if current_size < self.max_cache_size {
					break
				}
			}
		}
	}

	fn rotate_cache_if_needed(&mut self) {
		if self.cache_usage.is_empty() { return }

		if self.cache_usage[0].len() * self.bytes_per_cache_entry > self.pref_cache_size / COLLECTION_QUEUE_SIZE {
			if let Some(cache) = self.cache_usage.pop_back() {
				self.cache_usage.push_front(cache);
			}
		}
	}
}
