Aegisub/src/block_cache.h

// Copyright (c) 2009, Niels Martin Hansen
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/// @file block_cache.h
/// @ingroup utility
/// @brief Template class for creating caches for blocks of data

#pragma once

#include <algorithm>
#include <list>
#include <vector>

/// @class DataBlockCache
/// @brief Cache for blocks of data in a stream or similar
/// @tparam BlockT             Type of blocks to store
/// @tparam MacroblockExponent Controls the number of blocks per macroblock, for tuning memory usage
/// @tparam BlockFactoryT      Type of block factory, see BasicDataBlockFactory class for detail on these
template <typename BlockT, int MacroblockExponent, typename BlockFactoryT>
class DataBlockCache {
	/// Type of an array of blocks
	typedef std::vector<typename BlockFactoryT::BlockType> BlockArray;

	struct MacroBlock {
		/// This macroblock's position in the age list
		/// Is valid iff blocks.size() > 0
		typename std::list<MacroBlock*>::iterator position;

		/// The blocks contained in the macroblock
		BlockArray blocks;
	};

	/// Type of an array of macro blocks
	typedef std::vector<MacroBlock> MacroBlockArray;

	/// The data in the cache
	MacroBlockArray data;

	/// Type of a list of macro blocks
	typedef std::list<MacroBlock*> AgeList;

	/// The data sorted by how long it's been since they were used
	AgeList age;

	/// Number of blocks per macroblock
	size_t macroblock_size;

	/// Bitmask to extract the inside-macroblock index for a block by bitwise and
	size_t macroblock_index_mask;

	/// Current size of the cache in bytes
	size_t size = 0;

	/// Factory object for blocks
	BlockFactoryT factory;

	/// @brief Dispose of all blocks in a macroblock and mark it empty
	/// @param mb_index Index of macroblock to clear
	void KillMacroBlock(MacroBlock &mb)
	{
		if (mb.blocks.empty())
			return;

		auto& ba = mb.blocks;
		size -= (ba.size() - std::count(ba.begin(), ba.end(), nullptr)) * factory.GetBlockSize();

		ba.clear();
		age.erase(mb.position);
	}

public:
	/// @brief Constructor
	/// @param block_count Total number of blocks the cache will manage
	/// @param factory     Factory object to use for producing blocks
	///
	/// Note that the block_count is the maximum block index the cache will ever see,
	/// it is an error to request a block number greater than block_count.
	///
	/// The factory object passed must respond well to copying.
	DataBlockCache(size_t block_count, BlockFactoryT factory = BlockFactoryT())
	: factory(std::move(factory))
	{
		SetBlockCount(block_count);
	}


	/// @brief Change the number of blocks in cache
	/// @param block_count New number of blocks to hold
	///
	/// This will completely de-allocate the cache and re-allocate it with the new block count.
	void SetBlockCount(size_t block_count)
	{
		if (data.size() > 0)
			Age(0);

		macroblock_size = 1UL << MacroblockExponent;
		macroblock_index_mask = macroblock_size - 1;

		data.resize((block_count + macroblock_size - 1) >> MacroblockExponent);
		size = 0;
	}

	/// @brief Clean up the cache
	/// @param max_size Target maximum size of the cache in bytes
	///
	/// Passing a max_size of 0 (zero) causes the cache to be completely flushed
	/// in a fast manner.
	///
	/// The max_size is not a hard limit, the cache size might somewhat exceed the max
	/// after the aging operation, though it shouldn't be by much.
	void Age(size_t max_size)
	{
		// Quick way out: get rid of everything
		if (max_size == 0)
		{
			size_t block_count = data.size();
			data.clear();
			data.resize(block_count);
			age.clear();
			size = 0;
			return;
		}

		// Remove old entries until we're under the max size
		for (auto it = age.rbegin(); size > max_size && it != age.rend(); )
			KillMacroBlock(**it++);
	}

	/// @brief Obtain a data block from the cache
	/// @param      i       Index of the block to retrieve
	/// @param[out] created On return, tells whether the returned block was created during the operation
	/// @return A pointer to the block in cache
	///
	/// It is legal to pass 0 (null) for created, in this case nothing is returned in it.
	BlockT *Get(size_t i, bool *created = nullptr)
	{
		size_t mbi = i >> MacroblockExponent;
		assert(mbi < data.size());

		MacroBlock &mb = data[mbi];

		// Move this macroblock to the front of the age list
		if (mb.blocks.empty())
		{
			mb.blocks.resize(macroblock_size);
			age.push_front(&mb);
		}
		else if (mb.position != begin(age))
			age.splice(begin(age), age, mb.position);

		mb.position = age.begin();

		size_t block_index = i & macroblock_index_mask;
		assert(block_index < mb.blocks.size());

		BlockT *b = mb.blocks[block_index].get();

		if (!b)
		{
			mb.blocks[block_index] = factory.ProduceBlock(i);
			b = mb.blocks[block_index].get();
			assert(b != nullptr);
			size += factory.GetBlockSize();

			if (created) *created = true;
		}
		else
			if (created) *created = false;

		return b;
	}
};