dolphin/Source/Core/Common/LinearDiskCache.h

// Copyright 2010 Dolphin Emulator Project
// Licensed under GPLv2+
// Refer to the license.txt file included.

#pragma once

#include <cstring>
#include <fstream>
#include <string>
#include <type_traits>

#include "Common/CommonTypes.h"
#include "Common/FileUtil.h"

// On disk format:
//header{
// u32 'DCAC';
// u32 version;  // svn_rev
// u16 sizeof(key_type);
// u16 sizeof(value_type);
//}

//key_value_pair{
// u32 value_size;
// key_type   key;
// value_type[value_size]   value;
//}

template <typename K, typename V>
class LinearDiskCacheReader
{
public:
	virtual void Read(const K &key, const V *value, u32 value_size) = 0;
};

// Dead simple unsorted key-value store with append functionality.
// No random read functionality, all reading is done in OpenAndRead.
// Keys and values can contain any characters, including \0.
//
// Suitable for caching generated shader bytecode between executions.
// Not tuned for extreme performance but should be reasonably fast.
// Does not support keys or values larger than 2GB, which should be reasonable.
// Keys must have non-zero length; values can have zero length.

// K and V are some POD type
// K : the key type
// V : value array type
template <typename K, typename V>
class LinearDiskCache
{
public:
	// return number of read entries
	u32 OpenAndRead(const std::string& filename, LinearDiskCacheReader<K, V> &reader)
	{
		using std::ios_base;

		// Since we're reading/writing directly to the storage of K instances,
		// K must be trivially copyable. TODO: Remove #if once GCC 5.0 is a
		// minimum requirement.
#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5
		static_assert(std::has_trivial_copy_constructor<K>::value, "K must be a trivially copyable type");
#else
		static_assert(std::is_trivially_copyable<K>::value, "K must be a trivially copyable type");
#endif

		// close any currently opened file
		Close();
		m_num_entries = 0;

		// try opening for reading/writing
		OpenFStream(m_file, filename, ios_base::in | ios_base::out | ios_base::binary);

		m_file.seekg(0, std::ios::end);
		std::fstream::pos_type end_pos = m_file.tellg();
		m_file.seekg(0, std::ios::beg);
		std::fstream::pos_type start_pos = m_file.tellg();
		std::streamoff file_size = end_pos - start_pos;

		if (m_file.is_open() && ValidateHeader())
		{
			// good header, read some key/value pairs
			K key;

			V *value = nullptr;
			u32 value_size = 0;
			u32 entry_number = 0;

			std::fstream::pos_type last_pos = m_file.tellg();

			while (Read(&value_size))
			{
				std::streamoff next_extent = (last_pos - start_pos) + sizeof(value_size) + value_size;
				if (next_extent > file_size)
					break;

				delete[] value;
				value = new V[value_size];

				// read key/value and pass to reader
				if (Read(&key) &&
					Read(value, value_size) &&
					Read(&entry_number) &&
					entry_number == m_num_entries+1)
				{
					reader.Read(key, value, value_size);
				}
				else
				{
					break;
				}

				m_num_entries++;
				last_pos = m_file.tellg();
			}
			m_file.seekp(last_pos);
			m_file.clear();

			delete[] value;
			return m_num_entries;
		}

		// failed to open file for reading or bad header
		// close and recreate file
		Close();
		m_file.open(filename, ios_base::out | ios_base::trunc | ios_base::binary);
		WriteHeader();
		return 0;
	}

	void Sync()
	{
		m_file.flush();
	}

	void Close()
	{
		if (m_file.is_open())
			m_file.close();
		// clear any error flags
		m_file.clear();
	}

	// Appends a key-value pair to the store.
	void Append(const K &key, const V *value, u32 value_size)
	{
		// TODO: Should do a check that we don't already have "key"? (I think each caller does that already.)
		Write(&value_size);
		Write(&key);
		Write(value, value_size);
		m_num_entries++;
		Write(&m_num_entries);
	}

private:
	void WriteHeader()
	{
		Write(&m_header);
	}

	bool ValidateHeader()
	{
		char file_header[sizeof(Header)];

		return (Read(file_header, sizeof(Header)) &&
		        !memcmp((const char*)&m_header, file_header, sizeof(Header)));
	}

	template <typename D>
	bool Write(const D *data, u32 count = 1)
	{
		return m_file.write((const char*)data, count * sizeof(D)).good();
	}

	template <typename D>
	bool Read(const D *data, u32 count = 1)
	{
		return m_file.read((char*)data, count * sizeof(D)).good();
	}

	struct Header
	{
		Header()
			: key_t_size(sizeof(K))
			, value_t_size(sizeof(V))
		{
			// Null-terminator is intentionally not copied.
			std::memcpy(&id, "DCAC", sizeof(u32));
			std::memcpy(ver, scm_rev_git_str, 40);
		}

		u32 id;
		const u16 key_t_size, value_t_size;
		char ver[40];

	} m_header;

	std::fstream m_file;
	u32 m_num_entries;
};