citra/src/common/linear_disk_cache.h

// Copyright 2013 Dolphin Emulator Project / 2014 Citra Emulator Project
// Licensed under GPLv2 or any later version
// Refer to the license.txt file included.

#pragma once

#include "common/common_types.h"
#include <fstream>

// defined in Version.cpp
extern const char *scm_rev_git_str;

// 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 char *filename, LinearDiskCacheReader<K, V> &reader)
    {
        using std::ios_base;

        // 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;
            u32 entry_number;

            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()
            : id(*(u32*)"DCAC")
            , key_t_size(sizeof(K))
            , value_t_size(sizeof(V))
        {
            memcpy(ver, scm_rev_git_str, 40);
        }

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

    } m_header;

    std::fstream m_file;
    u32 m_num_entries;
};