gig.cpp

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/***************************************************************************
 *                                                                         *
 *   libgig - C++ cross-platform Gigasampler format file access library    *
 *                                                                         *
 *   Copyright (C) 2003-2007 by Christian Schoenebeck                      *
 *                              <cuse@users.sourceforge.net>               *
 *                                                                         *
 *   This library 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 2 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 *   This library 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 this library; if not, write to the Free Software           *
 *   Foundation, Inc., 59 Temple Place, Suite 330, Boston,                 *
 *   MA  02111-1307  USA                                                   *
 ***************************************************************************/

#include "gig.h"

#include "helper.h"

#include <algorithm>
#include <math.h>
#include <iostream>

#define INITIAL_SAMPLE_BUFFER_SIZE              512000 // 512 kB

#define GIG_EXP_DECODE(x)                       (pow(1.000000008813822, x))
#define GIG_EXP_ENCODE(x)                       (log(x) / log(1.000000008813822))
#define GIG_PITCH_TRACK_EXTRACT(x)              (!(x & 0x01))
#define GIG_PITCH_TRACK_ENCODE(x)               ((x) ? 0x00 : 0x01)
#define GIG_VCF_RESONANCE_CTRL_EXTRACT(x)       ((x >> 4) & 0x03)
#define GIG_VCF_RESONANCE_CTRL_ENCODE(x)        ((x & 0x03) << 4)
#define GIG_EG_CTR_ATTACK_INFLUENCE_EXTRACT(x)  ((x >> 1) & 0x03)
#define GIG_EG_CTR_DECAY_INFLUENCE_EXTRACT(x)   ((x >> 3) & 0x03)
#define GIG_EG_CTR_RELEASE_INFLUENCE_EXTRACT(x) ((x >> 5) & 0x03)
#define GIG_EG_CTR_ATTACK_INFLUENCE_ENCODE(x)   ((x & 0x03) << 1)
#define GIG_EG_CTR_DECAY_INFLUENCE_ENCODE(x)    ((x & 0x03) << 3)
#define GIG_EG_CTR_RELEASE_INFLUENCE_ENCODE(x)  ((x & 0x03) << 5)

namespace gig {

// *************** progress_t ***************
// *

    progress_t::progress_t() {
        callback    = NULL;
        custom      = NULL;
        __range_min = 0.0f;
        __range_max = 1.0f;
    }

    // private helper function to convert progress of a subprocess into the global progress
    static void __notify_progress(progress_t* pProgress, float subprogress) {
        if (pProgress && pProgress->callback) {
            const float totalrange    = pProgress->__range_max - pProgress->__range_min;
            const float totalprogress = pProgress->__range_min + subprogress * totalrange;
            pProgress->factor         = totalprogress;
            pProgress->callback(pProgress); // now actually notify about the progress
        }
    }

    // private helper function to divide a progress into subprogresses
    static void __divide_progress(progress_t* pParentProgress, progress_t* pSubProgress, float totalTasks, float currentTask) {
        if (pParentProgress && pParentProgress->callback) {
            const float totalrange    = pParentProgress->__range_max - pParentProgress->__range_min;
            pSubProgress->callback    = pParentProgress->callback;
            pSubProgress->custom      = pParentProgress->custom;
            pSubProgress->__range_min = pParentProgress->__range_min + totalrange * currentTask / totalTasks;
            pSubProgress->__range_max = pSubProgress->__range_min + totalrange / totalTasks;
        }
    }


// *************** Internal functions for sample decompression ***************
// *

namespace {

    inline int get12lo(const unsigned char* pSrc)
    {
        const int x = pSrc[0] | (pSrc[1] & 0x0f) << 8;
        return x & 0x800 ? x - 0x1000 : x;
    }

    inline int get12hi(const unsigned char* pSrc)
    {
        const int x = pSrc[1] >> 4 | pSrc[2] << 4;
        return x & 0x800 ? x - 0x1000 : x;
    }

    inline int16_t get16(const unsigned char* pSrc)
    {
        return int16_t(pSrc[0] | pSrc[1] << 8);
    }

    inline int get24(const unsigned char* pSrc)
    {
        const int x = pSrc[0] | pSrc[1] << 8 | pSrc[2] << 16;
        return x & 0x800000 ? x - 0x1000000 : x;
    }

    inline void store24(unsigned char* pDst, int x)
    {
        pDst[0] = x;
        pDst[1] = x >> 8;
        pDst[2] = x >> 16;
    }

    void Decompress16(int compressionmode, const unsigned char* params,
                      int srcStep, int dstStep,
                      const unsigned char* pSrc, int16_t* pDst,
                      unsigned long currentframeoffset,
                      unsigned long copysamples)
    {
        switch (compressionmode) {
            case 0: // 16 bit uncompressed
                pSrc += currentframeoffset * srcStep;
                while (copysamples) {
                    *pDst = get16(pSrc);
                    pDst += dstStep;
                    pSrc += srcStep;
                    copysamples--;
                }
                break;

            case 1: // 16 bit compressed to 8 bit
                int y  = get16(params);
                int dy = get16(params + 2);
                while (currentframeoffset) {
                    dy -= int8_t(*pSrc);
                    y  -= dy;
                    pSrc += srcStep;
                    currentframeoffset--;
                }
                while (copysamples) {
                    dy -= int8_t(*pSrc);
                    y  -= dy;
                    *pDst = y;
                    pDst += dstStep;
                    pSrc += srcStep;
                    copysamples--;
                }
                break;
        }
    }

    void Decompress24(int compressionmode, const unsigned char* params,
                      int dstStep, const unsigned char* pSrc, uint8_t* pDst,
                      unsigned long currentframeoffset,
                      unsigned long copysamples, int truncatedBits)
    {
        int y, dy, ddy, dddy;

#define GET_PARAMS(params)                      \
        y    = get24(params);                   \
        dy   = y - get24((params) + 3);         \
        ddy  = get24((params) + 6);             \
        dddy = get24((params) + 9)

#define SKIP_ONE(x)                             \
        dddy -= (x);                            \
        ddy  -= dddy;                           \
        dy   =  -dy - ddy;                      \
        y    += dy

#define COPY_ONE(x)                             \
        SKIP_ONE(x);                            \
        store24(pDst, y << truncatedBits);      \
        pDst += dstStep

        switch (compressionmode) {
            case 2: // 24 bit uncompressed
                pSrc += currentframeoffset * 3;
                while (copysamples) {
                    store24(pDst, get24(pSrc) << truncatedBits);
                    pDst += dstStep;
                    pSrc += 3;
                    copysamples--;
                }
                break;

            case 3: // 24 bit compressed to 16 bit
                GET_PARAMS(params);
                while (currentframeoffset) {
                    SKIP_ONE(get16(pSrc));
                    pSrc += 2;
                    currentframeoffset--;
                }
                while (copysamples) {
                    COPY_ONE(get16(pSrc));
                    pSrc += 2;
                    copysamples--;
                }
                break;

            case 4: // 24 bit compressed to 12 bit
                GET_PARAMS(params);
                while (currentframeoffset > 1) {
                    SKIP_ONE(get12lo(pSrc));
                    SKIP_ONE(get12hi(pSrc));
                    pSrc += 3;
                    currentframeoffset -= 2;
                }
                if (currentframeoffset) {
                    SKIP_ONE(get12lo(pSrc));
                    currentframeoffset--;
                    if (copysamples) {
                        COPY_ONE(get12hi(pSrc));
                        pSrc += 3;
                        copysamples--;
                    }
                }
                while (copysamples > 1) {
                    COPY_ONE(get12lo(pSrc));
                    COPY_ONE(get12hi(pSrc));
                    pSrc += 3;
                    copysamples -= 2;
                }
                if (copysamples) {
                    COPY_ONE(get12lo(pSrc));
                }
                break;

            case 5: // 24 bit compressed to 8 bit
                GET_PARAMS(params);
                while (currentframeoffset) {
                    SKIP_ONE(int8_t(*pSrc++));
                    currentframeoffset--;
                }
                while (copysamples) {
                    COPY_ONE(int8_t(*pSrc++));
                    copysamples--;
                }
                break;
        }
    }

    const int bytesPerFrame[] =      { 4096, 2052, 768, 524, 396, 268 };
    const int bytesPerFrameNoHdr[] = { 4096, 2048, 768, 512, 384, 256 };
    const int headerSize[] =         { 0, 4, 0, 12, 12, 12 };
    const int bitsPerSample[] =      { 16, 8, 24, 16, 12, 8 };
}



// *************** Internal CRC-32 (Cyclic Redundancy Check) functions  ***************
// *

    static uint32_t* __initCRCTable() {
        static uint32_t res[256];

        for (int i = 0 ; i < 256 ; i++) {
            uint32_t c = i;
            for (int j = 0 ; j < 8 ; j++) {
                c = (c & 1) ? 0xedb88320 ^ (c >> 1) : c >> 1;
            }
            res[i] = c;
        }
        return res;
    }

    static const uint32_t* __CRCTable = __initCRCTable();

    inline static void __resetCRC(uint32_t& crc) {
        crc = 0xffffffff;
    }

    static void __calculateCRC(unsigned char* buf, int bufSize, uint32_t& crc) {
        for (int i = 0 ; i < bufSize ; i++) {
            crc = __CRCTable[(crc ^ buf[i]) & 0xff] ^ (crc >> 8);
        }
    }

    inline static uint32_t __encodeCRC(const uint32_t& crc) {
        return crc ^ 0xffffffff;
    }



// *************** Other Internal functions  ***************
// *

    static split_type_t __resolveSplitType(dimension_t dimension) {
        return (
            dimension == dimension_layer ||
            dimension == dimension_samplechannel ||
            dimension == dimension_releasetrigger ||
            dimension == dimension_keyboard ||
            dimension == dimension_roundrobin ||
            dimension == dimension_random ||
            dimension == dimension_smartmidi ||
            dimension == dimension_roundrobinkeyboard
        ) ? split_type_bit : split_type_normal;
    }

    static int __resolveZoneSize(dimension_def_t& dimension_definition) {
        return (dimension_definition.split_type == split_type_normal)
        ? int(128.0 / dimension_definition.zones) : 0;
    }



// *************** Sample ***************
// *

    unsigned int Sample::Instances = 0;
    buffer_t     Sample::InternalDecompressionBuffer;

    Sample::Sample(File* pFile, RIFF::List* waveList, unsigned long WavePoolOffset, unsigned long fileNo) : DLS::Sample((DLS::File*) pFile, waveList, WavePoolOffset) {
        static const DLS::Info::string_length_t fixedStringLengths[] = {
            { CHUNK_ID_INAM, 64 },
            { 0, 0 }
        };
        pInfo->SetFixedStringLengths(fixedStringLengths);
        Instances++;
        FileNo = fileNo;

        __resetCRC(crc);

        pCk3gix = waveList->GetSubChunk(CHUNK_ID_3GIX);
        if (pCk3gix) {
            uint16_t iSampleGroup = pCk3gix->ReadInt16();
            pGroup = pFile->GetGroup(iSampleGroup);
        } else { // '3gix' chunk missing
            // by default assigned to that mandatory "Default Group"
            pGroup = pFile->GetGroup(0);
        }

        pCkSmpl = waveList->GetSubChunk(CHUNK_ID_SMPL);
        if (pCkSmpl) {
            Manufacturer  = pCkSmpl->ReadInt32();
            Product       = pCkSmpl->ReadInt32();
            SamplePeriod  = pCkSmpl->ReadInt32();
            MIDIUnityNote = pCkSmpl->ReadInt32();
            FineTune      = pCkSmpl->ReadInt32();
            pCkSmpl->Read(&SMPTEFormat, 1, 4);
            SMPTEOffset   = pCkSmpl->ReadInt32();
            Loops         = pCkSmpl->ReadInt32();
            pCkSmpl->ReadInt32(); // manufByt
            LoopID        = pCkSmpl->ReadInt32();
            pCkSmpl->Read(&LoopType, 1, 4);
            LoopStart     = pCkSmpl->ReadInt32();
            LoopEnd       = pCkSmpl->ReadInt32();
            LoopFraction  = pCkSmpl->ReadInt32();
            LoopPlayCount = pCkSmpl->ReadInt32();
        } else { // 'smpl' chunk missing
            // use default values
            Manufacturer  = 0;
            Product       = 0;
            SamplePeriod  = uint32_t(1000000000.0 / SamplesPerSecond + 0.5);
            MIDIUnityNote = 60;
            FineTune      = 0;
            SMPTEFormat   = smpte_format_no_offset;
            SMPTEOffset   = 0;
            Loops         = 0;
            LoopID        = 0;
            LoopType      = loop_type_normal;
            LoopStart     = 0;
            LoopEnd       = 0;
            LoopFraction  = 0;
            LoopPlayCount = 0;
        }

        FrameTable                 = NULL;
        SamplePos                  = 0;
        RAMCache.Size              = 0;
        RAMCache.pStart            = NULL;
        RAMCache.NullExtensionSize = 0;

        if (BitDepth > 24) throw gig::Exception("Only samples up to 24 bit supported");

        RIFF::Chunk* ewav = waveList->GetSubChunk(CHUNK_ID_EWAV);
        Compressed        = ewav;
        Dithered          = false;
        TruncatedBits     = 0;
        if (Compressed) {
            uint32_t version = ewav->ReadInt32();
            if (version == 3 && BitDepth == 24) {
                Dithered = ewav->ReadInt32();
                ewav->SetPos(Channels == 2 ? 84 : 64);
                TruncatedBits = ewav->ReadInt32();
            }
            ScanCompressedSample();
        }

        // we use a buffer for decompression and for truncating 24 bit samples to 16 bit
        if ((Compressed || BitDepth == 24) && !InternalDecompressionBuffer.Size) {
            InternalDecompressionBuffer.pStart = new unsigned char[INITIAL_SAMPLE_BUFFER_SIZE];
            InternalDecompressionBuffer.Size   = INITIAL_SAMPLE_BUFFER_SIZE;
        }
        FrameOffset = 0; // just for streaming compressed samples

        LoopSize = LoopEnd - LoopStart + 1;
    }

    void Sample::UpdateChunks() {
        // first update base class's chunks
        DLS::Sample::UpdateChunks();

        // make sure 'smpl' chunk exists
        pCkSmpl = pWaveList->GetSubChunk(CHUNK_ID_SMPL);
        if (!pCkSmpl) {
            pCkSmpl = pWaveList->AddSubChunk(CHUNK_ID_SMPL, 60);
            memset(pCkSmpl->LoadChunkData(), 0, 60);
        }
        // update 'smpl' chunk
        uint8_t* pData = (uint8_t*) pCkSmpl->LoadChunkData();
        SamplePeriod = uint32_t(1000000000.0 / SamplesPerSecond + 0.5);
        store32(&pData[0], Manufacturer);
        store32(&pData[4], Product);
        store32(&pData[8], SamplePeriod);
        store32(&pData[12], MIDIUnityNote);
        store32(&pData[16], FineTune);
        store32(&pData[20], SMPTEFormat);
        store32(&pData[24], SMPTEOffset);
        store32(&pData[28], Loops);

        // we skip 'manufByt' for now (4 bytes)

        store32(&pData[36], LoopID);
        store32(&pData[40], LoopType);
        store32(&pData[44], LoopStart);
        store32(&pData[48], LoopEnd);
        store32(&pData[52], LoopFraction);
        store32(&pData[56], LoopPlayCount);

        // make sure '3gix' chunk exists
        pCk3gix = pWaveList->GetSubChunk(CHUNK_ID_3GIX);
        if (!pCk3gix) pCk3gix = pWaveList->AddSubChunk(CHUNK_ID_3GIX, 4);
        // determine appropriate sample group index (to be stored in chunk)
        uint16_t iSampleGroup = 0; // 0 refers to default sample group
        File* pFile = static_cast<File*>(pParent);
        if (pFile->pGroups) {
            std::list<Group*>::iterator iter = pFile->pGroups->begin();
            std::list<Group*>::iterator end  = pFile->pGroups->end();
            for (int i = 0; iter != end; i++, iter++) {
                if (*iter == pGroup) {
                    iSampleGroup = i;
                    break; // found
                }
            }
        }
        // update '3gix' chunk
        pData = (uint8_t*) pCk3gix->LoadChunkData();
        store16(&pData[0], iSampleGroup);
    }

    void Sample::ScanCompressedSample() {
        //TODO: we have to add some more scans here (e.g. determine compression rate)
        this->SamplesTotal = 0;
        std::list<unsigned long> frameOffsets;

        SamplesPerFrame = BitDepth == 24 ? 256 : 2048;
        WorstCaseFrameSize = SamplesPerFrame * FrameSize + Channels; // +Channels for compression flag

        // Scanning
        pCkData->SetPos(0);
        if (Channels == 2) { // Stereo
            for (int i = 0 ; ; i++) {
                // for 24 bit samples every 8:th frame offset is
                // stored, to save some memory
                if (BitDepth != 24 || (i & 7) == 0) frameOffsets.push_back(pCkData->GetPos());

                const int mode_l = pCkData->ReadUint8();
                const int mode_r = pCkData->ReadUint8();
                if (mode_l > 5 || mode_r > 5) throw gig::Exception("Unknown compression mode");
                const unsigned long frameSize = bytesPerFrame[mode_l] + bytesPerFrame[mode_r];

                if (pCkData->RemainingBytes() <= frameSize) {
                    SamplesInLastFrame =
                        ((pCkData->RemainingBytes() - headerSize[mode_l] - headerSize[mode_r]) << 3) /
                        (bitsPerSample[mode_l] + bitsPerSample[mode_r]);
                    SamplesTotal += SamplesInLastFrame;
                    break;
                }
                SamplesTotal += SamplesPerFrame;
                pCkData->SetPos(frameSize, RIFF::stream_curpos);
            }
        }
        else { // Mono
            for (int i = 0 ; ; i++) {
                if (BitDepth != 24 || (i & 7) == 0) frameOffsets.push_back(pCkData->GetPos());

                const int mode = pCkData->ReadUint8();
                if (mode > 5) throw gig::Exception("Unknown compression mode");
                const unsigned long frameSize = bytesPerFrame[mode];

                if (pCkData->RemainingBytes() <= frameSize) {
                    SamplesInLastFrame =
                        ((pCkData->RemainingBytes() - headerSize[mode]) << 3) / bitsPerSample[mode];
                    SamplesTotal += SamplesInLastFrame;
                    break;
                }
                SamplesTotal += SamplesPerFrame;
                pCkData->SetPos(frameSize, RIFF::stream_curpos);
            }
        }
        pCkData->SetPos(0);

        // Build the frames table (which is used for fast resolving of a frame's chunk offset)
        if (FrameTable) delete[] FrameTable;
        FrameTable = new unsigned long[frameOffsets.size()];
        std::list<unsigned long>::iterator end  = frameOffsets.end();
        std::list<unsigned long>::iterator iter = frameOffsets.begin();
        for (int i = 0; iter != end; i++, iter++) {
            FrameTable[i] = *iter;
        }
    }

    buffer_t Sample::LoadSampleData() {
        return LoadSampleDataWithNullSamplesExtension(this->SamplesTotal, 0); // 0 amount of NullSamples
    }

    buffer_t Sample::LoadSampleData(unsigned long SampleCount) {
        return LoadSampleDataWithNullSamplesExtension(SampleCount, 0); // 0 amount of NullSamples
    }

    buffer_t Sample::LoadSampleDataWithNullSamplesExtension(uint NullSamplesCount) {
        return LoadSampleDataWithNullSamplesExtension(this->SamplesTotal, NullSamplesCount);
    }

    buffer_t Sample::LoadSampleDataWithNullSamplesExtension(unsigned long SampleCount, uint NullSamplesCount) {
        if (SampleCount > this->SamplesTotal) SampleCount = this->SamplesTotal;
        if (RAMCache.pStart) delete[] (int8_t*) RAMCache.pStart;
        unsigned long allocationsize = (SampleCount + NullSamplesCount) * this->FrameSize;
        RAMCache.pStart            = new int8_t[allocationsize];
        RAMCache.Size              = Read(RAMCache.pStart, SampleCount) * this->FrameSize;
        RAMCache.NullExtensionSize = allocationsize - RAMCache.Size;
        // fill the remaining buffer space with silence samples
        memset((int8_t*)RAMCache.pStart + RAMCache.Size, 0, RAMCache.NullExtensionSize);
        return GetCache();
    }

    buffer_t Sample::GetCache() {
        // return a copy of the buffer_t structure
        buffer_t result;
        result.Size              = this->RAMCache.Size;
        result.pStart            = this->RAMCache.pStart;
        result.NullExtensionSize = this->RAMCache.NullExtensionSize;
        return result;
    }

    void Sample::ReleaseSampleData() {
        if (RAMCache.pStart) delete[] (int8_t*) RAMCache.pStart;
        RAMCache.pStart = NULL;
        RAMCache.Size   = 0;
    }

    void Sample::Resize(int iNewSize) {
        if (Compressed) throw gig::Exception("There is no support for modifying compressed samples (yet)");
        DLS::Sample::Resize(iNewSize);
    }

    unsigned long Sample::SetPos(unsigned long SampleCount, RIFF::stream_whence_t Whence) {
        if (Compressed) {
            switch (Whence) {
                case RIFF::stream_curpos:
                    this->SamplePos += SampleCount;
                    break;
                case RIFF::stream_end:
                    this->SamplePos = this->SamplesTotal - 1 - SampleCount;
                    break;
                case RIFF::stream_backward:
                    this->SamplePos -= SampleCount;
                    break;
                case RIFF::stream_start: default:
                    this->SamplePos = SampleCount;
                    break;
            }
            if (this->SamplePos > this->SamplesTotal) this->SamplePos = this->SamplesTotal;

            unsigned long frame = this->SamplePos / 2048; // to which frame to jump
            this->FrameOffset   = this->SamplePos % 2048; // offset (in sample points) within that frame
            pCkData->SetPos(FrameTable[frame]);           // set chunk pointer to the start of sought frame
            return this->SamplePos;
        }
        else { // not compressed
            unsigned long orderedBytes = SampleCount * this->FrameSize;
            unsigned long result = pCkData->SetPos(orderedBytes, Whence);
            return (result == orderedBytes) ? SampleCount
                                            : result / this->FrameSize;
        }
    }

    unsigned long Sample::GetPos() {
        if (Compressed) return SamplePos;
        else            return pCkData->GetPos() / FrameSize;
    }

    unsigned long Sample::ReadAndLoop(void* pBuffer, unsigned long SampleCount, playback_state_t* pPlaybackState,
                                      DimensionRegion* pDimRgn, buffer_t* pExternalDecompressionBuffer) {
        unsigned long samplestoread = SampleCount, totalreadsamples = 0, readsamples, samplestoloopend;
        uint8_t* pDst = (uint8_t*) pBuffer;

        SetPos(pPlaybackState->position); // recover position from the last time

        if (pDimRgn->SampleLoops) { // honor looping if there are loop points defined

            const DLS::sample_loop_t& loop = pDimRgn->pSampleLoops[0];
            const uint32_t loopEnd = loop.LoopStart + loop.LoopLength;

            if (GetPos() <= loopEnd) {
                switch (loop.LoopType) {

                    case loop_type_bidirectional: { //TODO: not tested yet!
                        do {
                            // if not endless loop check if max. number of loop cycles have been passed
                            if (this->LoopPlayCount && !pPlaybackState->loop_cycles_left) break;

                            if (!pPlaybackState->reverse) { // forward playback
                                do {
                                    samplestoloopend  = loopEnd - GetPos();
                                    readsamples       = Read(&pDst[totalreadsamples * this->FrameSize], Min(samplestoread, samplestoloopend), pExternalDecompressionBuffer);
                                    samplestoread    -= readsamples;
                                    totalreadsamples += readsamples;
                                    if (readsamples == samplestoloopend) {
                                        pPlaybackState->reverse = true;
                                        break;
                                    }
                                } while (samplestoread && readsamples);
                            }
                            else { // backward playback

                                // as we can only read forward from disk, we have to
                                // determine the end position within the loop first,
                                // read forward from that 'end' and finally after
                                // reading, swap all sample frames so it reflects
                                // backward playback

                                unsigned long swapareastart       = totalreadsamples;
                                unsigned long loopoffset          = GetPos() - loop.LoopStart;
                                unsigned long samplestoreadinloop = Min(samplestoread, loopoffset);
                                unsigned long reverseplaybackend  = GetPos() - samplestoreadinloop;

                                SetPos(reverseplaybackend);

                                // read samples for backward playback
                                do {
                                    readsamples          = Read(&pDst[totalreadsamples * this->FrameSize], samplestoreadinloop, pExternalDecompressionBuffer);
                                    samplestoreadinloop -= readsamples;
                                    samplestoread       -= readsamples;
                                    totalreadsamples    += readsamples;
                                } while (samplestoreadinloop && readsamples);

                                SetPos(reverseplaybackend); // pretend we really read backwards

                                if (reverseplaybackend == loop.LoopStart) {
                                    pPlaybackState->loop_cycles_left--;
                                    pPlaybackState->reverse = false;
                                }

                                // reverse the sample frames for backward playback
                                SwapMemoryArea(&pDst[swapareastart * this->FrameSize], (totalreadsamples - swapareastart) * this->FrameSize, this->FrameSize);
                            }
                        } while (samplestoread && readsamples);
                        break;
                    }

                    case loop_type_backward: { // TODO: not tested yet!
                        // forward playback (not entered the loop yet)
                        if (!pPlaybackState->reverse) do {
                            samplestoloopend  = loopEnd - GetPos();
                            readsamples       = Read(&pDst[totalreadsamples * this->FrameSize], Min(samplestoread, samplestoloopend), pExternalDecompressionBuffer);
                            samplestoread    -= readsamples;
                            totalreadsamples += readsamples;
                            if (readsamples == samplestoloopend) {
                                pPlaybackState->reverse = true;
                                break;
                            }
                        } while (samplestoread && readsamples);

                        if (!samplestoread) break;

                        // as we can only read forward from disk, we have to
                        // determine the end position within the loop first,
                        // read forward from that 'end' and finally after
                        // reading, swap all sample frames so it reflects
                        // backward playback

                        unsigned long swapareastart       = totalreadsamples;
                        unsigned long loopoffset          = GetPos() - loop.LoopStart;
                        unsigned long samplestoreadinloop = (this->LoopPlayCount) ? Min(samplestoread, pPlaybackState->loop_cycles_left * loop.LoopLength - loopoffset)
                                                                                  : samplestoread;
                        unsigned long reverseplaybackend  = loop.LoopStart + Abs((loopoffset - samplestoreadinloop) % loop.LoopLength);

                        SetPos(reverseplaybackend);

                        // read samples for backward playback
                        do {
                            // if not endless loop check if max. number of loop cycles have been passed
                            if (this->LoopPlayCount && !pPlaybackState->loop_cycles_left) break;
                            samplestoloopend     = loopEnd - GetPos();
                            readsamples          = Read(&pDst[totalreadsamples * this->FrameSize], Min(samplestoreadinloop, samplestoloopend), pExternalDecompressionBuffer);
                            samplestoreadinloop -= readsamples;
                            samplestoread       -= readsamples;
                            totalreadsamples    += readsamples;
                            if (readsamples == samplestoloopend) {
                                pPlaybackState->loop_cycles_left--;
                                SetPos(loop.LoopStart);
                            }
                        } while (samplestoreadinloop && readsamples);

                        SetPos(reverseplaybackend); // pretend we really read backwards

                        // reverse the sample frames for backward playback
                        SwapMemoryArea(&pDst[swapareastart * this->FrameSize], (totalreadsamples - swapareastart) * this->FrameSize, this->FrameSize);
                        break;
                    }

                    default: case loop_type_normal: {
                        do {
                            // if not endless loop check if max. number of loop cycles have been passed
                            if (this->LoopPlayCount && !pPlaybackState->loop_cycles_left) break;
                            samplestoloopend  = loopEnd - GetPos();
                            readsamples       = Read(&pDst[totalreadsamples * this->FrameSize], Min(samplestoread, samplestoloopend), pExternalDecompressionBuffer);
                            samplestoread    -= readsamples;
                            totalreadsamples += readsamples;
                            if (readsamples == samplestoloopend) {
                                pPlaybackState->loop_cycles_left--;
                                SetPos(loop.LoopStart);
                            }
                        } while (samplestoread && readsamples);