First working PCM decoder
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emeric
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/*
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* Copyright (C) 2026 Emeric Poupon
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*
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* This file is part of LMS.
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*
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* LMS is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* LMS is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with LMS. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include "PcmDecoder.hpp"
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#include <array>
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extern "C"
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{
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#include <libavcodec/avcodec.h>
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#include <libavcodec/packet.h>
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#include <libavformat/avformat.h>
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#include <libavutil/channel_layout.h>
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#include <libavutil/samplefmt.h>
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#include <libswresample/swresample.h>
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}
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#include "core/ILogger.hpp"
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#include "audio/Exception.hpp"
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#include "audio/IPcmDecoder.hpp"
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#include "Exception.hpp"
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namespace lms::audio
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{
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std::unique_ptr<IPcmDecoder> createPcmDecoder(const std::filesystem::path& filePath, const PcmDecoderParameters& parameters)
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{
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return std::make_unique<ffmpeg::PcmDecoder>(filePath, parameters);
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}
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} // namespace lms::audio
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namespace lms::audio::ffmpeg
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{
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namespace
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{
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::AVSampleFormat toAvSampleFormat(PcmDecodeSampleType type, bool planar)
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{
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switch (type)
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{
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case PcmDecodeSampleType::Signed16:
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return planar ? AV_SAMPLE_FMT_S16P : AV_SAMPLE_FMT_S16;
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case PcmDecodeSampleType::Signed32:
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return planar ? AV_SAMPLE_FMT_S32P : AV_SAMPLE_FMT_S32;
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case PcmDecodeSampleType::Float32:
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return planar ? AV_SAMPLE_FMT_FLTP : AV_SAMPLE_FMT_FLT;
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case PcmDecodeSampleType::Float64:
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return planar ? AV_SAMPLE_FMT_DBLP : AV_SAMPLE_FMT_DBL;
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}
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throw Exception("Unsupported PcmDecodeSampleType");
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}
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} // namespace
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PcmDecoder::PcmDecoder(const std::filesystem::path& filePath, const PcmDecoderParameters& parameters)
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: _parameters{ parameters }
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{
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if (_parameters.channelCount > AV_NUM_DATA_POINTERS)
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throw Exception("Channel count exceeds maximum supported channels");
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{
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::AVFormatContext* context{};
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int error{ ::avformat_open_input(&context, filePath.c_str(), nullptr, nullptr) };
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if (error < 0)
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{
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LMS_LOG(AUDIO, ERROR, "Cannot open " << filePath << ": " << utils::averrorToString(error));
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throw FFmpegException{ "Cannot open '" + filePath.string() + "'", error };
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}
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_context = AVFormatContextPtr{ context };
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}
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{
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int error{ ::avformat_find_stream_info(_context.get(), nullptr) };
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if (error < 0)
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{
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LMS_LOG(AUDIO, ERROR, "Cannot find stream information in " << filePath << ": " << utils::averrorToString(error));
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throw FFmpegException{ "Cannot find stream information in '" + filePath.string() + "'", error };
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}
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}
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const ::AVCodec* decoder{};
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_inputStreamIndex = ::av_find_best_stream(_context.get(),
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AVMEDIA_TYPE_AUDIO,
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-1, // auto
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-1, // auto
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&decoder,
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0);
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if (_inputStreamIndex < 0)
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{
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LMS_LOG(AUDIO, ERROR, "Cannot find best audio stream in " << filePath << ": " << utils::averrorToString(_inputStreamIndex));
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throw FFmpegException{ "Cannot find best audio stream in '" + filePath.string() + "'", _inputStreamIndex };
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}
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_decoderContext = AVCodecContextPtr{ ::avcodec_alloc_context3(decoder) };
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if (!_decoderContext)
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throw Exception{ "Cannot allocate decoder context" };
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{
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int error{ ::avcodec_parameters_to_context(_decoderContext.get(), _context->streams[_inputStreamIndex]->codecpar) };
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if (error < 0)
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throw FFmpegException{ "Cannot init decoder parameters", error };
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}
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{
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int error{ ::avcodec_open2(_decoderContext.get(), decoder, nullptr) };
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if (error < 0)
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throw FFmpegException("Cannot open decoder", error);
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}
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_decodedFrame = AVFramePtr{ av_frame_alloc() };
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if (!_decodedFrame)
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throw Exception{ "Cannot allocate decoded frame" };
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_inputPacket = AVPacketPtr{ ::av_packet_alloc() };
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if (!_inputPacket)
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throw Exception{ "Cannot allocate input packet" };
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// Resampler
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const ::AVSampleFormat outFmt{ toAvSampleFormat(_parameters.sampleType, _parameters.planar) };
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AVChannelLayout outLayout;
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::av_channel_layout_default(&outLayout, _parameters.channelCount);
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{
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::SwrContext* context{};
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::swr_alloc_set_opts2(
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&context, // existing context
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&outLayout, // out layout
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outFmt, // out format
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static_cast<int>(_parameters.sampleRate), // out rate
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&_decoderContext->ch_layout, // in layout
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_decoderContext->sample_fmt, // in format
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_decoderContext->sample_rate, // in rate
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0, // log offset
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nullptr);
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::av_channel_layout_uninit(&outLayout);
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if (!context)
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throw Exception{ "Cannot allocate resampler context" };
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_resampleContext = SwrContextPtr{ context };
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}
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{
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int error{ ::swr_init(_resampleContext.get()) };
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if (error < 0)
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throw FFmpegException{ "Cannot initialize resampler", error };
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}
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}
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PcmDecoder::~PcmDecoder() = default;
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std::size_t PcmDecoder::readSamples(std::span<WritableBuffer> outputChannelBuffers, std::size_t maxSamplesPerChannel)
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{
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assert(outputChannelBuffers.size() <= AV_NUM_DATA_POINTERS);
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if (_finished)
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return 0;
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if (_parameters.planar)
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{
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if (outputChannelBuffers.size() != _parameters.channelCount)
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throw Exception{ "Expected " + std::to_string(_parameters.channelCount) + " buffers for planar output" };
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}
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else
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{
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if (outputChannelBuffers.size() != 1)
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throw Exception{ "Expected a single buffer for interleaved output" };
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}
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std::array<uint8_t*, AV_NUM_DATA_POINTERS> outData{};
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for (size_t i = 0; i < outputChannelBuffers.size(); ++i)
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outData[i] = reinterpret_cast<uint8_t*>(outputChannelBuffers[i].data());
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while (true)
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{
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if (!_eof)
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feedDecoder();
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// Try to receive a decoded frame
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int recvErr{ ::avcodec_receive_frame(_decoderContext.get(), _decodedFrame.get()) };
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if (recvErr == AVERROR(EAGAIN))
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{
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if (!_eof)
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continue; // need more input
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_draining = true;
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}
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else if (recvErr == AVERROR_EOF)
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{
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_draining = true;
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}
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else if (recvErr < 0)
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{
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throw FFmpegException{ "avcodec_receive_frame failed", recvErr };
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}
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else
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{
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// Resample decoded audio
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const int outSampleCount{ ::swr_convert(
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_resampleContext.get(),
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outData.data(),
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static_cast<int>(maxSamplesPerChannel),
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(const uint8_t**)_decodedFrame->data,
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_decodedFrame->nb_samples) };
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::av_frame_unref(_decodedFrame.get());
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if (outSampleCount < 0)
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throw FFmpegException{ "swr_convert failed", outSampleCount };
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if (outSampleCount > 0)
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return static_cast<std::size_t>(outSampleCount);
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continue; // Rare but legal: frame produced no output (delay accumulation)
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}
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// Drain resampler once decoder is drained
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if (_draining)
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{
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const int outSampleCount = ::swr_convert(_resampleContext.get(),
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outData.data(),
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static_cast<int>(maxSamplesPerChannel),
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nullptr,
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0);
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if (outSampleCount < 0)
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throw FFmpegException{ "swr_convert (drain) failed", outSampleCount };
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if (outSampleCount > 0)
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return outSampleCount;
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_finished = true;
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return 0;
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}
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}
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return 0;
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}
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bool PcmDecoder::finished() const
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{
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return _finished;
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}
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void PcmDecoder::feedDecoder()
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{
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assert(!_eof);
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const int readError{ ::av_read_frame(_context.get(), _inputPacket.get()) };
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if (readError == AVERROR_EOF)
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{
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_eof = true;
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// flush decoder
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::avcodec_send_packet(_decoderContext.get(), nullptr);
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}
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else if (readError < 0)
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{
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throw FFmpegException{ "av_read_frame failed", readError };
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}
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else
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{
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if (_inputPacket->stream_index == _inputStreamIndex)
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{
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const int sendError{ ::avcodec_send_packet(_decoderContext.get(), _inputPacket.get()) };
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::av_packet_unref(_inputPacket.get());
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if (sendError < 0)
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throw FFmpegException{ "avcodec_send_packet failed", sendError };
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}
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else
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::av_packet_unref(_inputPacket.get());
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}
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}
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} // namespace lms::audio::ffmpeg
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