LibAudio: Implement a basic FLAC loader

This commit adds a loader for the FLAC audio codec, the Free Lossless
Audio codec by the Xiph.Org foundation. LibAudio will automatically
read and parse FLAC files, so users do not need to adjust.

This implementation is bare-bones and needs to be improved upon.
There are many bugs, verbatim subframes and any kind of seeking is
not supported. However, stereo files exported by libavcodec on
highest compression setting seem to work well.

5 files changed, 1069 insertions, 1 deletions

diff --git a/Userland/Libraries/LibAudio/CMakeLists.txt b/Userland/Libraries/LibAudio/CMakeLists.txt
index bde29e3aa7..8497c0cb5b 100644
--- a/Userland/Libraries/LibAudio/CMakeLists.txt
+++ b/Userland/Libraries/LibAudio/CMakeLists.txt
@@ -3,6 +3,7 @@ set(SOURCES
     ClientConnection.cpp
     Loader.cpp
     WavLoader.cpp
+    FlacLoader.cpp
     WavWriter.cpp
 )
 
diff --git a/Userland/Libraries/LibAudio/FlacLoader.cpp b/Userland/Libraries/LibAudio/FlacLoader.cpp
new file mode 100644
index 0000000000..bdc24906c5
--- /dev/null
+++ b/Userland/Libraries/LibAudio/FlacLoader.cpp
@@ -0,0 +1,837 @@
+/*
+ * Copyright (c) 2021, kleines Filmröllchen <malu.bertsch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "FlacLoader.h"
+#include "Buffer.h"
+#include <AK/BitStream.h>
+#include <AK/Debug.h>
+#include <AK/FlyString.h>
+#include <AK/Format.h>
+#include <AK/Stream.h>
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <LibCore/File.h>
+#include <LibCore/FileStream.h>
+#include <math.h>
+
+namespace Audio {
+
+FlacLoaderPlugin::FlacLoaderPlugin(const StringView& path)
+    : m_file(Core::File::construct(path))
+{
+    if (!m_file->open(Core::OpenMode::ReadOnly)) {
+        m_error_string = String::formatted("Can't open file: {}", m_file->error_string());
+        return;
+    }
+
+    m_valid = parse_header();
+    if (!m_valid)
+        return;
+
+    m_stream = make<FlacInputStream>(Core::InputFileStream(*m_file));
+    reset();
+
+    m_resampler = make<ResampleHelper<double>>(m_sample_rate, 44100);
+}
+
+FlacLoaderPlugin::FlacLoaderPlugin(const ByteBuffer& buffer)
+{
+    m_stream = make<FlacInputStream>(InputMemoryStream(buffer));
+    if (!m_stream) {
+        m_error_string = String::formatted("Can't open memory stream");
+        return;
+    }
+
+    m_valid = parse_header();
+    if (!m_valid)
+        return;
+    reset();
+
+    m_resampler = make<ResampleHelper<double>>(m_sample_rate, 44100);
+}
+
+bool FlacLoaderPlugin::sniff()
+{
+    return m_valid;
+}
+
+bool FlacLoaderPlugin::parse_header()
+{
+    Optional<Core::InputFileStream> fis;
+    bool ok = true;
+
+    InputBitStream bit_input = [&]() -> InputBitStream {
+        if (m_file) {
+            fis = Core::InputFileStream(*m_file);
+            return InputBitStream(*fis);
+        }
+        return InputBitStream(m_stream->get<InputMemoryStream>());
+    }();
+
+#define CHECK_OK(msg)                                                      \
+    do {                                                                   \
+        if (!ok) {                                                         \
+            m_error_string = String::formatted("Parsing failed: {}", msg); \
+            return {};                                                     \
+        }                                                                  \
+    } while (0)
+
+    // Magic number
+    u32 flac = bit_input.read_bits_big_endian(32);
+    m_data_start_location += 4;
+    ok = ok && flac == 0x664C6143; // "flaC"
+    CHECK_OK("FLAC magic number");
+
+    // Receive the streaminfo block
+    FlacRawMetadataBlock streaminfo = next_meta_block(bit_input);
+    // next_meta_block sets the error string if something goes wrong
+    ok = ok && m_error_string.is_empty();
+    CHECK_OK(m_error_string);
+    ok = ok && (streaminfo.type == FlacMetadataBlockType::STREAMINFO);
+    CHECK_OK("First block type");
+    InputMemoryStream streaminfo_data_memory(streaminfo.data.bytes());
+    InputBitStream streaminfo_data(streaminfo_data_memory);
+
+    // STREAMINFO block
+    m_min_block_size = streaminfo_data.read_bits_big_endian(16);
+    ok = ok && (m_min_block_size >= 16);
+    CHECK_OK("Minimum block size");
+    m_max_block_size = streaminfo_data.read_bits_big_endian(16);
+    ok = ok && (m_max_block_size >= 16);
+    CHECK_OK("Maximum block size");
+    m_min_frame_size = streaminfo_data.read_bits_big_endian(24);
+    m_max_frame_size = streaminfo_data.read_bits_big_endian(24);
+    m_sample_rate = streaminfo_data.read_bits_big_endian(20);
+    ok = ok && (m_sample_rate <= 655350);
+    CHECK_OK("Sample rate");
+    m_num_channels = streaminfo_data.read_bits_big_endian(3) + 1; // 0 ^= one channel
+
+    u8 bits_per_sample = streaminfo_data.read_bits_big_endian(5) + 1;
+    if (bits_per_sample == 8) {
+        // FIXME: Signed/Unsigned issues?
+        m_sample_format = PcmSampleFormat::Uint8;
+    } else if (bits_per_sample == 16) {
+        m_sample_format = PcmSampleFormat::Int16;
+    } else if (bits_per_sample == 24) {
+        m_sample_format = PcmSampleFormat::Int24;
+    } else if (bits_per_sample == 32) {
+        m_sample_format = PcmSampleFormat::Int32;
+    } else {
+        ok = false;
+        CHECK_OK("Sample bit depth");
+    }
+
+    m_total_samples = streaminfo_data.read_bits_big_endian(36);
+    // Parse checksum into a buffer first
+    ByteBuffer md5_checksum = ByteBuffer::create_uninitialized(128 / 8);
+    streaminfo_data.read(md5_checksum);
+    md5_checksum.bytes().copy_to({ m_md5_checksum, sizeof(m_md5_checksum) });
+
+    // Parse other blocks
+    // TODO: For a simple first implementation, all other blocks are skipped as allowed by the FLAC specification.
+    // Especially the SEEKTABLE block may become useful in a more sophisticated version.
+    [[maybe_unused]] u16 meta_blocks_parsed = 1;
+    [[maybe_unused]] u16 total_meta_blocks = meta_blocks_parsed;
+    FlacRawMetadataBlock block = streaminfo;
+    while (!block.is_last_block) {
+        block = next_meta_block(bit_input);
+        ++total_meta_blocks;
+        ok = ok && m_error_string.is_empty();
+        CHECK_OK(m_error_string);
+    }
+
+    if constexpr (AFLACLOADER_DEBUG) {
+        // HACK: u128 should be able to format itself
+        StringBuilder checksum_string;
+        for (unsigned int i = 0; i < md5_checksum.size(); ++i) {
+            checksum_string.appendff("{:0X}", md5_checksum[i]);
+        }
+        dbgln("Parsed FLAC header: blocksize {}-{}{}, framesize {}-{}, {}Hz, {}bit, {} channels, {} samples total ({:.2f}s), MD5 {}, data start at {:x} bytes, {} headers total (skipped {})", m_min_block_size, m_max_block_size, is_fixed_blocksize_stream() ? " (constant)" : "", m_min_frame_size, m_max_frame_size, m_sample_rate, pcm_bits_per_sample(m_sample_format), m_num_channels, m_total_samples, m_total_samples / static_cast<double>(m_sample_rate), checksum_string.to_string(), m_data_start_location, total_meta_blocks, total_meta_blocks - meta_blocks_parsed);
+    }
+
+    return true;
+#undef CHECK_OK
+}
+
+FlacRawMetadataBlock FlacLoaderPlugin::next_meta_block(InputBitStream& bit_input)
+{
+#define CHECK_IO_ERROR()                    \
+    do {                                    \
+        if (bit_input.handle_any_error()) { \
+            m_error_string = "Read error";  \
+            return FlacRawMetadataBlock {}; \
+        }                                   \
+    } while (0)
+
+    bool is_last_block = bit_input.read_bit_big_endian();
+    CHECK_IO_ERROR();
+    // The block type enum constants agree with the specification
+    FlacMetadataBlockType type = (FlacMetadataBlockType)bit_input.read_bits_big_endian(7);
+    CHECK_IO_ERROR();
+    if (type == FlacMetadataBlockType::INVALID) {
+        m_error_string = "Invalid metadata block";
+        return FlacRawMetadataBlock {};
+    }
+    m_data_start_location += 1;
+
+    u32 block_length = bit_input.read_bits_big_endian(24);
+    m_data_start_location += 3;
+    CHECK_IO_ERROR();
+    ByteBuffer block_data = ByteBuffer::create_uninitialized(block_length);
+    // Reads exactly the bytes necessary into the Bytes container
+    bit_input.read(block_data);
+    m_data_start_location += block_length;
+    CHECK_IO_ERROR();
+    return FlacRawMetadataBlock {
+        is_last_block,
+        type,
+        block_length,
+        block_data,
+    };
+
+#undef CHECK_IO_ERROR
+}
+
+void FlacLoaderPlugin::reset()
+{
+    seek(m_data_start_location);
+    m_current_frame.clear();
+}
+
+void FlacLoaderPlugin::seek(const int position)
+{
+    m_stream->seek(position);
+}
+
+// TODO implement these
+RefPtr<Buffer> FlacLoaderPlugin::get_more_samples([[maybe_unused]] size_t max_bytes_to_read_from_input)
+{
+    Vector<Frame> samples;
+    size_t remaining_samples = max_bytes_to_read_from_input;
+    while (remaining_samples > 0) {
+        if (!m_current_frame.has_value()) {
+            next_frame();
+            if (!m_error_string.is_empty()) {
+                dbgln("Frame parsing error: {}", m_error_string);
+                return nullptr;
+            }
+            // HACK: Test the start of the next subframe
+            // auto input = m_stream->bit_stream();
+            // u64 next = input.read_bits_big_endian(64);
+            // dbgln("After frame end: {}", next);
+        }
+        samples.append(m_current_frame_data.take_first());
+        if (m_current_frame_data.size() == 0) {
+            m_current_frame.clear();
+        }
+        --remaining_samples;
+    }
+
+    return Buffer::create_with_samples(move(samples));
+}
+
+void FlacLoaderPlugin::next_frame()
+{
+    bool ok = true;
+    InputBitStream bit_stream = m_stream->bit_stream();
+#define CHECK_OK(msg)                                                                                                      \
+    do {                                                                                                                   \
+        if (!ok) {                                                                                                         \
+            m_error_string = String::formatted("Frame parsing failed: {}", msg);                                           \
+            bit_stream.align_to_byte_boundary();                                                                           \
+            dbgln_if(AFLACLOADER_DEBUG, "Crash in FLAC loader: next bytes are {:x}", bit_stream.read_bits_big_endian(32)); \
+            return;                                                                                                        \
+        }                                                                                                                  \
+    } while (0)
+
+#define CHECK_ERROR_STRING                                             \
+    do {                                                               \
+        if (!m_error_string.is_null() && !m_error_string.is_empty()) { \
+            ok = false;                                                \
+            CHECK_OK(m_error_string);                                  \
+        }                                                              \
+    } while (0)
+
+    // TODO: Check the CRC-16 checksum (and others) by keeping track of read data
+
+    // FLAC frame sync code starts header
+    u16 sync_code = bit_stream.read_bits_big_endian(14);
+    ok = ok && (sync_code == 0b11111111111110);
+    CHECK_OK("Sync code");
+    bool reserved_bit = bit_stream.read_bit_big_endian();
+    ok = ok && (reserved_bit == 0);
+    CHECK_OK("Reserved frame header bit");
+    [[maybe_unused]] bool blocking_strategy = bit_stream.read_bit_big_endian();
+
+    u32 sample_count = convert_sample_count_code(bit_stream.read_bits_big_endian(4));
+    CHECK_ERROR_STRING;
+
+    u32 frame_sample_rate = convert_sample_rate_code(bit_stream.read_bits_big_endian(4));
+    CHECK_ERROR_STRING;
+
+    u8 channel_type_num = bit_stream.read_bits_big_endian(4);
+    if (channel_type_num >= 0b1011) {
+        ok = false;
+        CHECK_OK("Channel assignment");
+    }
+    FlacFrameChannelType channel_type = (FlacFrameChannelType)channel_type_num;
+
+    PcmSampleFormat bit_depth = convert_bit_depth_code(bit_stream.read_bits_big_endian(3));
+    CHECK_ERROR_STRING;
+
+    reserved_bit = bit_stream.read_bit_big_endian();
+    ok = ok && (reserved_bit == 0);
+    CHECK_OK("Reserved frame header end bit");
+
+    // FIXME: sample number can be 8-56 bits, frame number can be 8-48 bits
+    m_current_sample_or_frame = read_utf8_char(bit_stream);
+
+    // Conditional header variables
+    if (sample_count == FLAC_BLOCKSIZE_AT_END_OF_HEADER_8) {
+        sample_count = bit_stream.read_bits(8) + 1;
+    } else if (sample_count == FLAC_BLOCKSIZE_AT_END_OF_HEADER_16) {
+        sample_count = bit_stream.read_bits(16) + 1;
+    }
+
+    if (frame_sample_rate == FLAC_SAMPLERATE_AT_END_OF_HEADER_8) {
+        frame_sample_rate = bit_stream.read_bits(8) * 1000;
+    } else if (frame_sample_rate == FLAC_SAMPLERATE_AT_END_OF_HEADER_16) {
+        frame_sample_rate = bit_stream.read_bits(16);
+    } else if (frame_sample_rate == FLAC_SAMPLERATE_AT_END_OF_HEADER_16X10) {
+        frame_sample_rate = bit_stream.read_bits(16) * 10;
+    }
+
+    // TODO: check header checksum, see above
+    [[maybe_unused]] u8 checksum = bit_stream.read_bits(8);
+
+    dbgln_if(AFLACLOADER_DEBUG, "Frame: {} samples, {}bit {}Hz, channeltype {:x}, {} number {}, header checksum {}", sample_count, pcm_bits_per_sample(bit_depth), frame_sample_rate, channel_type_num, blocking_strategy ? "sample" : "frame", m_current_sample_or_frame, checksum);
+
+    m_current_frame = FlacFrameHeader {
+        sample_count,
+        frame_sample_rate,
+        channel_type,
+        bit_depth,
+    };
+
+    u8 subframe_count = frame_channel_type_to_channel_count(channel_type);
+    Vector<Vector<i32>> current_subframes;
+    current_subframes.ensure_capacity(subframe_count);
+
+    for (u8 i = 0; i < subframe_count; ++i) {
+        FlacSubframeHeader new_subframe = next_subframe_header(bit_stream, i);
+        CHECK_ERROR_STRING;
+        Vector<i32> subframe_samples = parse_subframe(new_subframe, bit_stream);
+        // HACK: Test the start of the next subframe
+        CHECK_ERROR_STRING;
+        current_subframes.append(move(subframe_samples));
+    }
+
+    bit_stream.align_to_byte_boundary();
+
+    // TODO: check checksum, see above
+    [[maybe_unused]] u16 footer_checksum = bit_stream.read_bits_big_endian(16);
+
+    Vector<i32> left, right;
+
+    switch (channel_type) {
+    case FlacFrameChannelType::Mono:
+        left = right = current_subframes[0];
+        break;
+    case FlacFrameChannelType::Stereo:
+    // TODO mix together surround channels on each side?
+    case FlacFrameChannelType::StereoCenter:
+    case FlacFrameChannelType::Surround4p0:
+    case FlacFrameChannelType::Surround5p0:
+    case FlacFrameChannelType::Surround5p1:
+    case FlacFrameChannelType::Surround6p1:
+    case FlacFrameChannelType::Surround7p1:
+        left = current_subframes[0];
+        right = current_subframes[1];
+        break;
+    case FlacFrameChannelType::LeftSideStereo:
+        // channels are left (0) and side (1)
+        left = current_subframes[0];
+        right.ensure_capacity(left.size());
+        for (size_t i = 0; i < left.size(); ++i) {
+            // right = left - side
+            right.unchecked_append(left[i] - current_subframes[1][i]);
+        }
+        break;
+    case FlacFrameChannelType::RightSideStereo:
+        // channels are side (0) and right (1)
+        right = current_subframes[1];
+        left.ensure_capacity(right.size());
+        for (size_t i = 0; i < right.size(); ++i) {
+            // left = right + side
+            left.unchecked_append(right[i] + current_subframes[0][i]);
+        }
+        break;
+    case FlacFrameChannelType::MidSideStereo:
+        // channels are mid (0) and side (1)
+        left.ensure_capacity(current_subframes[0].size());
+        right.ensure_capacity(current_subframes[0].size());
+        for (size_t i = 0; i < current_subframes[0].size(); ++i) {
+            i64 mid = current_subframes[0][i];
+            i64 side = current_subframes[1][i];
+            mid *= 2;
+            // prevent integer division errors
+            left.unchecked_append(static_cast<i32>((mid + side) / 2));
+            right.unchecked_append(static_cast<i32>((mid - side) / 2));
+        }
+        break;
+    }
+
+    VERIFY(left.size() == right.size());
+
+    // TODO: find the correct rescale offset
+    double sample_rescale = static_cast<double>(1 << pcm_bits_per_sample(m_current_frame->bit_depth));
+    dbgln_if(AFLACLOADER_DEBUG, "Sample rescaled from {} bits: factor {:.1f}", pcm_bits_per_sample(m_current_frame->bit_depth), sample_rescale);
+
+    m_current_frame_data.clear_with_capacity();
+    m_current_frame_data.ensure_capacity(left.size());
+    // zip together channels
+    for (size_t i = 0; i < left.size(); ++i) {
+        Frame frame = { left[i] / sample_rescale, right[i] / sample_rescale };
+        m_current_frame_data.unchecked_append(frame);
+    }
+
+#undef CHECK_OK
+#undef CHECK_ERROR_STRING
+}
+
+u32 FlacLoaderPlugin::convert_sample_count_code(u8 sample_count_code)
+{
+    // single codes
+    switch (sample_count_code) {
+    case 0:
+        m_error_string = "Reserved block size";
+        return 0;
+    case 1:
+        return 192;
+    case 6:
+        return FLAC_BLOCKSIZE_AT_END_OF_HEADER_8;
+    case 7:
+        return FLAC_BLOCKSIZE_AT_END_OF_HEADER_16;
+    }
+    if (sample_count_code >= 2 && sample_count_code <= 5) {
+        return 576 * pow(2, (sample_count_code - 2));
+    }
+    return 256 * pow(2, (sample_count_code - 8));
+}
+
+u32 FlacLoaderPlugin::convert_sample_rate_code(u8 sample_rate_code)
+{
+    switch (sample_rate_code) {
+    case 0:
+        return m_sample_rate;
+    case 1:
+        return 88200;
+    case 2:
+        return 176400;
+    case 3:
+        return 192000;
+    case 4:
+        return 8000;
+    case 5:
+        return 16000;
+    case 6:
+        return 22050;
+    case 7:
+        return 24000;
+    case 8:
+        return 32000;
+    case 9:
+        return 44100;
+    case 10:
+        return 48000;
+    case 11:
+        return 96000;
+    case 12:
+        return FLAC_SAMPLERATE_AT_END_OF_HEADER_8;
+    case 13:
+        return FLAC_SAMPLERATE_AT_END_OF_HEADER_16;
+    case 14:
+        return FLAC_SAMPLERATE_AT_END_OF_HEADER_16X10;
+    default:
+        m_error_string = "Invalid sample rate code";
+        return 0;
+    }
+}
+
+PcmSampleFormat FlacLoaderPlugin::convert_bit_depth_code(u8 bit_depth_code)
+{
+    switch (bit_depth_code) {
+    case 0:
+        return m_sample_format;
+    case 1:
+        return PcmSampleFormat::Uint8;
+    case 4:
+        return PcmSampleFormat::Int16;
+    case 6:
+        return PcmSampleFormat::Int24;
+    case 3:
+    case 7:
+        m_error_string = "Reserved sample size";
+        return PcmSampleFormat::Float64;
+    default:
+        m_error_string = String::formatted("Unsupported sample size {}", bit_depth_code);
+        return PcmSampleFormat::Float64;
+    }
+}
+
+u8 frame_channel_type_to_channel_count(FlacFrameChannelType channel_type)
+{
+    if (channel_type <= 7)
+        return channel_type + 1;
+    return 2;
+}
+
+FlacSubframeHeader FlacLoaderPlugin::next_subframe_header(InputBitStream& bit_stream, u8 channel_index)
+{
+    u8 bits_per_sample = pcm_bits_per_sample(m_current_frame->bit_depth);
+
+    // For inter-channel correlation, the side channel needs an extra bit for its samples
+    switch (m_current_frame->channels) {
+    case LeftSideStereo:
+    case MidSideStereo:
+        if (channel_index == 1) {
+            ++bits_per_sample;
+        }
+        break;
+    case RightSideStereo:
+        if (channel_index == 0) {
+            ++bits_per_sample;
+        }
+        break;
+    // "normal" channel types
+    default:
+        break;
+    }
+
+    // zero-bit padding
+    bit_stream.read_bit_big_endian();
+
+    // subframe type (encoding)
+    u8 subframe_code = bit_stream.read_bits_big_endian(6);
+    if ((subframe_code >= 0b000010 && subframe_code <= 0b000111) || (subframe_code > 0b001100 && subframe_code < 0b100000)) {
+        m_error_string = "Subframe type";
+        return {};
+    }
+
+    FlacSubframeType subframe_type;
+    u8 order = 0;
+    //LPC has the highest bit set
+    if ((subframe_code & 0b100000) > 0) {
+        subframe_type = FlacSubframeType::LPC;
+        order = (subframe_code & 0b011111) + 1;
+    } else if ((subframe_code & 0b001000) > 0) {
+        // Fixed has the third-highest bit set
+        subframe_type = FlacSubframeType::Fixed;
+        order = (subframe_code & 0b000111);
+    } else {
+        subframe_type = (FlacSubframeType)subframe_code;
+    }
+
+    // wasted bits per sample (unary encoding)
+    bool has_wasted_bits = bit_stream.read_bit_big_endian();
+    u8 k = 0;
+    if (has_wasted_bits) {
+        bool current_k_bit = 0;
+        u8 k = 0;
+        do {
+            current_k_bit = bit_stream.read_bit_big_endian();
+            ++k;
+        } while (current_k_bit != 1);
+    }
+
+    return FlacSubframeHeader {
+        subframe_type,
+        order,
+        k,
+        bits_per_sample
+    };
+}
+
+Vector<i32> FlacLoaderPlugin::parse_subframe(FlacSubframeHeader& subframe_header, InputBitStream& bit_input)
+{
+    Vector<i32> samples;
+
+    switch (subframe_header.type) {
+    case FlacSubframeType::Constant: {
+        u64 constant_value = bit_input.read_bits_big_endian(subframe_header.bits_per_sample - subframe_header.wasted_bits_per_sample);
+        dbgln_if(AFLACLOADER_DEBUG, "Constant subframe: {}", constant_value);
+
+        samples.ensure_capacity(m_current_frame->sample_count);
+        for (u32 i = 0; i < m_current_frame->sample_count; ++i) {
+            samples.unchecked_append(constant_value);
+        }
+        break;
+    }
+    case FlacSubframeType::Fixed: {
+        dbgln_if(AFLACLOADER_DEBUG, "Fixed LPC subframe order {}", subframe_header.order);
+        samples = decode_fixed_lpc(subframe_header, bit_input);
+        break;
+    }
+    case FlacSubframeType::Verbatim: {
+        dbgln_if(AFLACLOADER_DEBUG, "Verbatim subframe");
+        samples = decode_verbatim(subframe_header, bit_input);
+        break;
+    }
+    case FlacSubframeType::LPC: {
+        dbgln_if(AFLACLOADER_DEBUG, "Custom LPC subframe order {}", subframe_header.order);
+        samples = decode_custom_lpc(subframe_header, bit_input);
+        break;
+    }
+    default:
+        m_error_string = "Unhandled FLAC subframe type";
+        return {};
+    }
+    if (!m_error_string.is_empty()) {
+        return {};
+    }
+
+    for (size_t i = 0; i < samples.size(); ++i) {
+        samples[i] <<= subframe_header.wasted_bits_per_sample;
+    }
+
+    ResampleHelper<i32> resampler(m_current_frame->sample_rate, m_sample_rate);
+    return resampler.resample(samples);
+}
+
+// Decode a subframe that isn't actually encoded
+Vector<i32> FlacLoaderPlugin::decode_verbatim([[maybe_unused]] FlacSubframeHeader& subframe, [[maybe_unused]] InputBitStream& bit_input)
+{
+    TODO();
+}
+
+// Decode a subframe encoded with a custom linear predictor coding, i.e. the subframe provides the polynomial order and coefficients
+Vector<i32> FlacLoaderPlugin::decode_custom_lpc(FlacSubframeHeader& subframe, InputBitStream& bit_input)
+{
+    Vector<i32> decoded;
+    decoded.ensure_capacity(m_current_frame->sample_count);
+
+    // warm-up samples
+    for (auto i = 0; i < subframe.order; ++i) {
+        decoded.unchecked_append(sign_extend(bit_input.read_bits_big_endian(subframe.bits_per_sample - subframe.wasted_bits_per_sample), subframe.bits_per_sample));
+        decoded[i] <<= subframe.wasted_bits_per_sample;
+    }
+
+    // precision of the coefficients
+    u8 lpc_precision = bit_input.read_bits_big_endian(4);
+    if (lpc_precision == 0b1111) {
+        m_error_string = "Invalid linear predictor coefficient precision";
+        return {};
+    }
+    lpc_precision += 1;
+
+    // shift needed on the data (signed!)
+    i8 lpc_shift = sign_extend(bit_input.read_bits_big_endian(5), 5);
+
+    Vector<i32> coefficients;
+    coefficients.ensure_capacity(subframe.order);
+    // read coefficients
+    for (auto i = 0; i < subframe.order; ++i) {
+        u32 raw_coefficient = bit_input.read_bits_big_endian(lpc_precision);
+        i32 coefficient = sign_extend(raw_coefficient, lpc_precision);
+        coefficients.unchecked_append(coefficient);
+    }
+
+    if constexpr (AFLACLOADER_DEBUG) {
+        StringBuilder coefficients_formatted;
+        coefficients_formatted.append("[ ");
+        for (auto coeff : coefficients) {
+            coefficients_formatted.append(String::formatted("{}, ", coeff));
+        }
+        coefficients_formatted.append("]");
+        dbgln("{}-bit {} shift coefficients: {}", lpc_precision, lpc_shift, coefficients_formatted.to_string());
+    }
+
+    // decode residual
+    // FIXME: This order may be incorrect, the LPC is applied to the residual, probably leading to incorrect results.
+    decoded = decode_residual(decoded, subframe, bit_input);
+
+    // approximate the waveform with the predictor
+    for (size_t i = subframe.order; i < m_current_frame->sample_count; ++i) {
+        i32 sample = 0;
+        for (size_t t = 0; t < subframe.order; ++t) {
+            sample += coefficients[t] * decoded[i - t - 1];
+        }
+        decoded[i] += sample >> lpc_shift;
+    }
+
+    return decoded;
+}
+
+// Decode a subframe encoded with one of the fixed linear predictor codings
+Vector<i32> FlacLoaderPlugin::decode_fixed_lpc(FlacSubframeHeader& subframe, InputBitStream& bit_input)
+{
+    Vector<i32> decoded;
+    decoded.ensure_capacity(m_current_frame->sample_count);
+
+    // warm-up samples
+    for (auto i = 0; i < subframe.order; ++i) {
+        decoded.unchecked_append(bit_input.read_bits_big_endian(subframe.bits_per_sample - subframe.wasted_bits_per_sample));
+    }
+
+    decode_residual(decoded, subframe, bit_input);
+    if (!m_error_string.is_empty())
+        return {};
+    dbgln_if(AFLACLOADER_DEBUG, "decoded length {}, {} order predictor", decoded.size(), subframe.order);
+
+    switch (subframe.order) {
+    case 0:
+        // s_0(t) = 0
+        for (auto i = subframe.order; i < m_current_frame->sample_count; ++i)
+            decoded[i] += 0;
+        break;
+    case 1:
+        // s_1(t) = s(t-1)
+        for (auto i = subframe.order; i < m_current_frame->sample_count; ++i)
+            decoded[i] += decoded[i - 1];
+        break;
+    case 2:
+        // s_2(t) = 2s(t-1) - s(t-2)
+        for (auto i = subframe.order; i < m_current_frame->sample_count; ++i)
+            decoded[i] += 2 * decoded[i - 1] - decoded[i - 2];
+        break;
+    case 3:
+        // s_3(t) = 3s(t-1) - 3s(t-2) + s(t-3)
+        for (auto i = subframe.order; i < m_current_frame->sample_count; ++i)
+            decoded[i] += 3 * decoded[i - 1] - 3 * decoded[i - 2] + decoded[i - 3];
+        break;
+    case 4:
+        // s_4(t) = 4s(t-1) - 6s(t-2) + 4s(t-3) - s(t-4)
+        for (auto i = subframe.order; i < m_current_frame->sample_count; ++i)
+            decoded[i] += 4 * decoded[i - 1] - 6 * decoded[i - 2] + 4 * decoded[i - 3] - decoded[i - 4];
+        break;
+    default:
+        m_error_string = String::formatted("Unrecognized predictor order {}", subframe.order);
+        break;
+    }
+    return decoded;
+}
+
+// Decode the residual, the "error" between the function approximation and the actual audio data
+Vector<i32> FlacLoaderPlugin::decode_residual(Vector<i32>& decoded, FlacSubframeHeader& subframe, InputBitStream& bit_input)
+{
+    u8 residual_mode = bit_input.read_bits_big_endian(2);
+    u8 partition_order = bit_input.read_bits_big_endian(4);
+    u32 partitions = 1 << partition_order;
+
+    if (residual_mode == FlacResidualMode::Rice4Bit) {
+        // decode a single Rice partition with four bits for the order k
+        for (u32 i = 0; i < partitions; ++i) {
+            auto rice_partition = decode_rice_partition(4, partitions, i, subframe, bit_input);
+            decoded.extend(move(rice_partition));
+        }
+    } else if (residual_mode == FlacResidualMode::Rice5Bit) {
+        // five bits equivalent
+        for (u32 i = 0; i < partitions; ++i) {
+            auto rice_partition = decode_rice_partition(5, partitions, i, subframe, bit_input);
+            decoded.extend(move(rice_partition));
+        }
+    } else {
+        m_error_string = "Reserved residual coding method";
+        return {};
+    }
+
+    return decoded;
+}
+
+// Decode a single Rice partition as part of the residual, every partition can have its own Rice parameter k
+ALWAYS_INLINE Vector<i32> FlacLoaderPlugin::decode_rice_partition(u8 partition_type, u32 partitions, u32 partition_index, FlacSubframeHeader& subframe, InputBitStream& bit_input)
+{
+    // Rice parameter / Exp-Golomb order
+    u8 k = bit_input.read_bits_big_endian(partition_type);
+
+    u32 residual_sample_count;
+    if (partitions == 0)
+        residual_sample_count = m_current_frame->sample_count - subframe.order;
+    else
+        residual_sample_count = m_current_frame->sample_count / partitions;
+    if (partition_index == 0)
+        residual_sample_count -= subframe.order;
+
+    Vector<i32> rice_partition;
+    rice_partition.resize(residual_sample_count);
+
+    // escape code for unencoded binary partition
+    if (k == (1 << partition_type) - 1) {
+        u8 unencoded_bps = bit_input.read_bits_big_endian(5);
+        for (u32 r = 0; r < residual_sample_count; ++r) {
+            rice_partition[r] = bit_input.read_bits_big_endian(unencoded_bps);
+        }
+    } else {
+        for (u32 r = 0; r < residual_sample_count; ++r) {
+            rice_partition[r] = decode_unsigned_exp_golomb(k, bit_input);
+        }
+    }
+
+    return rice_partition;
+}
+
+// Decode a single number encoded with Rice/Exponential-Golomb encoding (the unsigned variant)
+ALWAYS_INLINE i32 decode_unsigned_exp_golomb(u8 k, InputBitStream& bit_input)
+{
+    u8 q = 0;
+    while (bit_input.read_bit_big_endian() == 0)
+        ++q;
+
+    // least significant bits (remainder)
+    u32 rem = bit_input.read_bits_big_endian(k);
+    u32 value = (u32)(q << k | rem);
+
+    return rice_to_signed(value);
+}
+
+u64 read_utf8_char(InputStream& input)
+{
+    u64 character;
+    ByteBuffer single_byte_buffer = ByteBuffer::create_uninitialized(1);
+    input.read(single_byte_buffer);
+    u8 start_byte = single_byte_buffer[0];
+    // Signal byte is zero: ASCII character
+    if ((start_byte & 0b10000000) == 0) {
+        return start_byte;
+    } else if ((start_byte & 0b11000000) == 0b10000000) {
+        // illegal continuation byte
+        return 0;
+    }
+    // This algorithm is too good and supports the theoretical max 0xFF start byte
+    u8 length = 1;
+    while (((start_byte << length) & 0b10000000) == 0b10000000)
+        ++length;
+    u8 bits_from_start_byte = 8 - (length + 1);
+    u8 start_byte_bitmask = pow(2, bits_from_start_byte) - 1;
+    character = start_byte_bitmask & start_byte;
+    for (u8 i = length; i > 0; --i) {
+        input.read(single_byte_buffer);
+        u8 current_byte = single_byte_buffer[0];
+        character = (character << 6) | (current_byte & 0b00111111);
+    }
+    return character;
+}
+
+i64 sign_extend(u32 n, u8 size)
+{
+    // negative
+    if ((n & (1 << (size - 1))) > 0) {
+        return static_cast<i64>(n | (0xffffffff << size));
+    }
+    // positive
+    return n;
+}
+
+i32 rice_to_signed(u32 x)
+{
+    // positive numbers are even, negative numbers are odd
+    // bitmask for conditionally inverting the entire number, thereby "negating" it
+    i32 sign = -(x & 1);
+    // copies the sign's sign onto the actual magnitude of x
+    return (i32)(sign ^ (x >> 1));
+}
+
+}
diff --git a/Userland/Libraries/LibAudio/FlacLoader.h b/Userland/Libraries/LibAudio/FlacLoader.h
new file mode 100644
index 0000000000..693146c5c7
--- /dev/null
+++ b/Userland/Libraries/LibAudio/FlacLoader.h
@@ -0,0 +1,131 @@
+/*
+ * Copyright (c) 2021, kleines Filmröllchen <malu.bertsch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Buffer.h"
+#include "FlacTypes.h"
+#include "Loader.h"
+#include <AK/BitStream.h>
+#include <AK/Stream.h>
+#include <AK/Types.h>
+#include <AK/Variant.h>
+#include <LibCore/FileStream.h>
+
+namespace Audio {
+
+class FlacInputStream : public Variant<Core::InputFileStream, InputMemoryStream> {
+
+public:
+    using Variant<Core::InputFileStream, InputMemoryStream>::Variant;
+
+    void seek(size_t pos)
+    {
+        this->visit(
+            [&](auto& stream) {
+                stream.seek(pos);
+            });
+    }
+
+    InputBitStream bit_stream()
+    {
+        return this->visit(
+            [&](auto& stream) {
+                return InputBitStream(stream);
+            });
+    }
+};
+
+ALWAYS_INLINE u8 frame_channel_type_to_channel_count(FlacFrameChannelType channel_type);
+// Sign-extend an arbitrary-size signed number to 64 bit signed
+ALWAYS_INLINE i64 sign_extend(u32 n, u8 size);
+// Decodes the sign representation method used in Rice coding.
+// Numbers alternate between positive and negative: 0, 1, -1, 2, -2, 3, -3, 4, -4, 5, -5, ...
+ALWAYS_INLINE i32 rice_to_signed(u32 x);
+
+// decoders
+// read a UTF-8 encoded number, even if it is not a valid codepoint
+ALWAYS_INLINE u64 read_utf8_char(InputStream& input);
+// decode a single number encoded with exponential golomb encoding of the specified order
+ALWAYS_INLINE i32 decode_unsigned_exp_golomb(u8 order, InputBitStream& bit_input);
+
+class FlacLoaderPlugin : public LoaderPlugin {
+public:
+    FlacLoaderPlugin(const StringView& path);
+    FlacLoaderPlugin(const ByteBuffer& buffer);
+
+    virtual bool sniff() override;
+
+    virtual bool has_error() override { return !m_error_string.is_null(); }
+    virtual const String& error_string() override { return m_error_string; }
+
+    virtual RefPtr<Buffer> get_more_samples(size_t max_bytes_to_read_from_input = 128 * KiB) override;
+
+    virtual void reset() override;
+    virtual void seek(const int position) override;
+
+    // FIXME
+    virtual int loaded_samples() override { return 0; }
+    virtual int total_samples() override { return m_total_samples; }
+    virtual u32 sample_rate() override { return m_sample_rate; }
+    virtual u16 num_channels() override { return m_num_channels; }
+    virtual PcmSampleFormat pcm_format() override { return m_sample_format; }
+    virtual RefPtr<Core::File> file() override { return m_file; }
+
+    bool is_fixed_blocksize_stream() const { return m_min_block_size == m_max_block_size; }
+    bool sample_count_unknown() const { return m_total_samples == 0; }
+
+private:
+    bool parse_header();
+    // Either returns the metadata block or sets error message.
+    // Additionally, increments m_data_start_location past the read meta block.
+    FlacRawMetadataBlock next_meta_block(InputBitStream& bit_input);
+    // Fetches and sets the next FLAC frame
+    void next_frame();
+    // Helper of next_frame that fetches a sub frame's header
+    FlacSubframeHeader next_subframe_header(InputBitStream& bit_input, u8 channel_index);
+    // Helper of next_frame that decompresses a subframe
+    Vector<i32> parse_subframe(FlacSubframeHeader& subframe_header, InputBitStream& bit_input);
+    // Subframe-internal data decoders (heavy lifting)
+    Vector<i32> decode_fixed_lpc(FlacSubframeHeader& subframe, InputBitStream& bit_input);
+    Vector<i32> decode_verbatim(FlacSubframeHeader& subframe, InputBitStream& bit_input);
+    Vector<i32> decode_custom_lpc(FlacSubframeHeader& subframe, InputBitStream& bit_input);
+    Vector<i32> decode_residual(Vector<i32>& decoded, FlacSubframeHeader& subframe, InputBitStream& bit_input);
+    // decode a single rice partition that has its own rice parameter
+    ALWAYS_INLINE Vector<i32> decode_rice_partition(u8 partition_type, u32 partitions, u32 partition_index, FlacSubframeHeader& subframe, InputBitStream& bit_input);
+
+    // Converters for special coding used in frame headers
+    ALWAYS_INLINE u32 convert_sample_count_code(u8 sample_count_code);
+    ALWAYS_INLINE u32 convert_sample_rate_code(u8 sample_rate_code);
+    ALWAYS_INLINE PcmSampleFormat convert_bit_depth_code(u8 bit_depth_code);
+
+    bool m_valid { false };
+    RefPtr<Core::File> m_file;
+    String m_error_string;
+    OwnPtr<ResampleHelper<double>> m_resampler;
+
+    // Data obtained directly from the FLAC metadata: many values have specific bit counts
+    u32 m_sample_rate { 0 };         // 20 bit
+    u8 m_num_channels { 0 };         // 3 bit
+    PcmSampleFormat m_sample_format; // 5 bits for the integer bit depth
+    // Blocks are units of decoded audio data
+    u16 m_min_block_size { 0 };
+    u16 m_max_block_size { 0 };
+    // Frames are units of encoded audio data, both of these are 24-bit
+    u32 m_min_frame_size { 0 }; //24 bit
+    u32 m_max_frame_size { 0 }; // 24 bit
+    u64 m_total_samples { 0 };  // 36 bit
+    u8 m_md5_checksum[128 / 8]; // 128 bit (!)
+
+    // keep track of the start of the data in the FLAC stream to seek back more easily
+    u64 m_data_start_location { 0 };
+    OwnPtr<FlacInputStream> m_stream;
+    Optional<FlacFrameHeader> m_current_frame;
+    Vector<Frame> m_current_frame_data;
+    u64 m_current_sample_or_frame { 0 };
+};
+
+}
diff --git a/Userland/Libraries/LibAudio/FlacTypes.h b/Userland/Libraries/LibAudio/FlacTypes.h
new file mode 100644
index 0000000000..b67602e879
--- /dev/null
+++ b/Userland/Libraries/LibAudio/FlacTypes.h
@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2021, kleines Filmröllchen <malu.bertsch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Buffer.h"
+#include <AK/ByteBuffer.h>
+#include <AK/Types.h>
+#include <AK/Variant.h>
+
+namespace Audio {
+
+// Temporary constants for header blocksize/sample rate spec
+#define FLAC_BLOCKSIZE_AT_END_OF_HEADER_8 0xffffffff
+#define FLAC_BLOCKSIZE_AT_END_OF_HEADER_16 0xfffffffe
+#define FLAC_SAMPLERATE_AT_END_OF_HEADER_8 0xffffffff
+#define FLAC_SAMPLERATE_AT_END_OF_HEADER_16 0xfffffffe
+#define FLAC_SAMPLERATE_AT_END_OF_HEADER_16X10 0xfffffffd
+
+// Metadata block type, 7 bits.
+enum FlacMetadataBlockType : u8 {
+    STREAMINFO = 0,     // Important data about the audio format
+    PADDING = 1,        // Non-data block to be ignored
+    APPLICATION = 2,    // Ignored
+    SEEKTABLE = 3,      // Seeking info, maybe to be used later
+    VORBIS_COMMENT = 4, // Ignored
+    CUESHEET = 5,       // Ignored
+    PICTURE = 6,        // Ignored
+    INVALID = 127,      // Error
+};
+
+// follows FLAC codes
+enum FlacFrameChannelType : u8 {
+    Mono = 0,
+    Stereo = 1,
+    StereoCenter = 2,    // left, right, center
+    Surround4p0 = 3,     // front left/right, back left/right
+    Surround5p0 = 4,     // front left/right, center, back left/right
+    Surround5p1 = 5,     // front left/right, center, LFE, back left/right
+    Surround6p1 = 6,     // front left/right, center, LFE, back center, side left/right
+    Surround7p1 = 7,     // front left/right, center, LFE, back left/right, side left/right
+    LeftSideStereo = 8,  // channel coupling: left and difference
+    RightSideStereo = 9, // channel coupling: difference and right
+    MidSideStereo = 10,  // channel coupling: center and difference
+    // others are reserved
+};
+
+// follows FLAC codes
+enum FlacSubframeType : u8 {
+    Constant = 0,
+    Verbatim = 1,
+    Fixed = 0b001000,
+    LPC = 0b100000,
+    // others are reserved
+};
+
+// follows FLAC codes
+enum FlacResidualMode : u8 {
+    Rice4Bit = 0,
+    Rice5Bit = 1,
+};
+
+// Simple wrapper around any kind of metadata block
+struct FlacRawMetadataBlock {
+    bool is_last_block;
+    FlacMetadataBlockType type;
+    u32 length; // 24 bits
+    ByteBuffer data;
+};
+
+// An abstract, parsed and validated FLAC frame
+struct FlacFrameHeader {
+    u32 sample_count;
+    u32 sample_rate;
+    FlacFrameChannelType channels;
+    PcmSampleFormat bit_depth;
+};
+
+struct FlacSubframeHeader {
+    FlacSubframeType type;
+    // order for fixed and LPC subframes
+    u8 order;
+    u8 wasted_bits_per_sample;
+    u8 bits_per_sample;
+};
+
+}
diff --git a/Userland/Libraries/LibAudio/Loader.cpp b/Userland/Libraries/LibAudio/Loader.cpp
index 07a45a8d29..f3b54f8ffa 100644
--- a/Userland/Libraries/LibAudio/Loader.cpp
+++ b/Userland/Libraries/LibAudio/Loader.cpp
@@ -1,9 +1,10 @@
 /*
- * Copyright (c) 2018-2020, the SerenityOS developers.
+ * Copyright (c) 2018-2021, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibAudio/FlacLoader.h>
 #include <LibAudio/WavLoader.h>
 
 namespace Audio {
@@ -13,6 +14,9 @@ Loader::Loader(const StringView& path)
     m_plugin = make<WavLoaderPlugin>(path);
     if (m_plugin->sniff())
         return;
+    m_plugin = make<FlacLoaderPlugin>(path);
+    if (m_plugin->sniff())
+        return;
     m_plugin = nullptr;
 }
 
@@ -21,6 +25,11 @@ Loader::Loader(const ByteBuffer& buffer)
     m_plugin = make<WavLoaderPlugin>(buffer);
     if (m_plugin->sniff())
         return;
+    m_plugin = make<FlacLoaderPlugin>(buffer);
+    if (m_plugin->sniff()) {
+        dbgln("FLAC sniff successful");
+        return;
+    }
     m_plugin = nullptr;
 }