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https://github.com/RGBCube/serenity
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LibAudio: Add a FLAC encoder
This encoder can handle all integer formats and sample rates, though only two channels well. It uses fixed LPC and performs a close-to-optimal parameter search on the LPC order and residual Rice parameter, leading to decent compression already.
This commit is contained in:
kleines Filmröllchen
Andrew Kaster
parent
252b1328ea
commit
625aac2367
7 changed files with 795 additions and 0 deletions
624
Userland/Libraries/LibAudio/FlacWriter.cpp
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624
Userland/Libraries/LibAudio/FlacWriter.cpp
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/*
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* Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include "FlacWriter.h"
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#include <AK/BitStream.h>
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#include <AK/Endian.h>
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#include <AK/IntegralMath.h>
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#include <AK/MemoryStream.h>
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#include <LibCrypto/Checksum/ChecksummingStream.h>
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namespace Audio {
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ErrorOr<NonnullOwnPtr<FlacWriter>> FlacWriter::create(NonnullOwnPtr<SeekableStream> stream, u32 sample_rate, u8 num_channels, u16 bits_per_sample)
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{
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auto writer = TRY(AK::adopt_nonnull_own_or_enomem(new (nothrow) FlacWriter(move(stream))));
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TRY(writer->set_bits_per_sample(bits_per_sample));
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TRY(writer->set_sample_rate(sample_rate));
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TRY(writer->set_num_channels(num_channels));
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return writer;
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}
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FlacWriter::FlacWriter(NonnullOwnPtr<SeekableStream> stream)
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: m_stream(move(stream))
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{
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}
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FlacWriter::~FlacWriter()
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{
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if (m_state != WriteState::FullyFinalized)
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(void)finalize();
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}
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ErrorOr<void> FlacWriter::finalize()
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{
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if (m_state == WriteState::FullyFinalized)
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return Error::from_string_view("File is already finalized"sv);
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// TODO: Write missing sample data instead of discarding it.
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if (m_state == WriteState::HeaderUnwritten)
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TRY(finalize_header_format());
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{
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// 1 byte metadata block header + 3 bytes size + 2*2 bytes min/max block size
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TRY(m_stream->seek(m_streaminfo_start_index + 8, AK::SeekMode::SetPosition));
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BigEndianOutputBitStream bit_stream { MaybeOwned<Stream> { *m_stream } };
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TRY(bit_stream.write_bits(m_min_frame_size, 24));
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TRY(bit_stream.write_bits(m_max_frame_size, 24));
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TRY(bit_stream.write_bits(m_sample_rate, 20));
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TRY(bit_stream.write_bits(m_num_channels - 1u, 3));
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TRY(bit_stream.write_bits(m_bits_per_sample - 1u, 5));
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TRY(bit_stream.write_bits(m_sample_count, 36));
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TRY(bit_stream.align_to_byte_boundary());
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}
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// TODO: Write the audio data MD5 to the header.
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m_stream->close();
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m_state = WriteState::FullyFinalized;
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return {};
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}
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ErrorOr<void> FlacWriter::finalize_header_format()
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{
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if (m_state != WriteState::HeaderUnwritten)
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return Error::from_string_view("Header format is already finalized"sv);
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TRY(write_header());
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m_state = WriteState::FormatFinalized;
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return {};
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}
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ErrorOr<void> FlacWriter::set_num_channels(u8 num_channels)
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{
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if (m_state != WriteState::HeaderUnwritten)
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return Error::from_string_view("Header format is already finalized"sv);
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if (num_channels > 8)
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return Error::from_string_view("FLAC doesn't support more than 8 channels"sv);
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m_num_channels = num_channels;
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return {};
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}
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ErrorOr<void> FlacWriter::set_sample_rate(u32 sample_rate)
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{
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if (m_state != WriteState::HeaderUnwritten)
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return Error::from_string_view("Header format is already finalized"sv);
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m_sample_rate = sample_rate;
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return {};
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}
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ErrorOr<void> FlacWriter::set_bits_per_sample(u16 bits_per_sample)
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{
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if (m_state != WriteState::HeaderUnwritten)
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return Error::from_string_view("Header format is already finalized"sv);
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if (bits_per_sample < 8 || bits_per_sample > 32)
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return Error::from_string_view("FLAC only supports bits per sample between 8 and 32"sv);
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m_bits_per_sample = bits_per_sample;
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return {};
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}
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ErrorOr<void> FlacWriter::write_header()
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{
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TRY(m_stream->write_until_depleted(flac_magic.bytes()));
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m_streaminfo_start_index = TRY(m_stream->tell());
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ByteBuffer data;
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// STREAMINFO is always exactly 34 bytes long.
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TRY(data.try_resize(34));
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BigEndianOutputBitStream header_stream { TRY(try_make<FixedMemoryStream>(data.bytes())) };
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// Duplication on purpose:
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// Minimum frame size.
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TRY(header_stream.write_bits(block_size, 16));
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// Maximum frame size.
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TRY(header_stream.write_bits(block_size, 16));
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// Leave the frame sizes as unknown for now.
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TRY(header_stream.write_bits(0u, 24));
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TRY(header_stream.write_bits(0u, 24));
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TRY(header_stream.write_bits(m_sample_rate, 20));
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TRY(header_stream.write_bits(m_num_channels - 1u, 3));
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TRY(header_stream.write_bits(m_bits_per_sample - 1u, 5));
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// Leave the sample count as unknown for now.
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TRY(header_stream.write_bits(0u, 36));
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// TODO: Calculate the MD5 signature of all of the audio data.
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auto md5 = TRY(ByteBuffer::create_zeroed(128u / 8u));
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TRY(header_stream.write_until_depleted(md5));
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FlacRawMetadataBlock streaminfo_block = {
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.is_last_block = true,
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.type = FlacMetadataBlockType::STREAMINFO,
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.length = static_cast<u32>(data.size()),
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.data = data,
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};
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TRY(m_stream->write_value(streaminfo_block));
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return {};
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}
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ErrorOr<void> FlacRawMetadataBlock::write_to_stream(Stream& stream) const
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{
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BigEndianOutputBitStream bit_stream { MaybeOwned<Stream> { stream } };
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TRY(bit_stream.write_bits(static_cast<u8>(is_last_block), 1));
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TRY(bit_stream.write_bits(to_underlying(type), 7));
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TRY(bit_stream.write_bits(length, 24));
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VERIFY(data.size() == length);
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TRY(bit_stream.write_until_depleted(data));
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return {};
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}
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// If the given sample count is uncommon, this function will return one of the uncommon marker block sizes.
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// The caller has to handle and add these later manually.
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static BlockSizeCategory to_common_block_size(u16 sample_count)
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{
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switch (sample_count) {
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case 192:
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return BlockSizeCategory::S192;
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case 576:
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return BlockSizeCategory::S576;
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case 1152:
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return BlockSizeCategory::S1152;
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case 2304:
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return BlockSizeCategory::S2304;
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case 4608:
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return BlockSizeCategory::S4608;
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case 256:
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return BlockSizeCategory::S256;
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case 512:
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return BlockSizeCategory::S512;
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case 1024:
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return BlockSizeCategory::S1024;
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case 2048:
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return BlockSizeCategory::S2048;
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case 4096:
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return BlockSizeCategory::S4096;
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case 8192:
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return BlockSizeCategory::S8192;
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case 16384:
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return BlockSizeCategory::S16384;
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case 32768:
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return BlockSizeCategory::S32768;
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}
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if (sample_count - 1 <= 0xff)
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return BlockSizeCategory::Uncommon8Bits;
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// Data type guarantees that 16-bit storage is possible.
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return BlockSizeCategory::Uncommon16Bits;
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}
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static ByteBuffer to_utf8(u64 value)
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{
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ByteBuffer buffer;
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if (value < 0x7f) {
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buffer.append(static_cast<u8>(value));
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} else if (value < 0x7ff) {
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buffer.append(static_cast<u8>(0b110'00000 | (value >> 6)));
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buffer.append(static_cast<u8>(0b10'000000 | (value & 0b111111)));
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} else if (value < 0xffff) {
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buffer.append(static_cast<u8>(0b1110'0000 | (value >> 12)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 6) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 0) & 0b111111)));
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} else if (value < 0x1f'ffff) {
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buffer.append(static_cast<u8>(0b11110'000 | (value >> 18)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 12) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 6) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 0) & 0b111111)));
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} else if (value < 0x3ff'ffff) {
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buffer.append(static_cast<u8>(0b111110'00 | (value >> 24)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 18) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 12) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 6) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 0) & 0b111111)));
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} else if (value < 0x7fff'ffff) {
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buffer.append(static_cast<u8>(0b1111110'0 | (value >> 30)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 24) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 18) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 12) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 6) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 0) & 0b111111)));
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} else if (value < 0xf'ffff'ffff) {
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buffer.append(static_cast<u8>(0b11111110));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 30) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 24) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 18) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 12) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 6) & 0b111111)));
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buffer.append(static_cast<u8>(0b10'000000 | ((value >> 0) & 0b111111)));
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} else {
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// Anything larger is illegal even in expanded UTF-8, but FLAC only passes 32-bit values anyways.
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VERIFY_NOT_REACHED();
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}
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return buffer;
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}
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ErrorOr<void> FlacFrameHeader::write_to_stream(Stream& stream) const
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{
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Crypto::Checksum::ChecksummingStream<FlacFrameHeaderCRC> checksumming_stream { MaybeOwned<Stream> { stream } };
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BigEndianOutputBitStream bit_stream { MaybeOwned<Stream> { checksumming_stream } };
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TRY(bit_stream.write_bits(0b11111111111110u, 14));
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TRY(bit_stream.write_bits(0u, 1));
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TRY(bit_stream.write_bits(to_underlying(blocking_strategy), 1));
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auto common_block_size = to_common_block_size(sample_count);
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TRY(bit_stream.write_bits(to_underlying(common_block_size), 4));
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// We always store sample rate in the file header.
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TRY(bit_stream.write_bits(0u, 4));
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TRY(bit_stream.write_bits(to_underlying(channels), 4));
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// We always store bit depth in the file header.
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TRY(bit_stream.write_bits(0u, 3));
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// Reserved zero bit.
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TRY(bit_stream.write_bits(0u, 1));
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auto coded_number = to_utf8(sample_or_frame_index);
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TRY(bit_stream.write_until_depleted(coded_number));
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if (common_block_size == BlockSizeCategory::Uncommon8Bits)
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TRY(bit_stream.write_value(static_cast<u8>(sample_count - 1)));
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if (common_block_size == BlockSizeCategory::Uncommon16Bits)
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TRY(bit_stream.write_value(BigEndian<u16>(static_cast<u16>(sample_count - 1))));
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// Ensure that the checksum is calculated correctly.
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TRY(bit_stream.align_to_byte_boundary());
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auto checksum = checksumming_stream.digest();
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TRY(bit_stream.write_value(checksum));
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return {};
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}
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ErrorOr<void> FlacWriter::write_samples(ReadonlySpan<Sample> samples)
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{
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if (m_state == WriteState::FullyFinalized)
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return Error::from_string_view("File is already finalized"sv);
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auto remaining_samples = samples;
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while (remaining_samples.size() > 0) {
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if (m_sample_buffer.size() == block_size) {
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TRY(write_frame());
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m_sample_buffer.clear();
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}
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auto amount_to_copy = min(remaining_samples.size(), m_sample_buffer.capacity() - m_sample_buffer.size());
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auto current_buffer_size = m_sample_buffer.size();
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TRY(m_sample_buffer.try_resize_and_keep_capacity(current_buffer_size + amount_to_copy));
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remaining_samples.copy_trimmed_to(m_sample_buffer.span().slice(current_buffer_size));
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remaining_samples = remaining_samples.slice(amount_to_copy);
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}
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// Ensure that the buffer is flushed if possible.
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if (m_sample_buffer.size() == block_size) {
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TRY(write_frame());
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m_sample_buffer.clear();
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}
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return {};
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}
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ErrorOr<void> FlacWriter::write_frame()
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{
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auto frame_samples = move(m_sample_buffer);
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// De-interleave and integer-quantize subframes.
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float sample_rescale = static_cast<float>(1 << (m_bits_per_sample - 1));
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auto subframe_samples = Vector<Vector<i64, block_size>>();
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TRY(subframe_samples.try_resize_and_keep_capacity(m_num_channels));
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for (auto const& sample : frame_samples) {
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TRY(subframe_samples[0].try_append(static_cast<i64>(sample.left * sample_rescale)));
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// FIXME: We don't have proper data for any channels past 2.
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for (auto i = 1; i < m_num_channels; ++i)
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TRY(subframe_samples[i].try_append(static_cast<i64>(sample.right * sample_rescale)));
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}
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FlacFrameHeader header {
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.sample_rate = m_sample_rate,
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.sample_count = static_cast<u16>(frame_samples.size()),
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.sample_or_frame_index = static_cast<u32>(m_current_frame),
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.blocking_strategy = BlockingStrategy::Fixed,
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// FIXME: We should brute-force channel coupling for stereo.
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.channels = static_cast<FlacFrameChannelType>(m_num_channels - 1),
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.bit_depth = static_cast<u8>(m_bits_per_sample),
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// Calculated for us during header write.
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.checksum = 0,
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};
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auto frame_stream = Crypto::Checksum::ChecksummingStream<IBMCRC> { MaybeOwned<Stream> { *m_stream } };
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auto frame_start_offset = TRY(m_stream->tell());
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TRY(frame_stream.write_value(header));
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BigEndianOutputBitStream bit_stream { MaybeOwned<Stream> { frame_stream } };
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for (auto const& subframe : subframe_samples)
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TRY(write_subframe(subframe.span(), bit_stream));
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TRY(bit_stream.align_to_byte_boundary());
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auto frame_crc = frame_stream.digest();
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dbgln_if(FLAC_ENCODER_DEBUG, "Frame {:4} CRC: {:04x}", m_current_frame, frame_crc);
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TRY(frame_stream.write_value<AK::BigEndian<u16>>(frame_crc));
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auto frame_end_offset = TRY(m_stream->tell());
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auto frame_size = frame_end_offset - frame_start_offset;
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m_max_frame_size = max(m_max_frame_size, frame_size);
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m_min_frame_size = min(m_min_frame_size, frame_size);
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m_current_frame++;
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m_sample_count += frame_samples.size();
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return {};
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}
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ErrorOr<void> FlacWriter::write_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream)
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{
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// The current subframe encoding strategy is as follows:
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// - Check if the subframe is constant; use constant encoding in this case.
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// - Try all fixed predictors and record the resulting residuals.
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// - Estimate their encoding cost by taking the sum of all absolute logarithmic residuals,
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// which is an accurate estimate of the final encoded size of the residuals.
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// - Accurately estimate the encoding cost of a verbatim subframe.
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// - Select the encoding strategy with the lowest cost out of this selection.
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auto constant_value = subframe[0];
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auto is_constant = true;
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for (auto const sample : subframe) {
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if (sample != constant_value) {
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is_constant = false;
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break;
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}
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}
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if (is_constant) {
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dbgln_if(FLAC_ENCODER_DEBUG, "Encoding constant frame with value {}", constant_value);
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TRY(bit_stream.write_bits(1u, 0));
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TRY(bit_stream.write_bits(to_underlying(FlacSubframeType::Constant), 6));
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TRY(bit_stream.write_bits(1u, 0));
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TRY(bit_stream.write_bits(bit_cast<u64>(constant_value), m_bits_per_sample));
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return {};
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}
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auto verbatim_cost_bits = subframe.size() * m_bits_per_sample;
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Optional<FlacLPCEncodedSubframe> best_lpc_subframe;
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auto current_min_cost = verbatim_cost_bits;
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for (auto order : { FlacFixedLPC::Zero, FlacFixedLPC::One, FlacFixedLPC::Two, FlacFixedLPC::Three, FlacFixedLPC::Four }) {
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// Too many warm-up samples would be required; the lower-level encoding procedures assume that this was checked.
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if (to_underlying(order) > subframe.size())
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continue;
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auto encode_result = TRY(encode_fixed_lpc(order, subframe, current_min_cost));
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if (encode_result.has_value() && encode_result.value().residual_cost_bits < current_min_cost) {
|
||||
current_min_cost = encode_result.value().residual_cost_bits;
|
||||
best_lpc_subframe = encode_result.release_value();
|
||||
}
|
||||
}
|
||||
|
||||
// No LPC encoding was better than verbatim.
|
||||
if (!best_lpc_subframe.has_value()) {
|
||||
dbgln_if(FLAC_ENCODER_DEBUG, "Best subframe type was Verbatim; encoding {} samples at {} bps = {} bits", subframe.size(), m_bits_per_sample, verbatim_cost_bits);
|
||||
TRY(write_verbatim_subframe(subframe, bit_stream));
|
||||
} else {
|
||||
dbgln_if(FLAC_ENCODER_DEBUG, "Best subframe type was Fixed LPC order {} (estimated cost {} bits); encoding {} samples", to_underlying(best_lpc_subframe->coefficients.get<FlacFixedLPC>()), best_lpc_subframe->residual_cost_bits, subframe.size());
|
||||
TRY(write_lpc_subframe(best_lpc_subframe.release_value(), bit_stream));
|
||||
}
|
||||
|
||||
return {};
|
||||
}
|
||||
|
||||
ErrorOr<Optional<FlacLPCEncodedSubframe>> FlacWriter::encode_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> subframe, size_t current_min_cost)
|
||||
{
|
||||
FlacLPCEncodedSubframe lpc {
|
||||
.warm_up_samples = Vector<i64> { subframe.trim(to_underlying(order)) },
|
||||
.coefficients = order,
|
||||
.residuals {},
|
||||
// Warm-up sample cost.
|
||||
.residual_cost_bits = to_underlying(order) * m_bits_per_sample,
|
||||
.single_partition_optimal_order {},
|
||||
};
|
||||
TRY(lpc.residuals.try_ensure_capacity(subframe.size() - to_underlying(order)));
|
||||
|
||||
Vector<i64> predicted;
|
||||
TRY(predicted.try_resize_and_keep_capacity(subframe.size()));
|
||||
lpc.warm_up_samples.span().copy_trimmed_to(predicted);
|
||||
|
||||
// NOTE: Although we can't interrupt the prediction if the corresponding residuals would become too bad,
|
||||
// we don't need to branch on the order in every loop during prediction, meaning this shouldn't cost us much.
|
||||
predict_fixed_lpc(order, subframe, predicted);
|
||||
|
||||
// There isn’t really a way of computing an LPC’s cost without performing most of the calculations, including a Rice parameter search.
|
||||
// This is nevertheless optimized in multiple ways, so that we always bail out once we are sure no improvements can be made.
|
||||
auto extra_residual_cost = NumericLimits<size_t>::max();
|
||||
// Keep track of when we want to estimate costs again. We don't do this for every new residual since it's an expensive procedure.
|
||||
// The likelihood for misprediction is pretty high for large orders; start with a later index for them.
|
||||
auto next_cost_estimation_index = min(subframe.size() - 1, first_residual_estimation * (to_underlying(order) + 1));
|
||||
for (auto i = to_underlying(order); i < subframe.size(); ++i) {
|
||||
auto residual = subframe[i] - predicted[i];
|
||||
if (!AK::is_within_range<i32>(residual)) {
|
||||
dbgln_if(FLAC_ENCODER_DEBUG, "Bailing from Fixed LPC order {} due to residual overflow ({} is outside the 32-bit range)", to_underlying(order), residual);
|
||||
return Optional<FlacLPCEncodedSubframe> {};
|
||||
}
|
||||
lpc.residuals.append(residual);
|
||||
|
||||
if (i >= next_cost_estimation_index) {
|
||||
// Find best exponential Golomb order.
|
||||
// Storing this in the LPC data allows us to automatically reuse the computation during LPC encoding.
|
||||
// FIXME: Use more than one partition to improve compression.
|
||||
// FIXME: Investigate whether this can be estimated “good enough” to improve performance at the cost of compression strength.
|
||||
// Especially at larger sample counts, it is unlikely that we will find a different optimal order.
|
||||
// Therefore, use a zig-zag search around the previous optimal order.
|
||||
extra_residual_cost = NumericLimits<size_t>::max();
|
||||
auto start_order = lpc.single_partition_optimal_order;
|
||||
size_t useless_parameters = 0;
|
||||
size_t steps = 0;
|
||||
constexpr auto max_rice_parameter = AK::exp2(4) - 1;
|
||||
for (auto offset = 0; start_order + offset < max_rice_parameter || start_order - offset >= 0; ++offset) {
|
||||
for (auto factor : { -1, 1 }) {
|
||||
auto k = start_order + factor * offset;
|
||||
if (k >= max_rice_parameter || k < 0)
|
||||
continue;
|
||||
|
||||
auto order_cost = count_exp_golomb_bits_in(k, lpc.residuals);
|
||||
if (order_cost < extra_residual_cost) {
|
||||
extra_residual_cost = order_cost;
|
||||
lpc.single_partition_optimal_order = k;
|
||||
} else {
|
||||
useless_parameters++;
|
||||
}
|
||||
steps++;
|
||||
// Don’t do 0 twice.
|
||||
if (offset == 0)
|
||||
break;
|
||||
}
|
||||
// If we found enough useless parameters, we probably won't find useful ones anymore.
|
||||
// The only exception is the first ever parameter search, where we search everything.
|
||||
if (useless_parameters >= useless_parameter_threshold && start_order != 0)
|
||||
break;
|
||||
}
|
||||
|
||||
// Min cost exceeded; bail out.
|
||||
if (lpc.residual_cost_bits + extra_residual_cost > current_min_cost) {
|
||||
dbgln_if(FLAC_ENCODER_DEBUG, " Bailing from Fixed LPC order {} at sample index {} and cost {} (best {})", to_underlying(order), i, lpc.residual_cost_bits + extra_residual_cost, current_min_cost);
|
||||
return Optional<FlacLPCEncodedSubframe> {};
|
||||
}
|
||||
|
||||
// Figure out when to next estimate costs.
|
||||
auto estimated_bits_per_residual = static_cast<double>(extra_residual_cost) / static_cast<double>(i);
|
||||
auto estimated_residuals_for_min_cost = static_cast<double>(current_min_cost) / estimated_bits_per_residual;
|
||||
auto unchecked_next_cost_estimation_index = AK::round_to<size_t>(estimated_residuals_for_min_cost * (1 - residual_cost_margin));
|
||||
// Check either at the estimated residual, or the next residual if that is in the past, or the last residual.
|
||||
next_cost_estimation_index = min(subframe.size() - 1, max(unchecked_next_cost_estimation_index, i + min_residual_estimation_step));
|
||||
dbgln_if(FLAC_ENCODER_DEBUG, " {} {:4} Estimate cost/residual {:.1f} (param {:2} after {:2} steps), will hit at {:6.1f}, jumping to {:4} (sanitized to {:4})", to_underlying(order), i, estimated_bits_per_residual, lpc.single_partition_optimal_order, steps, estimated_residuals_for_min_cost, unchecked_next_cost_estimation_index, next_cost_estimation_index);
|
||||
}
|
||||
}
|
||||
|
||||
lpc.residual_cost_bits += extra_residual_cost;
|
||||
return lpc;
|
||||
}
|
||||
|
||||
void predict_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> samples, Span<i64> predicted_output)
|
||||
{
|
||||
switch (order) {
|
||||
case FlacFixedLPC::Zero:
|
||||
// s_0(t) = 0
|
||||
for (auto i = to_underlying(order); i < predicted_output.size(); ++i)
|
||||
predicted_output[i] += 0;
|
||||
break;
|
||||
case FlacFixedLPC::One:
|
||||
// s_1(t) = s(t-1)
|
||||
for (auto i = to_underlying(order); i < predicted_output.size(); ++i)
|
||||
predicted_output[i] += samples[i - 1];
|
||||
break;
|
||||
case FlacFixedLPC::Two:
|
||||
// s_2(t) = 2s(t-1) - s(t-2)
|
||||
for (auto i = to_underlying(order); i < predicted_output.size(); ++i)
|
||||
predicted_output[i] += 2 * samples[i - 1] - samples[i - 2];
|
||||
break;
|
||||
case FlacFixedLPC::Three:
|
||||
// s_3(t) = 3s(t-1) - 3s(t-2) + s(t-3)
|
||||
for (auto i = to_underlying(order); i < predicted_output.size(); ++i)
|
||||
predicted_output[i] += 3 * samples[i - 1] - 3 * samples[i - 2] + samples[i - 3];
|
||||
break;
|
||||
case FlacFixedLPC::Four:
|
||||
// s_4(t) = 4s(t-1) - 6s(t-2) + 4s(t-3) - s(t-4)
|
||||
for (auto i = to_underlying(order); i < predicted_output.size(); ++i)
|
||||
predicted_output[i] += 4 * samples[i - 1] - 6 * samples[i - 2] + 4 * samples[i - 3] - samples[i - 4];
|
||||
break;
|
||||
default:
|
||||
VERIFY_NOT_REACHED();
|
||||
}
|
||||
}
|
||||
|
||||
// https://www.ietf.org/archive/id/draft-ietf-cellar-flac-08.html#name-verbatim-subframe
|
||||
ErrorOr<void> FlacWriter::write_verbatim_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream)
|
||||
{
|
||||
TRY(bit_stream.write_bits(0u, 1));
|
||||
TRY(bit_stream.write_bits(to_underlying(FlacSubframeType::Verbatim), 6));
|
||||
TRY(bit_stream.write_bits(0u, 1));
|
||||
for (auto const& sample : subframe)
|
||||
TRY(bit_stream.write_bits(bit_cast<u64>(sample), m_bits_per_sample));
|
||||
|
||||
return {};
|
||||
}
|
||||
|
||||
// https://www.ietf.org/archive/id/draft-ietf-cellar-flac-08.html#name-fixed-predictor-subframe
|
||||
ErrorOr<void> FlacWriter::write_lpc_subframe(FlacLPCEncodedSubframe lpc_subframe, BigEndianOutputBitStream& bit_stream)
|
||||
{
|
||||
// Reserved.
|
||||
TRY(bit_stream.write_bits(0u, 1));
|
||||
// 9.2.1 Subframe header (https://www.ietf.org/archive/id/draft-ietf-cellar-flac-08.html#name-subframe-header)
|
||||
u8 encoded_type;
|
||||
if (lpc_subframe.coefficients.has<FlacFixedLPC>())
|
||||
encoded_type = to_underlying(lpc_subframe.coefficients.get<FlacFixedLPC>()) + to_underlying(FlacSubframeType::Fixed);
|
||||
else
|
||||
encoded_type = lpc_subframe.coefficients.get<Vector<i64>>().size() - 1 + to_underlying(FlacSubframeType::LPC);
|
||||
|
||||
TRY(bit_stream.write_bits(encoded_type, 6));
|
||||
// No wasted bits per sample (unnecessary for the vast majority of data).
|
||||
TRY(bit_stream.write_bits(0u, 1));
|
||||
|
||||
for (auto const& warm_up_sample : lpc_subframe.warm_up_samples)
|
||||
TRY(bit_stream.write_bits(bit_cast<u64>(warm_up_sample), m_bits_per_sample));
|
||||
|
||||
// 4-bit Rice parameters.
|
||||
TRY(bit_stream.write_bits(0b00u, 2));
|
||||
// Only one partition (2^0 = 1).
|
||||
TRY(bit_stream.write_bits(0b0000u, 4));
|
||||
TRY(write_rice_partition(lpc_subframe.single_partition_optimal_order, lpc_subframe.residuals, bit_stream));
|
||||
|
||||
return {};
|
||||
}
|
||||
|
||||
ErrorOr<void> FlacWriter::write_rice_partition(u8 k, ReadonlySpan<i64> residuals, BigEndianOutputBitStream& bit_stream)
|
||||
{
|
||||
TRY(bit_stream.write_bits(k, 4));
|
||||
|
||||
for (auto const& residual : residuals)
|
||||
TRY(encode_unsigned_exp_golomb(k, static_cast<i32>(residual), bit_stream));
|
||||
|
||||
return {};
|
||||
}
|
||||
|
||||
u32 signed_to_rice(i32 x)
|
||||
{
|
||||
// Implements (x < 0 ? -1 : 0) + 2 * abs(x) in about half as many instructions.
|
||||
// The reference encoder’s implementation is known to be the fastest on -O2/3 clang and gcc:
|
||||
// x << 1 = multiply by 2.
|
||||
// For negative numbers, x >> 31 will create an all-ones XOR mask, meaning that the number will be inverted.
|
||||
// In two's complement this is -value - 1, exactly what we need.
|
||||
// For positive numbers, x >> 31 == 0.
|
||||
return static_cast<u32>((x << 1) ^ (x >> 31));
|
||||
}
|
||||
|
||||
// Adopted from https://github.com/xiph/flac/blob/28e4f0528c76b296c561e922ba67d43751990599/src/libFLAC/bitwriter.c#L727
|
||||
ErrorOr<void> encode_unsigned_exp_golomb(u8 k, i32 value, BigEndianOutputBitStream& bit_stream)
|
||||
{
|
||||
auto zigzag_encoded = signed_to_rice(value);
|
||||
auto msbs = zigzag_encoded >> k;
|
||||
auto pattern = 1u << k;
|
||||
pattern |= zigzag_encoded & ((1 << k) - 1);
|
||||
|
||||
TRY(bit_stream.write_bits(0u, msbs));
|
||||
TRY(bit_stream.write_bits(pattern, k + 1));
|
||||
|
||||
return {};
|
||||
}
|
||||
|
||||
// Adopted from count_rice_bits_in_partition():
|
||||
// https://github.com/xiph/flac/blob/28e4f0528c76b296c561e922ba67d43751990599/src/libFLAC/stream_encoder.c#L4299
|
||||
size_t count_exp_golomb_bits_in(u8 k, ReadonlySpan<i64> residuals)
|
||||
{
|
||||
// Exponential Golomb order size (4).
|
||||
// One unary stop bit and the entire exponential Golomb parameter for every residual.
|
||||
size_t partition_bits = 4 + (1 + k) * residuals.size();
|
||||
|
||||
// Bit magic to compute the amount of leading unary bits.
|
||||
for (auto const& residual : residuals)
|
||||
partition_bits += (static_cast<u32>((residual << 1) ^ (residual >> 31)) >> k);
|
||||
|
||||
return partition_bits;
|
||||
}
|
||||
|
||||
}
|
||||
Loading…
Add table
Add a link
Reference in a new issue