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Add the v1 audio signature to the pipeline (IR-004, IR-005, IR-007, IR-008)

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Implements the content-derived spectral-peak signature from
JRay-public-server/SPEC.md §3 so a truth file is self-identifying: 120 s
window centred on the media midpoint, mono at 11025 Hz, 4096/1024 Hann
STFT, 32 log-spaced bins over 300-3000 Hz, one byte per frame (5-bit peak
band + 2-bit energy class), base64, `v1:` prefix.

Audio decode is a second stream from the FFmpeg libraries the pipeline
already links for video; libswresample is added to the existing
ffmpeg_libs interface target. The FFT is written out rather than pulled
from a library for the same reason the plugin vendors one: the output has
to be bit-identical across two languages, so a dependency whose version
could change the numerics is a liability.

The server spec fixes the geometry but not enough to reproduce a byte
stream — Hann periodicity, band aggregation, the energy-class definition,
tie-breaking and the base64 alphabet are all unconstrained by it. Those
are pinned in audio_signature.hpp and mirrored in the golden fixture, so
the plugin can be implemented from the fixture alone.

IR-005: tests/fixtures/audio/ carries a deterministic 120 s tone (FLAC —
lossless, so identical PCM to the WAV make_fixture.py emits, and 3.5x
smaller in git) plus the signature it must produce, the decoded-PCM
checksum and the full parameter contract. That directory is the artefact
shared with the plugin repo; the PCM checksum is separate from the
signature so a codec-level difference is distinguishable from a DSP one.

IR-007: media under 120 s emits no signature. Same for a file with no
audio stream or one that will not open — UR-9 is an enhancement and must
never be able to break a fetch.

Verified against an independent Python reference implementation: same
bytes. All 32 bands and all 4 energy classes appear in the golden vector,
and the window-centring test wraps the fixture in 90 s of silence either
side and requires the golden value back.

Not wired into the truth-file output yet — that is the schema_version
bump under IR-002/IR-003 and is deliberately out of scope here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
dtourolle
2026-07-30 18:25:37 +02:00
co-authored by Claude Opus 5
parent 43d2c976c3
commit 45ef7c1916
9 changed files with 1283 additions and 9 deletions
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src/audio_signature.cpp
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// ── JRay audio signature, v1 — implementation ────────────────────────────────
//
/// TRACES: IR-004, IR-007, IR-008 | SR-003
//
// The contract this implements is documented in full in audio_signature.hpp;
// read that before changing anything here. Every constant is load-bearing: the
// JRay Jellyfin plugin computes the same bytes in C#, and a signature that
// differs in any parameter simply does not match.
//
// Audio decode is a *second stream from an existing dependency* — the pipeline
// already links libavformat/libavcodec/libavutil for video (ffmpeg_decoder.hpp);
// this adds libswresample for the downmix+resample, no new project dependency.
// The FFT is written out here rather than pulled from a library for the same
// reason the plugin vendors one: it is a fixed, fully specified transform, and
// a dependency whose version could change the numerics is a liability when the
// output has to be bit-identical across two languages.
#include "audio_signature.hpp"
extern "C" {
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavutil/avutil.h>
#include <libavutil/channel_layout.h>
#include <libavutil/opt.h>
#include <libavutil/samplefmt.h>
#include <libswresample/swresample.h>
}
#include <algorithm>
#include <cmath>
#include <cstring>
#include <vector>
namespace sae::audio {
namespace {
constexpr double kPi = 3.14159265358979323846;
// ── Band table ───────────────────────────────────────────────────────────────
// edge[b] = 300 * 10^(b/32); band b owns FFT bins [k_lo[b], k_lo[b+1]).
// ceil() of the edge in bins, so membership is decided once by integers rather
// than by a float comparison per bin per frame. The bands tile [112, 1115)
// contiguously with no gap and no overlap, which is what lets the frame energy
// below be accumulated from the per-band sums.
std::array<std::pair<int, int>, kNumBands> build_band_table() {
const double hz_per_bin = static_cast<double>(kSampleRate) / kFrameSize;
std::array<int, kNumBands + 1> k{};
for (int b = 0; b <= kNumBands; ++b) {
const double edge = kBandLoHz * std::pow(kBandHiHz / kBandLoHz,
static_cast<double>(b) / kNumBands);
k[b] = static_cast<int>(std::ceil(edge / hz_per_bin));
}
std::array<std::pair<int, int>, kNumBands> tbl{};
for (int b = 0; b < kNumBands; ++b) tbl[b] = {k[b], k[b + 1]};
return tbl;
}
// Hann, periodic: w[n] = 0.5 * (1 - cos(2*pi*n/N)). Not the symmetric (N-1)
// variant — the two differ, and the difference is observable.
const std::vector<double>& hann_window() {
static const std::vector<double> w = [] {
std::vector<double> v(kFrameSize);
for (int n = 0; n < kFrameSize; ++n)
v[n] = 0.5 * (1.0 - std::cos(2.0 * kPi * n / kFrameSize));
return v;
}();
return w;
}
// ── Radix-2 decimation-in-time complex FFT, in place, no normalisation ──────
// Twiddles are precomputed per stage from cos/sin of -2*pi*j/len so the angle
// is an exactly reproducible double in any language and only the libm rounding
// of cos/sin (≤1 ulp) can differ — orders of magnitude below the decision
// margins in the golden fixture.
struct FftTables {
std::vector<int> rev; // bit-reversal permutation
std::vector<std::vector<double>> wr, wi; // per stage
};
const FftTables& fft_tables() {
static const FftTables t = [] {
FftTables f;
f.rev.resize(kFrameSize);
int bits = 0;
while ((1 << bits) < kFrameSize) ++bits;
for (int i = 0; i < kFrameSize; ++i) {
int r = 0;
for (int b = 0; b < bits; ++b)
if (i & (1 << b)) r |= 1 << (bits - 1 - b);
f.rev[i] = r;
}
for (int len = 2; len <= kFrameSize; len <<= 1) {
const int half = len / 2;
std::vector<double> cr(half), ci(half);
for (int j = 0; j < half; ++j) {
const double ang = -2.0 * kPi * j / len;
cr[j] = std::cos(ang);
ci[j] = std::sin(ang);
}
f.wr.push_back(std::move(cr));
f.wi.push_back(std::move(ci));
}
return f;
}();
return t;
}
void fft_4096(std::vector<double>& re, std::vector<double>& im) {
const FftTables& t = fft_tables();
for (int i = 0; i < kFrameSize; ++i) {
const int j = t.rev[i];
if (i < j) { std::swap(re[i], re[j]); std::swap(im[i], im[j]); }
}
int stage = 0;
for (int len = 2; len <= kFrameSize; len <<= 1, ++stage) {
const int half = len / 2;
const std::vector<double>& wr = t.wr[stage];
const std::vector<double>& wi = t.wi[stage];
for (int base = 0; base < kFrameSize; base += len) {
for (int j = 0; j < half; ++j) {
const int a = base + j;
const int b = a + half;
const double tr = re[b] * wr[j] - im[b] * wi[j];
const double ti = re[b] * wi[j] + im[b] * wr[j];
re[b] = re[a] - tr; im[b] = im[a] - ti;
re[a] = re[a] + tr; im[a] = im[a] + ti;
}
}
}
}
int energy_class(double r) {
if (r < kEnergyClassEdges[0]) return 0;
if (r < kEnergyClassEdges[1]) return 1;
if (r < kEnergyClassEdges[2]) return 2;
return 3;
}
// ── FFmpeg RAII ─────────────────────────────────────────────────────────────
struct DecodeCtx {
AVFormatContext* fmt = nullptr;
AVCodecContext* dec = nullptr;
SwrContext* swr = nullptr;
AVFrame* frm = nullptr;
AVPacket* pkt = nullptr;
~DecodeCtx() {
if (swr) swr_free(&swr);
if (frm) av_frame_free(&frm);
if (pkt) av_packet_free(&pkt);
if (dec) avcodec_free_context(&dec);
if (fmt) avformat_close_input(&fmt);
}
};
bool open_resampler(DecodeCtx& c, const AVFrame* f) {
#if LIBAVUTIL_VERSION_INT >= AV_VERSION_INT(57, 24, 100)
AVChannelLayout out_layout;
av_channel_layout_default(&out_layout, 1); // mono
AVChannelLayout in_layout;
if (av_channel_layout_copy(&in_layout, &f->ch_layout) < 0) return false;
if (in_layout.nb_channels <= 0) {
av_channel_layout_uninit(&in_layout);
av_channel_layout_default(&in_layout, 1);
}
const int rc = swr_alloc_set_opts2(
&c.swr,
&out_layout, AV_SAMPLE_FMT_FLT, kSampleRate,
&in_layout, static_cast<AVSampleFormat>(f->format),
f->sample_rate ? f->sample_rate : kSampleRate,
0, nullptr);
av_channel_layout_uninit(&in_layout);
av_channel_layout_uninit(&out_layout);
if (rc < 0 || !c.swr) return false;
#else
const int64_t in_layout = f->channel_layout
? static_cast<int64_t>(f->channel_layout)
: av_get_default_channel_layout(f->channels ? f->channels : 1);
c.swr = swr_alloc_set_opts(
nullptr,
AV_CH_LAYOUT_MONO, AV_SAMPLE_FMT_FLT, kSampleRate,
in_layout, static_cast<AVSampleFormat>(f->format),
f->sample_rate ? f->sample_rate : kSampleRate,
0, nullptr);
if (!c.swr) return false;
#endif
return swr_init(c.swr) >= 0;
}
// Push one decoded frame (or a flush) through the resampler, dropping the
// leading `to_skip` output samples, and append to `out`.
void drain(SwrContext* swr, const AVFrame* f, int in_rate,
std::size_t& to_skip, std::vector<float>& out) {
const int64_t delay = swr_get_delay(swr, in_rate ? in_rate : kSampleRate);
const int in_n = f ? f->nb_samples : 0;
const int max_out = static_cast<int>(av_rescale_rnd(
delay + in_n, kSampleRate, in_rate ? in_rate : kSampleRate, AV_ROUND_UP)) + 32;
if (max_out <= 0) return;
std::vector<float> buf(static_cast<std::size_t>(max_out));
uint8_t* dst = reinterpret_cast<uint8_t*>(buf.data());
const int n = swr_convert(swr, &dst, max_out,
f ? const_cast<const uint8_t**>(f->extended_data) : nullptr,
in_n);
if (n <= 0) return;
std::size_t produced = static_cast<std::size_t>(n);
std::size_t off = 0;
if (to_skip) {
const std::size_t drop = std::min(to_skip, produced);
to_skip -= drop;
off = drop;
produced -= drop;
}
if (produced)
out.insert(out.end(), buf.begin() + off, buf.begin() + off + produced);
}
} // namespace
// ── Public surface ──────────────────────────────────────────────────────────
const std::array<std::pair<int, int>, kNumBands>& band_fft_bins() {
static const std::array<std::pair<int, int>, kNumBands> tbl = build_band_table();
return tbl;
}
std::string base64_encode(const std::uint8_t* data, std::size_t n) {
static constexpr char kAlphabet[] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
std::string out;
out.reserve(((n + 2) / 3) * 4);
std::size_t i = 0;
for (; i + 3 <= n; i += 3) {
const std::uint32_t v = (std::uint32_t(data[i]) << 16) |
(std::uint32_t(data[i + 1]) << 8) |
std::uint32_t(data[i + 2]);
out += kAlphabet[(v >> 18) & 0x3F];
out += kAlphabet[(v >> 12) & 0x3F];
out += kAlphabet[(v >> 6) & 0x3F];
out += kAlphabet[v & 0x3F];
}
if (i < n) {
const bool two = (n - i) == 2;
const std::uint32_t v = (std::uint32_t(data[i]) << 16) |
(two ? (std::uint32_t(data[i + 1]) << 8) : 0u);
out += kAlphabet[(v >> 18) & 0x3F];
out += kAlphabet[(v >> 12) & 0x3F];
out += two ? kAlphabet[(v >> 6) & 0x3F] : '=';
out += '=';
}
return out;
}
std::uint64_t fnv1a64(const void* data, std::size_t n) {
const auto* p = static_cast<const std::uint8_t*>(data);
std::uint64_t h = 0xcbf29ce484222325ULL;
for (std::size_t i = 0; i < n; ++i) {
h ^= p[i];
h *= 0x100000001b3ULL;
}
return h;
}
/// TRACES: IR-004
std::vector<std::uint8_t> pack_frames(const std::vector<float>& mono) {
if (mono.size() < static_cast<std::size_t>(kFrameSize)) return {};
const std::size_t nframes = 1 + (mono.size() - kFrameSize) / kHopSize;
const auto& bands = band_fft_bins();
const auto& win = hann_window();
const int k_lo = bands.front().first;
const int k_hi = bands.back().second; // exclusive
const double bin_count = static_cast<double>(k_hi - k_lo);
std::vector<double> re(kFrameSize), im(kFrameSize);
std::vector<std::uint8_t> peak(nframes);
std::vector<double> energy(nframes);
for (std::size_t f = 0; f < nframes; ++f) {
const float* src = mono.data() + f * kHopSize;
for (int n = 0; n < kFrameSize; ++n) {
re[n] = static_cast<double>(src[n]) * win[n];
im[n] = 0.0;
}
fft_4096(re, im);
// Per-band mean magnitude; the bands tile the 300–3000 Hz range with no
// gaps, so the frame's band-limited energy is the sum of the band sums.
double best = -1.0, total = 0.0;
int best_b = 0;
for (int b = 0; b < kNumBands; ++b) {
double sum = 0.0;
for (int k = bands[b].first; k < bands[b].second; ++k)
sum += std::sqrt(re[k] * re[k] + im[k] * im[k]);
total += sum;
const double mean = sum / (bands[b].second - bands[b].first);
if (mean > best) { best = mean; best_b = b; } // ties → lowest index
}
peak[f] = static_cast<std::uint8_t>(best_b);
energy[f] = total / bin_count;
}
// Reference is the upper median of the frame energies: an actually observed
// value (no averaging of the two middle samples), so it is bit-reproducible,
// gain-invariant and barely moves when the window is trimmed.
std::vector<double> sorted = energy;
std::sort(sorted.begin(), sorted.end());
const double ref = sorted[sorted.size() / 2];
std::vector<std::uint8_t> out(nframes);
for (std::size_t f = 0; f < nframes; ++f) {
const double r = std::log10((energy[f] + kEnergyEps) / (ref + kEnergyEps));
out[f] = static_cast<std::uint8_t>(((peak[f] & 0x1F) << 2) |
(energy_class(r) & 0x03));
}
return out;
}
/// TRACES: IR-004, IR-008
std::optional<std::string> signature_from_mono(const std::vector<float>& mono) {
const std::vector<std::uint8_t> packed = pack_frames(mono);
if (packed.empty()) return std::nullopt;
return std::string(kVersionPrefix) + base64_encode(packed.data(), packed.size());
}
/// TRACES: IR-004, IR-007
std::optional<std::vector<float>> decode_centre_window(const std::string& path) {
av_log_set_level(AV_LOG_ERROR);
DecodeCtx c;
if (avformat_open_input(&c.fmt, path.c_str(), nullptr, nullptr) < 0)
return std::nullopt;
if (avformat_find_stream_info(c.fmt, nullptr) < 0) return std::nullopt;
if (c.fmt->duration == AV_NOPTS_VALUE) return std::nullopt;
const double duration = static_cast<double>(c.fmt->duration) / AV_TIME_BASE;
// IR-007 — the window underflows, so there is no signature and no sync
// offset downstream. The plugin applies the identical rule.
if (duration < kWindowSec) return std::nullopt;
const int idx = av_find_best_stream(c.fmt, AVMEDIA_TYPE_AUDIO, -1, -1, nullptr, 0);
if (idx < 0) return std::nullopt; // no audio → no signature
AVStream* st = c.fmt->streams[idx];
const AVCodec* codec = avcodec_find_decoder(st->codecpar->codec_id);
if (!codec) return std::nullopt;
c.dec = avcodec_alloc_context3(codec);
if (!c.dec) return std::nullopt;
if (avcodec_parameters_to_context(c.dec, st->codecpar) < 0) return std::nullopt;
c.dec->thread_count = 0;
if (avcodec_open2(c.dec, codec, nullptr) < 0) return std::nullopt;
const double start_sec = duration / 2.0 - kWindowSec / 2.0;
// Seek to a packet at or before the window start; the exact start is then
// reached by discarding the leading output samples, which is what
// `ffmpeg -ss <t> -i <file>` does and therefore what the plugin sees.
if (start_sec > 0.0) {
const int64_t tgt = av_rescale_q(
static_cast<int64_t>(start_sec * AV_TIME_BASE), AV_TIME_BASE_Q, st->time_base);
if (av_seek_frame(c.fmt, idx, tgt, AVSEEK_FLAG_BACKWARD) >= 0)
avcodec_flush_buffers(c.dec);
}
c.frm = av_frame_alloc();
c.pkt = av_packet_alloc();
if (!c.frm || !c.pkt) return std::nullopt;
std::vector<float> mono;
mono.reserve(kWindowSamples + kSampleRate);
std::size_t to_skip = 0;
bool have_swr = false;
int in_rate = kSampleRate;
bool eof = false;
while (mono.size() < kWindowSamples && !eof) {
const int rr = av_read_frame(c.fmt, c.pkt);
if (rr < 0) {
eof = true;
avcodec_send_packet(c.dec, nullptr); // flush the decoder
} else if (c.pkt->stream_index != idx) {
av_packet_unref(c.pkt);
continue;
} else {
avcodec_send_packet(c.dec, c.pkt);
av_packet_unref(c.pkt);
}
while (avcodec_receive_frame(c.dec, c.frm) == 0) {
if (!have_swr) {
if (!open_resampler(c, c.frm)) return std::nullopt;
have_swr = true;
in_rate = c.frm->sample_rate ? c.frm->sample_rate : kSampleRate;
int64_t pts = c.frm->best_effort_timestamp;
if (pts == AV_NOPTS_VALUE) pts = c.frm->pts;
const double t0 = (pts == AV_NOPTS_VALUE)
? start_sec : av_q2d(st->time_base) * static_cast<double>(pts);
const double lead = start_sec - t0;
to_skip = lead > 0.0
? static_cast<std::size_t>(std::llround(lead * kSampleRate)) : 0;
}
drain(c.swr, c.frm, in_rate, to_skip, mono);
av_frame_unref(c.frm);
if (mono.size() >= kWindowSamples) break;
}
}
if (have_swr && mono.size() < kWindowSamples)
drain(c.swr, nullptr, in_rate, to_skip, mono); // flush the resampler
if (mono.empty()) return std::nullopt;
// Truncate to exactly 120.000 s so the frame count is 1288 for every input
// and does not wobble with seek granularity or the resampler tail.
if (mono.size() > kWindowSamples) mono.resize(kWindowSamples);
return mono;
}
/// TRACES: IR-004, IR-005, IR-007, IR-008
std::optional<std::string> compute_signature(const std::string& path) {
const std::optional<std::vector<float>> mono = decode_centre_window(path);
if (!mono) return std::nullopt;
return signature_from_mono(*mono);
}
} // namespace sae::audio