v0.7.0 — the quality release

glint v0.7.0 — the quality release

124 commits since v0.6.0. This release takes glint from "~15 dB SNR at every
bitrate" (a single quantizer-curve bug) to measurably ahead of LAME on 8 of
10 test clips at 128 kbps (PEAQ ODG) and ahead on SNR across the board at
256 kbps — while cutting embedded RAM to ~64 KB, one third less than
Shine. Every claim below is measured; the full experiment log, including
every dead end, lives in PLAN.md.

Headline numbers (vs LAME 3.100 -q 2, identical inputs)

128 kbps joint stereo, PEAQ ODG (closer to 0 is better):

clip	glint	LAME		clip	glint	LAME
choir	−1.28	−2.06		piano	−0.63	−0.70
orchestral	−0.75	−1.09		torture	−0.32	−0.45
quartet	−0.46	−0.88		speech	−1.24	−1.29
industrial	−0.82	−1.00		electronic	−0.25	−0.22
drums	−1.18	−1.03		castanets	−0.62	−0.29

256 kbps joint: everything ODG-transparent; glint ahead on SNR throughout
(speech 38.4 vs 36.9 dB, electronic 44.8 / NMR −18.0 vs 44.5 / −15.8, quartet
44.9 vs 46.0 with NMR −13.7 vs −11.2). Castanets mean noise-to-mask is
glint's at both rates. 64 kbps stereo: glint ahead on ODG (−3.17 vs −3.32).
MPEG-2 22.05 kHz @ 64k: 21.6 dB vs LAME's 17.6.

Correctness fixes (wire format & math)

• pow34 quantizer curve: the old integer-grid table degenerated to
  identity on (0,1) — quantizer inputs are fractional — capping the whole
  encoder at ~15 dB SNR independent of bitrate. Root cause of everything.
• sfb21 out-of-bounds scalefactor read: a phantom ~29× HF boost with no
  decoder-side counterpart; fixing it was worth +8..17 dB SNR on music.
• MPEG-2 scalefac_compress: glint used an invented mapping — internally
  consistent, garbage for every real decoder. Now ISO 13818-3.
• Short-block scalefactor band tables: the 44.1 kHz table had an
  off-by-two boundary (138 vs ISO's 136), and the MPEG-2 tables were MPEG-1
  copies that scrambled the wire order — the real cause of the historical
  LSF short-block collapse.
• preemphasis[20]: 3 → 2 per ISO (desynced band 20 whenever preflag
  was set).
• LSF start/stop region boundary: decoders hardwire region0 = 54 lines
  (not 36) for LSF transition granules.
• VBR budget bugs: quantized under the caller's default frame budget
  instead of the max; assumed the CBR padding byte and overflowed the
  largest unpadded frame.
• Small-buffer frame overflow: a 320 kbps frame silently overflowed the
  1024-byte embedded frame buffer; buffers are now sized for every legal
  frame and glint_create rejects what cannot fit.

New encoder machinery

• Bit reservoir + rate control (CBR): continuous main-data stream with
  deferred emission, buffer-feedback constant-quality anchor, post-transient
  banking (joint mode). Worth +1..2.6 dB at all rates.
• Short blocks: start/stop transition windows, per-frame scheduler with
  one-granule lookahead, per-window subblock_gain, full per-(band,window)
  short scalefactors, 6× attack threshold, 2-granule attack-decay extension.
  Live for MPEG-1 and MPEG-2/LSF, on both signal paths (fixed-point
  now matches the double path to ≤0.06 dB on transients).
• Psychoacoustic allocation: Schroeder-Bark masking aligned with the
  measurement metric drives scalefactor noise-shaping loops for long and
  short granules, shaping ~9 dB below the mask under a total-noise guard,
  with tonality-adaptive masker offsets at ≤96 kbps/channel and
  scalefac_scale escalation. Runs in the VBR path too.
• Bitrate-scaled encoder lowpass with content-aware sfb21 keep at high
  rates: the no-scalefactor sfb21 region is zeroed where it would only
  collect quantizer spray, kept where it holds real content.
• Optimal Huffman region splits on finished granules (exhaustive
  region0/region1 search over per-table prefix costs); real per-region table
  selection by actual bit count; preflag folding.
• VBR: real variable-size frames, psy shaping, Xing header with seek
  TOC and gapless LAME tag — players show correct duration, seek, and trim
  the codec delay sample-exactly (ffmpeg decodes at a 0-sample offset). New
  API: glint_vbr_header().

Embedded

• GLINT_MODE=fixed RAM: ~64 KB (Shine: ~96 KB), measured as static/BSS
  • encoder context. Achieved via a mantissa cube-root table replacing a
    64 KB LUT (metrics-identical), single-slot per-samplerate model caches,
    float transition tables, and dead-code removal.
• Fixed path now has short blocks, transition windows, and the encoder
  lookahead — embedded output quality equals desktop.

Performance

Three profile-driven optimization passes (byte-identical where claimed):
Huffman pair-cost LUTs, fused region/table selection, gain-search state
reuse, MDCT table fusion, -O3/LTO, and an opt-in threaded scale-factor
search (-j N, byte-identical for any thread count). Encoder speed ~260× /
52× / 34× realtime (speed/normal/best, Apple M1, 256 kbps stereo).

Testing & tooling

• GitHub Actions CI: build matrix (double/fixed/both × Linux/macOS),
  unit tests, decode-based tests, full quality suite.
• Decode-based MPEG-2 regression test in ctest — the class of wire bug
  that unit tests provably missed.
• Encoder league harness (tests/compare_encoders.py): glint vs LAME vs
  Shine over a 10-clip lossless corpus with SNR, Bark-band NMR, PEAQ ODG,
  ViSQOL, PESQ and STOI, plus a regression gate (--check) against recorded
  baselines.
• 10-clip corpus: speech, four music genres from lossless sources
  (incl. 24-bit/96 kHz CC0 piano and CC-BY choir), synthetic castanets and a
  deterministic torture clip (generators in tests/).
• ABX listening tool (tests/abx.py): aligned, loudness-normalized,
  double-blind, binomial p-value.

Behavior changes

• Encoder latency is now 1104 samples (filterbank chain + one-granule
  lookahead) on both paths; VBR streams start with a placeholder frame that
  glint_vbr_header() finalizes (unrewritten streams decode with ~26 ms of
  leading silence).
• Content above the rate-dependent lowpass (15.8 kHz at ≥128 kbps/ch,
  44.1 kHz) is dropped unless the content-aware keep retains it.
• 320 kbps at 32 kHz and similar corner configs are rejected by
  glint_create in small-buffer builds only if they cannot fit (none of the
  legal ones are rejected anymore).