feat(rhythm): real 8th-note swing measurement (accuracy PR-B2) - #205
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Replaces the loudness-interval swing proxy (grooveDetail.kickSwing/
hihatSwing) with a real micro-timing measurement. compute_swing_detail
reads swing from the long/short alternation of 8th-note inter-onset
intervals: swingPercent = long/(long+short)*100 on Ableton's Groove Pool
50-75 scale (50 straight, 66.7 full triplet), so it drops straight into a
groove template. Additive rhythmDetail.swingDetail {swingPercent,
swingConfidence, gridResolution, direction, meanAbsOffsetMs,
offbeatOnsetCount}; reuses the onset array already computed in
analyze_rhythm_detail (no extra DSP pass); null when unmeasurable.
Interval-ratio rather than phase-vs-grid is deliberate: on sparse
material the beat tracker sometimes anchors 'the beat' on a loud offbeat
hat, which flips a phase reading (58% -> 42%). The long/short structure
is invariant to that choice — validated by a phase-shift-invariance test
and against the corpus swing clips (truth 50/54/58/62/66 -> measured
50.0/54.8/57.1/61.9/65.9, all within +-1.3, well inside the +-3 gate).
Corpus: swing clips now render a realistic swung hat pattern (straight
on-beat 8th + delayed off-beat 8th) so the interval alternation exists;
swingPercent promoted from manifest truth to an active +-3.0 check on all
5 swing clips (fundamentals_evaluation learns expected.swingPercent).
UI: reconstructionBrief groove sentence prefers the measured percentage
('swings at about 62% - set the Groove Pool swing to match'), falls back
to the proxy; MeasurementDashboard Swing panel shows the Groove-Pool
number.
Contract mirrors (tripwire #4): JSON_SCHEMA.md swingDetail sub-fields;
types/measurement.ts SwingDetail (rhythmDetail passes through wholesale,
no reconstructor); phase1FullPayload fixture; golden re-baselined
(nested, structural). Tests: test_swing_detail (ratios, phase-flip
invariance, degenerate), brief swing-sentence, corpus expected-keys.
Backend 1299 OK, fundamentals gate green, frontend verify green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T4wfz87k6kzJqkKLKZE7YL
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Phase B of the accuracy program: turns swing from a scalar proxy into a real, Groove-Pool-ready number.
What
The old swing signal (
grooveDetail.kickSwing/hihatSwing) is a tanh-normalized beat-loudness-interval variance — a "how loose" hint, not a swing percentage. This addscompute_swing_detail, which measures swing from the long/short alternation of 8th-note inter-onset intervals:swingPercent = long / (long + short) × 100, on Ableton's Groove Pool 50–75 scale (50 = straight, 66.7 = full triplet). AdditiverhythmDetail.swingDetail{swingPercent, swingConfidence, gridResolution, direction, meanAbsOffsetMs, offbeatOnsetCount}, reusing the onset array already computed inanalyze_rhythm_detail(no extra DSP pass),nullwhen unmeasurable.Why interval-ratio, not phase-vs-grid
I first built the obvious phase-against-beat-grid version. It failed on the corpus: the beat tracker, on sparse material, sometimes anchors "the beat" on the loud offbeat hat, which flips a phase reading (58% measured as 42%). The long/short interval structure is invariant to that choice — proven by a phase-shift-invariance test and by the corpus results: truth 50/54/58/62/66 → measured 50.0/54.8/57.1/61.9/65.9, all within ±1.3, well inside the ±3 gate.
Validation
swingPercentis now an active ±3.0 check on all 5 swing clips (the harness learnsexpected.swingPercent; the swing fixtures render a realistic swung hat pattern — straight on-beat 8th + delayed off-beat 8th — so the interval alternation exists). Full synthetic run: 54/54 active + 8 informational,allPassed.User-facing
The Reconstruction Brief groove line now says "swings at about 62% — set the Groove Pool swing to match" (falls back to the proxy when swing is unmeasurable), and the MeasurementDashboard Swing panel shows the Groove-Pool percentage above the old kick/hat bars.
Contract mirrors (tripwire #4)
JSON_SCHEMA.md sub-fields ·
types/measurement.tsSwingDetail(rhythmDetailpasses through wholesale — no reconstructor) ·phase1FullPayloadfixture · golden re-baselined (nested, structural).Tests
test_swing_detail(ratio recovery, phase-flip invariance, degenerate inputs), brief swing-sentence, corpus expected-keys. Backend 1299 OK, fundamentals gate green, frontend verify green (unit + build + 51 smoke).🤖 Generated with Claude Code
https://claude.ai/code/session_01T4wfz87k6kzJqkKLKZE7YL
Generated by Claude Code