FA-80892 / Music interval and transposition theory / Member archive

Interval quality and number naming: generic number counts letter gaps instead of letters · case 02

C4 to E4 is named AA2 instead of M3, because every generic number comes out one too small.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input [lower, upper] spelled pitches (letter, # or b accidentals, nonnegative octave). The generic number counts letter steps plus 7 per spelled octave plus one; quality compares the semitone span with the major/perfect reference (compounds add 12 per octave). Perfect class for simple numbers 1, 4, 5: P, A, d, AA, dd; others: M, m, A, d, AA, dd. Descending letter order or unrepresentable quality returns None.

Why this case matters

Pitch spelling and interval arithmetic decide the notes that notation, transposition and analysis tools display or play.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
oracle 0	`"AA2"`	`"M3"`	Failed

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