FA-80965 / Music interval and transposition theory / Member archive

Key signature size for a named tonic: signature folded through pitch class · case 05

Gb major reports six sharps and C# major reports one sharp: enharmonic keys collapse onto one pitch class.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input [tonic, mode] with tonic a letter optionally followed by one # or b, and mode major or minor. The signature is the letter position on the line of fifths (F -1, C 0, G 1, D 2, A 3, E 4, B 5) plus 7 per sharp and minus 7 per flat; minor keys use the relative major, three fifths flatward. Return the count (positive sharps, negative flats) or None if it lies outside -7..7 or the input is invalid.

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	`-7`	`5`	Failed

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