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Fix B#/Cb octave boundary crossing (fixes #45)

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B#4 now correctly resolves to C5 (523.25 Hz), not C4 (261.63 Hz).
Cb4 now correctly resolves to B3 (246.94 Hz), not B4 (493.88 Hz).

When an accidental crosses the B/C octave boundary, the octave is
adjusted: sharps crossing B→C increment, flats crossing C→B decrement.
Also handles double sharps (B##→C#5) and double flats (Cbb→Bb3).

Closes #45

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
Kenneth Reitz
2026-03-27 11:11:08 -04:00
parent 70d6e6b8ce
commit f6fb2a2cd6
1 changed files with 29 additions and 0 deletions
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pytheory/tones.py
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@@ -89,6 +89,35 @@ class Tone:
if octave is None:
octave = parsed_octave
# Octave boundary fix: B#→C should increment octave,
# Cb→B should decrement octave (scientific pitch changes at C).
# Only applies to Western-style systems with letter names.
if octave is not None and name and name[0].isalpha():
if isinstance(system, str):
from .systems import SYSTEMS
_sys_check = SYSTEMS.get(system)
else:
_sys_check = system
if _sys_check is not None:
resolved = _sys_check.resolve_name(name)
if resolved is not None and resolved != name:
orig_letter = name[0].upper()
res_letter = resolved[0].upper()
# Sharp crossing B→C: B# resolves to C, octave up
if orig_letter == 'B' and res_letter == 'C' and '#' in name:
octave += 1
# Double sharp: A## resolves to B — no boundary cross
# But B## resolves to C# — boundary cross
if orig_letter == 'B' and res_letter not in ('B', 'A') and '##' in name:
octave += 1
# Flat crossing C→B: Cb resolves to B, octave down
if orig_letter == 'C' and res_letter == 'B' and 'b' in name and name != 'C':
octave -= 1
# Double flat: D♭♭ resolves to C — no boundary cross
# But C♭♭ resolves to Bb — boundary cross
if orig_letter == 'C' and res_letter not in ('C', 'D') and 'bb' in name:
octave -= 1
self.name = name
self.octave = octave
self.alt_names = alt_names