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d09f52e | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 | """Cadence classification into DCML cadence codes.
music21 ships no cadence detector (only building blocks), so this is ours. It
looks at the last two chords of an Analysis sequence and classifies the terminal
cadence:
PAC perfect authentic V(7) -> I/i, both root position
IAC imperfect authentic V(7) -> I/i, dominant or tonic inverted
HC half phrase ends on a root-position diatonic V *triad*
(a terminal V7 demands resolution and is not an HC)
PC plagal IV/iv -> I/i
DC deceptive root-position V(7) -> vi/VI (an inverted dominant's
bass must resolve to the tonic, so V65 -> vi is a
voice-leading error, not a deceptive cadence)
None no terminal cadence
Simplification: with synthetic root-position chord symbols we have no soprano
voice, so PAC vs IAC is decided by inversion rather than by melodic closure.
"""
from __future__ import annotations
from typing import Optional
from .vocabulary import Analysis
_TONIC = frozenset({"I", "i"})
_DOMINANT = frozenset({"V"})
_SUBDOMINANT = frozenset({"IV", "iv"})
_SUBMEDIANT = frozenset({"vi", "VI"})
# figbass values that denote a root-position chord (triad '' or root seventh '7')
_ROOT_POSITION = frozenset({"", "7"})
def _is_diatonic(a: Analysis) -> bool:
return a.relativeroot is None
def _is_root_position(a: Analysis) -> bool:
return a.figbass in _ROOT_POSITION
def _is(a: Analysis, degrees: frozenset[str]) -> bool:
return _is_diatonic(a) and a.numeral in degrees
def classify_cadence(seq: list[Analysis]) -> Optional[str]:
if len(seq) < 2:
return None
penult, last = seq[-2], seq[-1]
if _is(last, _TONIC):
if _is(penult, _DOMINANT):
return "PAC" if _is_root_position(penult) and _is_root_position(last) else "IAC"
if _is(penult, _SUBDOMINANT):
return "PC"
return None
if _is(last, _SUBMEDIANT) and _is(penult, _DOMINANT) and _is_root_position(penult):
return "DC"
if _is(last, _DOMINANT) and last.figbass == "":
return "HC"
return None
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