Span

TECHNICAL_GUIDE.md

@@ -138,7 +138,7 @@ All three parts use octave-transposing notation: written notes sound one octave
 
 `pitch.py` resolves explicit MusicXML instrument transposition once before part generation. Explicit zero transposition is also authoritative. Octave-clef exports already encode sounding pitch in their note values, so they do not receive another implicit octave subtraction. Generated parts carry an explicit `transpose.octave_change: -1`; guitar placement and audio read that shared state. Internal concert-pitch normalization and written-pitch conversion cancel for ordinary source notation except for the intentional song electric melody octave increase. There is no automatic octave fitting, vocal-specific range shift, or key transposition.
 
-`fretboard.py` owns standard tuning E2 A2 D3 G3 B3 E4, fret positions, chord coverage, and pitch spelling. Both placement systems use frets 1-20 inclusive, exclude open strings, and share a maximum fret difference of three. `guitar.py` handles electric voicing and exact/held melody placement. Exact melody tones are never dropped or replaced, and sustained/tied tones retain their assignments. `acoustic_voicing.py` instead searches four named voices: a bass on string 5 or 6, and one voice on each of strings 4, 3, and 2. Acoustic assignments also require at least one occupied string at fret 12 or below. These are hard constraints: a failed voicing is not widened or reduced to two upper strings. Any defined chord tone, including thirds, sevenths, and extensions, can be the acoustic bass. Explicit slash basses and inversions override these choices; the bass remains the lowest sounding pitch.
+`fretboard.py` owns standard tuning E2 A2 D3 G3 B3 E4, fret positions, chord coverage, and pitch spelling. Both placement systems use frets 1-20 inclusive, exclude open strings, and require `max(frets) - min(frets) + 1 <= 3`. The inclusive limit accepts frets 1-3 and rejects 1-4; it applies to complete grips, staggered acoustic voices, and overlapping or tied notes. Complete acoustic and electric chords also require at least one occupied fret in 1-12, checked before candidate pruning. Other chord notes may exceed fret 12 within the span limit. `guitar.py` handles electric voicing and exact/held melody placement. Exact melody tones are never dropped or replaced, sustained/tied tones retain their assignments, and unharmonized melody notes are exempt from the anchor rule. `acoustic_voicing.py` instead searches four named voices: a bass on string 5 or 6, and one voice on each of strings 4, 3, and 2. These are hard constraints: a failed voicing is not widened or reduced to two upper strings. Any defined chord tone, including thirds, sevenths, and extensions, can be the acoustic bass. Explicit slash basses and inversions override these choices; the bass remains the lowest sounding pitch.
 
 Shared `ChordRequirements` separate mandatory pitch classes, allowed pitch classes, root-or-fifth alternatives, and bass constraints. Every complete generated guitar grip must contain the root, the fifth, or both, with no preference for the root itself. This rule applies equally to acoustic accompaniment, electric accompaniment, and electric melody chords, not isolated bass/upper attacks or unharmonized melody notes. The fifth follows the chord's actual degree, including diminished or augmented fifths. Explicitly removed degrees are never restored, and a harmony removing both root and fifth cannot generate a guitar chord. Defining thirds/suspensions, sixths/sevenths, alterations, and required extensions remain mandatory; an altered fifth cannot be omitted merely because the root is present.
 
@@ -156,7 +156,7 @@ Acoustic candidates are generated from chord-tone positions on the four voices,
 
 Acoustic transition cost is the sum of absolute fret changes for all four named voices, plus one when the bass switches between strings 5 and 6. For example, bass string 5 fret 7 to string 6 fret 9 costs three steps, not just a string-change charge. An unchanged assignment costs zero. Exact dynamic programming minimizes total travel per iteration; equal travel costs prefer fewer missing optional chord tones, followed by stable position ordering. The starting assignment has no position cost. Harmony spans, not rhythmic attacks, determine the transitions. There are no acoustic register, distinct-fret, span, or physical-reach scoring terms. The three-fret limit and fret-12 anchor are eligibility checks, not preferences.
 
-Electric effort remains `highest_fret + 2 * max(0, highest_fret - 12) + 2 * max(0, distinct_frets - 1) + 3 * fret_span`, with Manhattan movement between doubled fret/string position centers. Its small-grid distance transform and exact melody constraints are unchanged. Unharmonized melody placement retains `movement + 2 * max(0, highest_fret - 12) + fret_span`, and pure tie continuations add no fresh shape or movement cost. The acoustic Autumn Leaves lead-in can use Em7b5/G at `6:3, 4:2, 3:3, 2:3` followed by A7b9/G at `6:3, 4:2, 3:3, 2:2`, moving only the second string by one fret. This is a regression example, not a prescribed chord shape; generated bass inversions do not rewrite source chord labels.
+Electric chord effort is `2 * max(0, distinct_frets - 1) + 3 * fret_span`, plus Manhattan movement between doubled fret/string position centers. Here `fret_span` remains `max(frets) - min(frets)` for scoring; eligibility counts fret positions inclusively. Neither the highest fret nor crossing fret 12 adds a chord cost. The small-grid distance transform and exact melody constraints are unchanged. Unharmonized melody placement retains `movement + 2 * max(0, highest_fret - 12) + fret_span`, and pure tie continuations add no fresh shape or movement cost. The acoustic Autumn Leaves lead-in can use Em7b5/G at `6:3, 4:2, 3:3, 2:3` followed by A7b9/G at `6:3, 4:2, 3:3, 2:2`, moving only the second string by one fret. This is a regression example, not a prescribed chord shape; generated bass inversions do not rewrite source chord labels.
 
 There is no finger assignment, barre logic, or predefined chord-shape dictionary. These are proximity and fretboard-feasibility constraints, not a guarantee of comfortable fingering. Nonstandard tuning, capos, microtonal frets, and unplayable passages fail with source/part/event context; there is no slash fallback for a failed chord. Existing assets remain untouched after failure.