Fixed acoustic

TECHNICAL_GUIDE.md

@@ -135,7 +135,7 @@ 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.
 
-`guitar.py` searches standard tuning E2 A2 D3 G3 B3 E4, frets 1-20 inclusive, with at most four frets between the lowest and highest positions. Open strings are excluded from chords and single-note melodies alike. A pitch that only an open string could produce fails generation rather than being octave-shifted or omitted. Exact melody tones are never dropped or replaced. Simultaneous tones occupy distinct strings, and sustained/tied tones retain their assignments. Acoustic grips use one bass on string 5 or 6 and two or three consecutive strings within 2-4, with zero or one unused string between bass and upper voices. Both rhythmic voices must fit one complete grip, even when only one attacks. Each continuous acoustic harmony span keeps the same bass pitch, upper voicing, and string/fret positions. Root-bass and fifth-bass candidates compete equally; fifth choices are not merely fallbacks. Explicit acoustic slash basses and inversions override the default bass choices. There is no alternating-bass substitution.
+`guitar.py` searches standard tuning E2 A2 D3 G3 B3 E4, frets 1-20 inclusive, with at most four frets between the lowest and highest positions. Open strings are excluded from chords and single-note melodies alike. A pitch that only an open string could produce fails generation rather than being octave-shifted or omitted. Exact melody tones are never dropped or replaced. Simultaneous tones occupy distinct strings, and sustained/tied tones retain their assignments. Acoustic grips use one bass on string 5 or 6 and two or three consecutive strings within 2-4, with zero or one unused string between bass and upper voices. Both rhythmic voices must fit one complete grip, even when only one attacks. Each continuous acoustic harmony span keeps the same bass pitch, upper voicing, and string/fret positions. Any defined chord tone can serve as the acoustic bass, including thirds, sevenths, and extensions. Explicit acoustic slash basses and inversions override these choices, and the bass remains the lowest sounding pitch. There is no alternating-bass substitution.
 
 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.
 
@@ -145,13 +145,15 @@ Electric chord blocks use strings 1-3 or 1-4, including string 1. Rootless voici
 
 `melody.py` treats the full tied duration as one note when testing whether it exceeds one quarter-note beat. Qualifying notes receive a chord at their initial attack and at each subsequent changed harmony, even if a resulting fragment is less than one beat. Changed harmonies break ties and reattack the melody; repeated identical harmonies do not. N.C. releases the added voices while preserving the melody's sustain. Added tones through unchanged harmony retain their own ties. These melody rules are independent of accompaniment duration spelling. If no valid chord fits, the exact melody remains with a diagnostic; it is not octave-wrapped or omitted.
 
-Placement uses the ordered `VoicingCost` components: electric string shortfall from four, melody covered, number of tones above the melody, combined physical effort, and missing optional pitch classes. These costs accumulate over the whole passage, so physical effort cannot outweigh the earlier electric musical priorities. Melody height compares sounding pitch; unison doubling does not cover it. Electric accompaniment uses the same ranking without the melody-height terms. Acoustic does not receive electric fullness penalties: a simpler three-note grip can beat a more awkward four-note grip. Shape difficulty competes directly with fret height and movement, not as a later tie-breaker.
+Placement uses the ordered `VoicingCost` components: electric string shortfall from four, melody covered, number of tones above the melody, combined effort, and missing optional pitch classes. These costs accumulate over the whole passage, so effort cannot outweigh the earlier electric musical priorities. Melody height compares sounding pitch; unison doubling does not cover it. Electric accompaniment uses the same ranking without the melody-height terms. Acoustic does not receive electric fullness penalties: a simpler three-note grip can beat a more awkward four-note grip.
+
+Acoustic candidate selection separates harmonic validity from reach and sequence scoring. `physical_reach()` computes `round(1_000_000 * (2 ** (-lowest_fret / 12) - 2 ** (-highest_fret / 12)))`, normalized to scale length. The preferred limit is the reach from fret 7 to fret 10, inclusive. If any valid grip meets it, wider grips are excluded before movement optimization. Otherwise only the least-reaching valid grips remain, with a passage-specific warning. The hard four-fret difference and frets 1-20 still apply. Among the retained candidates, triads with exactly degrees 1, 3, and 5 prefer all three tones when feasible. Authored omissions are never restored; seventh, extended, and suspended chords retain their existing coverage rules. Complete triads do not justify exceeding the preferred reach when a reduced comfortable grip exists. Candidate filtering is cached independently of rhythmic events and does not alter electric placement.
 
 `electric.py` links song accompaniment attacks to harmonized melody events using authored source positions, not displayed measure numbers or a song-wide chord dictionary. References stay inside the corresponding source harmony span and must match its structured chord. Within a span, an attack uses the latest available melody grip at or before its source position, or the first later grip if it precedes all melody chords. This allows different shapes for the same chord in different passages. After guitar placement and melody tie resolution, linked stabs copy the actual melody pitches and TAB positions while retaining their own IDs, timing, and written durations; melody ties and markings are not copied. A melody that remains a single note provides no grip. Passages without a matching playable three- or four-string melody chord, including altered endings without a corresponding reference, retain independently generated electric shapes. No reference metadata reaches runtime JSON.
 
-Both generated chord profiles use the same shape-and-height cost: `highest_fret + 2 * max(0, highest_fret - 12) + 3 * fret_span + 2 * max(0, distinct_frets - 1)`. Electric adds its existing movement metric, the Manhattan distance between doubled fret/string position centers, with the exact small-grid distance transform used for independent attacks. Acoustic adds twice its per-string movement: fret changes on retained strings plus one unit per added or released string. The doubled weight balances transitions against shape difficulty so a lower isolated grip does not justify a large jump. An unchanged grip costs zero movement. Exact candidate-to-candidate dynamic programming retains all acoustic grips, including different grips with equal centers. Exact, unharmonized melody placement retains `movement + 2 * max(0, highest_fret - 12) + fret_span`.
+After candidate filtering, acoustic effort is `highest_fret + 2 * max(0, highest_fret - 12) + 2 * max(0, distinct_frets - 1)`. There is no raw-span penalty or continuing reward for smaller physical reach within the preferred range. Electric retains the same formula plus `3 * fret_span` and its existing movement metric, the Manhattan distance between doubled fret/string position centers, with the exact small-grid distance transform used for independent attacks. Acoustic still adds twice its per-string movement: fret changes on retained strings plus one unit per added or released string. An unchanged grip costs zero movement. Exact candidate-to-candidate dynamic programming retains all preferred acoustic grips, including different grips with equal centers. Exact, unharmonized melody placement retains `movement + 2 * max(0, highest_fret - 12) + fret_span`.
 
-Generated chord height contributes one point per fret throughout the allowed range. The shared `PREFERRED_MAX_FRET` remains 12, above which each excess fret adds two more points. Compact mid-neck grips can beat stretched lower ones, while lower positions still win when shape difficulty and movement are comparable. Autumn Leaves begins with Em7b5 at `5:7, 4:8, 3:7, 2:8` and A7b9 at `5:7, 4:8, 3:6, 2:8`; the nearby C7 at `5:10, 3:9, 2:11` remains preferred after Gm7 at `5:10, 3:10, 2:11`. These are regression examples, not a song-specific shape dictionary. Acoustic shape costs are charged once per harmony span, independent of rhythm density, and transitions compare complete grips rather than whichever lane last sounded. Pure melody tie continuations add no fresh shape or movement cost. There is no automatic octave fitting or finger-assignment model. Stable position ordering resolves equal costs.
+Generated chord height contributes one point per fret throughout the allowed range. The shared `PREFERRED_MAX_FRET` remains 12, above which each excess fret adds two more points. Lower positions still win when reach eligibility, shape simplicity, and movement are comparable. Autumn Leaves 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 one note by one fret. The original mid-neck grips at `5:7, 4:8, 3:7, 2:8` and `5:7, 4:8, 3:6, 2:8` remain valid nearby alternatives. Dm/F at `5:8, 3:7, 2:10` meets the preferred reach limit exactly. These are regression examples, not mandatory song-specific shapes, and generated inversions do not rewrite the source chord labels. Acoustic shape costs are charged once per harmony span, independent of rhythm density, and transitions compare complete grips rather than whichever lane last sounded. Pure melody tie continuations add no fresh shape or movement cost. There is no automatic octave fitting or finger-assignment model. Stable position ordering resolves equal costs.
 
 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.