Chord Families

generator/fretboard.py

@@ -12,7 +12,8 @@ STANDARD_TUNING = ((6, 40), (5, 45), (4, 50), (3, 55), (2, 59), (1, 64))
 MIN_FRET = 1
 MAX_FRET = 20
 MAX_SINGLE_FRET = 12
-MAX_FRET_SPAN = 3
+PREFERRED_FRET_SPAN = 3
+MAX_FRET_SPAN = 4
 MAX_ANCHOR_FRET = 12
 WRITTEN_GUITAR_OCTAVE_OFFSET = 12
 
@@ -21,10 +22,17 @@ class TabPosition(typing.NamedTuple):
   fret: int
   midi: int
 
-def within_fret_span(positions) -> bool:
-  """Count occupied fret positions inclusively: frets 1-3 fit, 1-4 do not."""
+def fret_span(positions) -> int:
+  """Inclusive occupied positions: frets 7-9 span three, 7-10 span four."""
   frets = [position.fret for position in positions]
-  return not frets or max(frets) - min(frets) + 1 <= MAX_FRET_SPAN
+  return max(frets) - min(frets) + 1 if frets else 0
+
+def span_penalty(positions) -> int:
+  """Two tiers for legal grips; all one-to-three-fret shapes share tier zero."""
+  return int(fret_span(positions) > PREFERRED_FRET_SPAN)
+
+def within_fret_span(positions) -> bool:
+  return fret_span(positions) <= MAX_FRET_SPAN
 
 def has_fret_anchor(positions) -> bool:
   return any(MIN_FRET <= position.fret <= MAX_ANCHOR_FRET for position in positions)