fix: support '.' and 'NC' in --prefix argument

_encode_prefix now handles hold ('.') and no-chord ('NC') tokens
alongside chord symbols, and returns (ids, n_positions) so that
pos_in_bar is tracked correctly regardless of token type.

Fixes ChordParseError when dots were passed in --prefix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/generate.py

@@ -145,22 +145,33 @@ def _transpose_to_key(period: ChordPeriod, target_key: str) -> ChordPeriod:
 # Prefix encoding
 # ---------------------------------------------------------------------------
 
-def _encode_prefix(chord_symbols: list[str], shift: int) -> list[int]:
-    """Encode chord symbols (in user key) as canonical body token IDs.
+def _encode_prefix(chord_symbols: list[str], shift: int) -> tuple[list[int], int]:
+    """Encode prefix tokens (chord symbols, '.', 'NC') as canonical body token IDs.
 
     *shift* is the semitone offset from user key to canonical (C/Am).
-    Returns a flat list: ROOT QUAL EXT BASS [ROOT QUAL EXT BASS ...].
+    '.' encodes as a single HOLD token; 'NC' encodes as a single NC token.
+    A chord symbol encodes as four tokens: ROOT QUAL EXT BASS.
+    Each element (chord, hold, or NC) counts as one bar position.
+
+    Returns (ids, n_positions).
     """
     ids: list[int] = []
+    n_positions = 0
     for sym in chord_symbols:
-        t = parse_chord_symbol(sym)
-        root = _transpose_note(t.root, shift)
-        bass = "root" if t.bass == "root" else _transpose_note(t.bass, shift)
-        ids.append(TOKEN_TO_ID[f"ROOT_{root}"])
-        ids.append(TOKEN_TO_ID[_qual_token(t.quality)])
-        ids.append(TOKEN_TO_ID[f"EXT_{t.extension}"])
-        ids.append(TOKEN_TO_ID[f"BASS_{bass}"])
-    return ids
+        if sym == ".":
+            ids.append(_HOLD)
+        elif sym == "NC":
+            ids.append(_NC)
+        else:
+            t = parse_chord_symbol(sym)
+            root = _transpose_note(t.root, shift)
+            bass = "root" if t.bass == "root" else _transpose_note(t.bass, shift)
+            ids.append(TOKEN_TO_ID[f"ROOT_{root}"])
+            ids.append(TOKEN_TO_ID[_qual_token(t.quality)])
+            ids.append(TOKEN_TO_ID[f"EXT_{t.extension}"])
+            ids.append(TOKEN_TO_ID[f"BASS_{bass}"])
+        n_positions += 1
+    return ids, n_positions
 
 
 # ---------------------------------------------------------------------------
@@ -233,14 +244,11 @@ def generate_period(
 
     positions_per_bar = _expected_positions(time, subdivision)
 
-    encoded_prefix: list[int] = []
+    pos_in_bar = 0
     if prefix:
-        encoded_prefix = _encode_prefix(prefix, shift_to_canonical)
+        encoded_prefix, n_prefix_positions = _encode_prefix(prefix, shift_to_canonical)
         ids.extend(encoded_prefix)
-
-    # Compute starting position-in-bar after the prefix
-    # Each chord in prefix = 4 tokens = 1 position
-    pos_in_bar = (len(encoded_prefix) // 4) % positions_per_bar
+        pos_in_bar = n_prefix_positions % positions_per_bar
 
     last_id = ids[-1]
     context_limit = model.max_seq_len - 1  # leave one slot so seq_len never hits max