feat: add vocabulary constants and tokenize/detokenize to tokenizer.py

Adds VOCAB (81 tokens), TOKEN_TO_ID, and ID_TO_TOKEN per spec §5.2.
tokenize_period() transposes to C/Am then emits BOS + metadata tokens +
per-bar chord/HOLD/NC tokens + BAR + EOS.  detokenize_to_period() is the
exact inverse, returning a ChordPeriod in canonical key.  The m(add9)
quality maps to QUAL_m_add9 in the vocab (parentheses not valid in token
names) via _qual_token/_token_qual helpers.

36 new tests cover vocabulary integrity, token sequence structure,
and full round-trip fidelity for all four valid fixture files.

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

src/tokenizer.py

@@ -1,10 +1,17 @@
-"""Parser and canonical transposer for .chord files.
+"""Parser, transposer, and tokenizer for .chord files.
 
-Public API (token-ID conversion will be added in the next step):
+Public API:
     parse_chord_file(path: Path) -> ChordPeriod
     transpose_to_canonical(period: ChordPeriod) -> ChordPeriod
+    tokenize_period(period: ChordPeriod) -> list[int]
+    detokenize_to_period(token_ids: list[int]) -> ChordPeriod
 
-See docs/chord_format_spec.md for the format specification.
+Vocabulary constants:
+    VOCAB       -- 81-token ordered list; index == token ID
+    TOKEN_TO_ID -- {token_string: id}
+    ID_TO_TOKEN -- alias for VOCAB
+
+See docs/chord_format_spec.md §5.2 for the vocabulary specification.
 """
 
 from __future__ import annotations
@@ -69,6 +76,44 @@ _VALID_FUNCTIONS: frozenset[str] = frozenset({
 })
 
 
+# ---------------------------------------------------------------------------
+# Token vocabulary  (§5.2)
+# ---------------------------------------------------------------------------
+
+VOCAB: list[str] = [
+    # Special (4)
+    "<BOS>", "<EOS>", "<PAD>", "<UNK>",
+    # Mode (2)
+    "MODE_major", "MODE_minor",
+    # Time signature (5)
+    "TIME_4/4", "TIME_3/4", "TIME_6/8", "TIME_2/4", "TIME_12/8",
+    # Subdivision (2)
+    "SUB_4", "SUB_8",
+    # Style (5)
+    "STYLE_user", "STYLE_jpop", "STYLE_classical", "STYLE_jazz", "STYLE_other",
+    # Function (9)
+    "FUNC_verse", "FUNC_prechorus", "FUNC_chorus", "FUNC_bridge",
+    "FUNC_intro", "FUNC_outro", "FUNC_interlude", "FUNC_other", "FUNC_unspecified",
+    # Chord root — 12 pitch classes, sharps only (12)
+    "ROOT_C", "ROOT_C#", "ROOT_D", "ROOT_D#", "ROOT_E", "ROOT_F",
+    "ROOT_F#", "ROOT_G", "ROOT_G#", "ROOT_A", "ROOT_A#", "ROOT_B",
+    # Chord quality (18)
+    "QUAL_maj", "QUAL_m", "QUAL_dim", "QUAL_aug", "QUAL_sus2", "QUAL_sus4",
+    "QUAL_maj7", "QUAL_m7", "QUAL_7", "QUAL_m7b5", "QUAL_dim7", "QUAL_mM7",
+    "QUAL_7sus4", "QUAL_aug7", "QUAL_6", "QUAL_m6", "QUAL_add9", "QUAL_m_add9",
+    # Extension (8)
+    "EXT_none", "EXT_9", "EXT_b9", "EXT_#9", "EXT_11", "EXT_#11", "EXT_13", "EXT_b13",
+    # Bass note — 'root' sentinel + 12 pitch classes (13)
+    "BASS_root", "BASS_C", "BASS_C#", "BASS_D", "BASS_D#", "BASS_E", "BASS_F",
+    "BASS_F#", "BASS_G", "BASS_G#", "BASS_A", "BASS_A#", "BASS_B",
+    # Structural (3)
+    "HOLD", "NC", "BAR",
+]
+
+TOKEN_TO_ID: dict[str, int] = {tok: i for i, tok in enumerate(VOCAB)}
+ID_TO_TOKEN: list[str] = VOCAB
+
+
 # ---------------------------------------------------------------------------
 # Internal helpers
 # ---------------------------------------------------------------------------
@@ -124,6 +169,17 @@ def _transpose_symbol(symbol: str, shift: int, fname: str, bar_no: int) -> str:
     return _tokens_to_symbol(ChordTokens(new_root, t.quality, t.extension, new_bass))
 
 
+def _qual_token(quality: str) -> str:
+    """Map canonical quality string → QUAL_x token name."""
+    return "QUAL_m_add9" if quality == "m(add9)" else f"QUAL_{quality}"
+
+
+def _token_qual(token: str) -> str:
+    """Map QUAL_x token name → canonical quality string."""
+    suffix = token[5:]  # strip "QUAL_"
+    return "m(add9)" if suffix == "m_add9" else suffix
+
+
 # ---------------------------------------------------------------------------
 # Public API
 # ---------------------------------------------------------------------------
@@ -277,3 +333,141 @@ def transpose_to_canonical(period: ChordPeriod) -> ChordPeriod:
 
     canonical_key = "C_major" if mode == "major" else "A_minor"
     return replace(period, key=canonical_key, bars=new_bars)
+
+
+def tokenize_period(period: ChordPeriod) -> list[int]:
+    """Transpose a period to canonical key and encode it as a token ID sequence.
+
+    Args:
+        period: A ChordPeriod as returned by parse_chord_file.
+
+    Returns:
+        List of integer token IDs: <BOS>, metadata tokens, per-bar chord
+        tokens interleaved with HOLD/NC, each bar closed by BAR, then <EOS>.
+
+    Raises:
+        ChordFormatError: If a chord symbol cannot be parsed during transposition.
+    """
+    p = transpose_to_canonical(period)
+    mode = "major" if p.key == "C_major" else "minor"
+
+    ids: list[int] = [TOKEN_TO_ID["<BOS>"]]
+    ids.append(TOKEN_TO_ID[f"MODE_{mode}"])
+    ids.append(TOKEN_TO_ID[f"TIME_{p.time}"])
+    ids.append(TOKEN_TO_ID[f"SUB_{p.subdivision}"])
+    ids.append(TOKEN_TO_ID[f"STYLE_{p.style}"])
+    ids.append(TOKEN_TO_ID[f"FUNC_{p.function}"])
+
+    for bar in p.bars:
+        for pos in bar:
+            if pos == ".":
+                ids.append(TOKEN_TO_ID["HOLD"])
+            elif pos == "NC":
+                ids.append(TOKEN_TO_ID["NC"])
+            elif pos == "?":
+                ids.append(TOKEN_TO_ID["<UNK>"])
+            else:
+                t = parse_chord_symbol(pos)
+                ids.append(TOKEN_TO_ID[f"ROOT_{t.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_{t.bass}"])
+        ids.append(TOKEN_TO_ID["BAR"])
+
+    ids.append(TOKEN_TO_ID["<EOS>"])
+    return ids
+
+
+def detokenize_to_period(token_ids: list[int]) -> ChordPeriod:
+    """Convert a token ID sequence back to a ChordPeriod in canonical key (C/Am).
+
+    Args:
+        token_ids: Sequence produced by tokenize_period.
+
+    Returns:
+        ChordPeriod with key='C_major' or 'A_minor', title='detokenized'.
+
+    Raises:
+        ChordFormatError: If the sequence is structurally malformed.
+    """
+    tokens = [ID_TO_TOKEN[i] for i in token_ids]
+    n = len(tokens)
+    idx = 0
+
+    def _consume(prefix: str) -> str:
+        nonlocal idx
+        if idx >= n:
+            raise ChordFormatError(
+                f"unexpected end of token sequence; expected '{prefix}...'"
+            )
+        tok = tokens[idx]
+        if not tok.startswith(prefix):
+            raise ChordFormatError(
+                f"expected token starting with '{prefix}', got {tok!r} at position {idx}"
+            )
+        idx += 1
+        return tok[len(prefix):]
+
+    if not tokens or tokens[0] != "<BOS>":
+        got = repr(tokens[0]) if tokens else "empty sequence"
+        raise ChordFormatError(f"token sequence must start with <BOS>, got {got}")
+    idx += 1
+
+    mode = _consume("MODE_")
+    time = _consume("TIME_")
+    subdivision = int(_consume("SUB_"))
+    style = _consume("STYLE_")
+    function = _consume("FUNC_")
+
+    key = "C_major" if mode == "major" else "A_minor"
+
+    bars: list[list[str]] = []
+    current_bar: list[str] = []
+
+    while idx < n:
+        tok = tokens[idx]
+        idx += 1
+
+        if tok == "<EOS>":
+            break
+        elif tok == "BAR":
+            bars.append(current_bar)
+            current_bar = []
+        elif tok == "HOLD":
+            current_bar.append(".")
+        elif tok == "NC":
+            current_bar.append("NC")
+        elif tok == "<UNK>":
+            current_bar.append("?")
+        elif tok.startswith("ROOT_"):
+            if idx + 3 > n:
+                raise ChordFormatError(
+                    "incomplete chord token group near end of sequence"
+                )
+            qual_tok = tokens[idx]; idx += 1
+            ext_tok = tokens[idx]; idx += 1
+            bass_tok = tokens[idx]; idx += 1
+            root = tok[5:]                          # strip "ROOT_"
+            quality = _token_qual(qual_tok)
+            extension = ext_tok[4:]                 # strip "EXT_"
+            bass = bass_tok[5:]                     # strip "BASS_"
+            current_bar.append(
+                _tokens_to_symbol(ChordTokens(root, quality, extension, bass))
+            )
+        else:
+            raise ChordFormatError(f"unexpected token in bar body: {tok!r}")
+
+    if current_bar:
+        raise ChordFormatError(
+            "token sequence ended without closing BAR before <EOS>"
+        )
+
+    return ChordPeriod(
+        title="detokenized",
+        key=key,
+        time=time,
+        subdivision=subdivision,
+        style=style,
+        function=function,
+        bars=bars,
+    )

tests/test_tokenizer.py

@@ -0,0 +1,210 @@
+"""Round-trip tests for tokenize_period / detokenize_to_period in src/tokenizer.py.
+
+Fixture files used (all from tests/fixtures/):
+  valid_c_major.chord     — already canonical; 8 bars, chorus, has F/A slash chord
+  valid_fsharp_major.chord — F# major → C major (shift=6); 4 bars
+  valid_b_minor.chord     — B minor → A minor (shift=10); 4 bars
+  valid_gsharp_minor.chord — G# minor → A minor (shift=1); 4 bars
+"""
+
+from pathlib import Path
+
+import pytest
+
+from src.chord_parser import ChordTokens, parse_chord_symbol
+from src.tokenizer import (
+    ID_TO_TOKEN,
+    TOKEN_TO_ID,
+    VOCAB,
+    ChordPeriod,
+    detokenize_to_period,
+    parse_chord_file,
+    tokenize_period,
+    transpose_to_canonical,
+)
+
+FIXTURES = Path(__file__).parent / "fixtures"
+
+VALID_FIXTURES = [
+    "valid_c_major.chord",
+    "valid_fsharp_major.chord",
+    "valid_b_minor.chord",
+    "valid_gsharp_minor.chord",
+]
+
+
+# ---------------------------------------------------------------------------
+# Vocabulary
+# ---------------------------------------------------------------------------
+
+
+class TestVocabulary:
+    def test_vocab_has_81_tokens(self):
+        assert len(VOCAB) == 81
+
+    def test_no_duplicate_tokens(self):
+        assert len(set(VOCAB)) == 81
+
+    def test_token_to_id_covers_all_vocab(self):
+        assert len(TOKEN_TO_ID) == 81
+
+    def test_id_to_token_covers_all_vocab(self):
+        assert len(ID_TO_TOKEN) == 81
+
+    def test_ids_are_contiguous_from_zero(self):
+        for i, tok in enumerate(VOCAB):
+            assert TOKEN_TO_ID[tok] == i
+
+    def test_id_to_token_is_inverse_of_token_to_id(self):
+        for i, tok in enumerate(VOCAB):
+            assert ID_TO_TOKEN[i] == tok
+
+    def test_special_tokens_at_front(self):
+        assert VOCAB[:4] == ["<BOS>", "<EOS>", "<PAD>", "<UNK>"]
+
+    def test_structural_tokens_at_end(self):
+        assert VOCAB[-3:] == ["HOLD", "NC", "BAR"]
+
+    def test_all_roots_present(self):
+        for note in ("C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"):
+            assert f"ROOT_{note}" in TOKEN_TO_ID
+
+    def test_all_qualities_present(self):
+        for qual_tok in (
+            "QUAL_maj", "QUAL_m", "QUAL_dim", "QUAL_aug",
+            "QUAL_sus2", "QUAL_sus4", "QUAL_maj7", "QUAL_m7",
+            "QUAL_7", "QUAL_m7b5", "QUAL_dim7", "QUAL_mM7",
+            "QUAL_7sus4", "QUAL_aug7", "QUAL_6", "QUAL_m6",
+            "QUAL_add9", "QUAL_m_add9",
+        ):
+            assert qual_tok in TOKEN_TO_ID
+
+    def test_all_extensions_present(self):
+        for ext in ("none", "9", "b9", "#9", "11", "#11", "13", "b13"):
+            assert f"EXT_{ext}" in TOKEN_TO_ID
+
+    def test_all_bass_notes_present(self):
+        assert "BASS_root" in TOKEN_TO_ID
+        for note in ("C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"):
+            assert f"BASS_{note}" in TOKEN_TO_ID
+
+
+# ---------------------------------------------------------------------------
+# Tokenize structure
+# ---------------------------------------------------------------------------
+
+
+class TestTokenizeStructure:
+    def test_starts_with_bos(self):
+        p = parse_chord_file(FIXTURES / "valid_c_major.chord")
+        assert tokenize_period(p)[0] == TOKEN_TO_ID["<BOS>"]
+
+    def test_ends_with_eos(self):
+        p = parse_chord_file(FIXTURES / "valid_c_major.chord")
+        assert tokenize_period(p)[-1] == TOKEN_TO_ID["<EOS>"]
+
+    def test_metadata_order_after_bos(self):
+        p = parse_chord_file(FIXTURES / "valid_c_major.chord")
+        toks = [ID_TO_TOKEN[i] for i in tokenize_period(p)]
+        assert toks[1] == "MODE_major"
+        assert toks[2] == "TIME_4/4"
+        assert toks[3] == "SUB_4"
+        assert toks[4] == "STYLE_user"
+        assert toks[5] == "FUNC_chorus"
+
+    def test_bar_token_count_matches_bar_count(self):
+        p = parse_chord_file(FIXTURES / "valid_c_major.chord")
+        ids = tokenize_period(p)
+        assert sum(1 for i in ids if i == TOKEN_TO_ID["BAR"]) == len(p.bars)
+
+    def test_minor_period_emits_mode_minor(self):
+        p = parse_chord_file(FIXTURES / "valid_b_minor.chord")
+        toks = [ID_TO_TOKEN[i] for i in tokenize_period(p)]
+        assert toks[1] == "MODE_minor"
+
+    def test_missing_function_emits_func_unspecified(self):
+        p = parse_chord_file(FIXTURES / "valid_b_minor.chord")
+        toks = [ID_TO_TOKEN[i] for i in tokenize_period(p)]
+        assert toks[5] == "FUNC_unspecified"
+
+    def test_all_ids_in_vocab_range(self):
+        for fixture_name in VALID_FIXTURES:
+            p = parse_chord_file(FIXTURES / fixture_name)
+            assert all(0 <= i < 81 for i in tokenize_period(p))
+
+    def test_non_canonical_key_transposed_before_encoding(self):
+        # F# major: first chord F#maj7 → Cmaj7 after shift=6; ROOT_C is at index 6.
+        p = parse_chord_file(FIXTURES / "valid_fsharp_major.chord")
+        ids = tokenize_period(p)
+        assert ID_TO_TOKEN[ids[6]] == "ROOT_C"
+
+
+# ---------------------------------------------------------------------------
+# Round-trip
+# ---------------------------------------------------------------------------
+
+
+class TestRoundTrip:
+    @pytest.mark.parametrize("fixture_name", VALID_FIXTURES)
+    def test_chord_symbols_survive_round_trip(self, fixture_name):
+        p = parse_chord_file(FIXTURES / fixture_name)
+        canonical = transpose_to_canonical(p)
+        recovered = detokenize_to_period(tokenize_period(p))
+
+        assert len(recovered.bars) == len(canonical.bars)
+        for bar_c, bar_r in zip(canonical.bars, recovered.bars):
+            assert len(bar_c) == len(bar_r)
+            for sym_c, sym_r in zip(bar_c, bar_r):
+                if sym_c in (".", "NC", "?"):
+                    assert sym_r == sym_c
+                else:
+                    assert parse_chord_symbol(sym_r) == parse_chord_symbol(sym_c)
+
+    @pytest.mark.parametrize("fixture_name", VALID_FIXTURES)
+    def test_metadata_survives_round_trip(self, fixture_name):
+        p = parse_chord_file(FIXTURES / fixture_name)
+        canonical = transpose_to_canonical(p)
+        recovered = detokenize_to_period(tokenize_period(p))
+
+        assert recovered.key == canonical.key
+        assert recovered.time == canonical.time
+        assert recovered.subdivision == canonical.subdivision
+        assert recovered.style == canonical.style
+        assert recovered.function == canonical.function
+
+    @pytest.mark.parametrize("fixture_name", VALID_FIXTURES)
+    def test_bar_count_survives_round_trip(self, fixture_name):
+        p = parse_chord_file(FIXTURES / fixture_name)
+        recovered = detokenize_to_period(tokenize_period(p))
+        assert len(recovered.bars) == len(p.bars)
+
+    def test_c_major_chord_identity(self):
+        # C major is already canonical — chord tokens must reproduce exactly.
+        p = parse_chord_file(FIXTURES / "valid_c_major.chord")
+        recovered = detokenize_to_period(tokenize_period(p))
+        assert recovered.key == "C_major"
+        for bar_p, bar_r in zip(p.bars, recovered.bars):
+            for sym_p, sym_r in zip(bar_p, bar_r):
+                if sym_p not in (".", "NC", "?"):
+                    assert parse_chord_symbol(sym_r) == parse_chord_symbol(sym_p)
+
+    def test_slash_chord_root_and_bass_survive(self):
+        # F/A in C major: root=F maj, bass=A must survive the round-trip.
+        p = parse_chord_file(FIXTURES / "valid_c_major.chord")
+        recovered = detokenize_to_period(tokenize_period(p))
+        t = parse_chord_symbol(recovered.bars[2][0])
+        assert t == ChordTokens("F", "maj", "none", "A")
+
+    def test_fsharp_major_tonic_becomes_c_after_round_trip(self):
+        p = parse_chord_file(FIXTURES / "valid_fsharp_major.chord")
+        recovered = detokenize_to_period(tokenize_period(p))
+        t = parse_chord_symbol(recovered.bars[0][0])
+        assert t.root == "C"
+        assert t.quality == "maj7"
+
+    def test_b_minor_tonic_becomes_am_after_round_trip(self):
+        p = parse_chord_file(FIXTURES / "valid_b_minor.chord")
+        recovered = detokenize_to_period(tokenize_period(p))
+        t = parse_chord_symbol(recovered.bars[0][0])
+        assert t.root == "A"
+        assert t.quality == "m"