# Design: reggaeton-composer

## Technical Approach

Extend the existing `SongDefinition → FLPBuilder` pipeline with a new selection +
melodic-generation layer. `SampleSelector` wraps `sample_index.json`; melodic
generators produce `{notes, sample_path}` dicts; `SongDefinition` gains a
`melodic_tracks` field; `FLPBuilder` appends audio-clip channels after existing
drum channels.

---

## Architecture Decisions

| Decision | Choice | Rejected | Rationale |
|----------|--------|----------|-----------|
| Selector load strategy | Load full index once at `__init__` | Lazy / streaming | 862 entries ≈ 50 MB max; random access needed for scoring |
| Note format | Reuse `{"pos","len","key","vel"}` from `rhythm.py` | New format | Already converted by `_convert_rhythm_notes`; zero friction |
| Schema extension | Add `melodic_tracks: list[MelodicTrack]` optional field | Separate class | Single source of truth; existing `validate()` extended, not replaced |
| Channel numbering | Melodic starts at ch 17 | Dynamic | `skeleton.py` already defines `EMPTY_SAMPLER_CHANNELS = {17,18,19}`; expanding naturally |
| Melodic channel type | `ChType = 21` → value `0` (sampler) | AudioClip type 1 | Reference FLP uses sampler channels for .wav; `skeleton.py` already patches them via `melodic_map` param (already present in `load()` signature) |
| Builder integration | `_build_melodic_channels()` inserted between channel_bytes and arrangement | New Builder subclass | Minimal diff; builder is not subclassed anywhere |
| CLI | `scripts/compose_track.py` using `argparse` | Click | Existing scripts use plain argparse |

---

## Data Flow

```
data/sample_index.json
        │
        ▼
  SampleSelector.select(role, key, bpm, character, is_tonal)
        │  list[SampleEntry]
        ▼
  melodic.py generators
  generate_bass / generate_lead / generate_chords / generate_pad
        │  MelodicTrackDef {role, sample_path, notes[], channel_hint}
        ▼
  SongDefinition (extended)
  .melodic_tracks: list[MelodicTrack]
        │
        ▼
  FLPBuilder.build(song)
        ├── _build_header          (unchanged)
        ├── _build_all_patterns    (unchanged)
        ├── ChannelSkeletonLoader  (pass melodic_map → already supported)
        ├── _build_melodic_notes() (NEW — PatNotes for ch 17+)
        └── _build_arrangement     (unchanged)
        │
        ▼
  output/my_track.flp
```

---

## File Changes

| File | Action | Description |
|------|--------|-------------|
| `src/selector/__init__.py` | Create | `SampleSelector` class |
| `src/composer/melodic.py` | Create | `generate_bass/lead/chords/pad` |
| `src/flp_builder/schema.py` | Modify | Add `MelodicTrack`, `Note` dataclasses; extend `SongDefinition`; extend `validate()` and `from_json()` |
| `src/flp_builder/builder.py` | Modify | Add `_build_melodic_notes()`; pass `melodic_map` to `ChannelSkeletonLoader.load()` |
| `scripts/compose_track.py` | Create | CLI entry point |

---

## Interfaces / Contracts

```python
# src/selector/__init__.py
@dataclass
class SampleEntry:
    path: str          # original_path (absolute)
    role: str
    key: str | None
    bpm: float         # 0 = unknown
    character: str
    is_tonal: bool
    score: float = 0.0

class SampleSelector:
    def __init__(self, index_path: str | Path): ...
    def select(
        self,
        role: str,
        key: str | None = None,
        bpm: float | None = None,
        character: str | None = None,
        is_tonal: bool | None = None,
        limit: int = 10,
    ) -> list[SampleEntry]: ...

# src/composer/melodic.py
@dataclass
class MelodicTrackDef:
    role: str                        # "bass" | "lead" | "chords" | "pad"
    sample_path: str
    notes: list[dict]                # {"pos","len","key","vel"}
    channel_hint: int                # suggested channel index (17+)

def generate_bass(key, scale, bpm, bars, kick_pattern=None, density=0.7) -> MelodicTrackDef
def generate_lead(key, scale, bpm, bars, density=0.5) -> MelodicTrackDef
def generate_chords(key, scale, bpm, bars, progression=None, voicing="closed") -> MelodicTrackDef
def generate_pad(key, scale, bpm, bars) -> MelodicTrackDef

# src/flp_builder/schema.py (additions)
_KNOWN_ROLES = frozenset({"bass","lead","chords","pad","arp","fx"})

@dataclass
class Note:                   # alias for PatternNote — same fields
    pos: float
    len: float
    key: int
    vel: int

@dataclass
class MelodicTrack:
    role: str
    sample_path: str
    notes: list[Note]
    channel_index: int        # >= 17
    volume: float = 0.78      # 0.0–1.0
    pan: float = 0.0          # -1.0–1.0

# SongDefinition gets:
melodic_tracks: list[MelodicTrack] = field(default_factory=list)
```

---

## Key Scoring Logic

```python
# Key compatibility (circle-of-fifths aware)
COMPAT = {
    "exact":        1.0,
    "relative":     0.8,   # Am ↔ C, Gm ↔ Bb
    "dominant":     0.6,   # Am → Em
    "subdominant":  0.6,   # Am → Dm
    "parallel":     0.5,   # Am ↔ A
}

# Character groups (fuzzy matching)
CHAR_GROUPS = [
    {"warm","soft","lush"},
    {"boomy","deep"},
    {"sharp","crisp","bright"},
    {"aggressive","dark"},
]

# Combined score = key_score * 0.5 + bpm_score * 0.3 + char_score * 0.2
```

---

## Tresillo / Dembow Bass Pattern

```
Bar positions (8 bars × 4 beats = 32 beats):
Tresillo: [0, 0.75, 1.5] per bar  (3+3+2 in 8th-note grid)
Kick-avoidance: skip notes within ±0.125 beats of a kick hit
```

---

## Validation Extensions

`SongDefinition.validate()` additions:
1. `melodic_track.role in _KNOWN_ROLES`
2. `Path(melodic_track.sample_path).exists()`
3. `melodic_track.channel_index >= 17`
4. No duplicate `channel_index` across melodic tracks

---

## Error Handling Strategy

| Layer | Strategy |
|-------|----------|
| `SampleSelector.select()` | Returns `[]` on no match — callers must check; never raises on empty |
| `generate_*` functions | Raise `ValueError` if `selector.select()` returns empty (required sample missing) |
| `SongDefinition.validate()` | Accumulate all errors, raise `ValueError` with full list |
| `FLPBuilder.build()` | Propagates `FileNotFoundError` if `sample_path` missing at write time |
| CLI | `sys.exit(1)` with human-readable message on any `ValueError` / `FileNotFoundError` |

---

## Testing Strategy

| Layer | What | Approach |
|-------|------|----------|
| Unit | `SampleSelector` scoring | Fixture with 5 hand-crafted entries; assert rank order |
| Unit | `generate_bass` tresillo | Assert note positions match `[0, 0.75, 1.5]` per bar |
| Unit | `MelodicTrack` schema validation | `validate()` returns expected errors for bad inputs |
| Integration | `FLPBuilder` with melodic tracks | Build to bytes, parse header, assert `num_channels == 17 + n` |
| Smoke | CLI end-to-end | `compose_track.py --key Am --bpm 95 --output /tmp/test.flp`; assert file exists and size > 0 |

---

## Open Questions

- [ ] `skeleton.py` `EMPTY_SAMPLER_CHANNELS` includes `{17,18,19}` — need to confirm that adding melodic channels beyond 19 doesn't require reference FLP changes or if we expand the sampler clone range.
- [ ] `sample_index.json` entries use `original_path` (absolute Windows paths). CLI on other machines will break. Decision needed: embed relative paths or make selector rebase paths against a configurable `library_root`.
