# T-006: `synesthesia-core` Crate Scaffold — Shared Constants, Types, Module Structure

**Type:** Task  
**Phase:** 0 — Foundation  
**Autonomy:** `agent:autonomous` — Pure Rust library, no external systems, no decisions required.  
**Stack:** `stack:rust`  
**Version:** v0.1  
**Iteration:** iter-1  
**Effort:** XS (1 hour)

---

> ⚠️ **Agent Scope:** Populate `crates/synesthesia-core/` with the canonical shared constants, a handful of foundational types, and stub module declarations. Do **not** implement any audio processing, model inference, or IPC logic here — those live in `kansas/` and `plugin/`. This crate is a library. Its job in v0.1 is to be the single source of truth for values that both targets must agree on.

---

## Context

T-001 created `crates/synesthesia-core/src/lib.rs` as a comment-only stub. T-005 added it as a dep to `plugin/`. T-006 gives it actual content.

The core problem this crate solves: `kansas/` and `plugin/` are separate compilation units. If `kansas/` defines `SAMPLE_RATE = 48_000` locally and `plugin/` defines it separately, they will inevitably diverge. `synesthesia-core` is the single place these constants live — both crates import from here, so a change propagates everywhere.

Beyond constants, this crate will eventually hold the generator traits, shared note event types, and latent vector types that the plugin's `process()` loop and the Tauri app's model inference loop both need to speak. T-006 lays the module skeleton so future tasks know where to add their types.

---

## Prerequisites

- [ ] T-001 merged — `crates/synesthesia-core/` exists with empty `lib.rs`
- [ ] `cargo check --workspace` passes before starting

---

## Acceptance Criteria

- [ ] `cargo check --workspace` — zero errors
- [ ] `cargo clippy --workspace -- -D warnings` — zero warnings
- [ ] `SAMPLE_RATE`, `CHANNELS`, `PPQN` are publicly exported from `synesthesia_core::audio`
- [ ] `NoteEvent`, `PitchClass` are publicly exported from `synesthesia_core::music`
- [ ] `LatentVector`, `MoodDescriptor` are publicly exported from `synesthesia_core::models`
- [ ] All types derive `Debug`, `Clone`, `PartialEq`; numeric newtype wrappers also derive `Copy`
- [ ] `pub use` re-exports at crate root mean `synesthesia_core::SAMPLE_RATE` works without importing submodules
- [ ] No `#[allow(dead_code)]` suppressions — if something is unused, the doc comment explains what T-NNN will use it
- [ ] `kansas/src/audio/engine.rs` local `SAMPLE_RATE` and `CHANNELS` constants are replaced with imports from this crate

---

## `crates/synesthesia-core/Cargo.toml`

```toml
[package]
name    = "synesthesia-core"
version = "0.1.0"
edition.workspace = true

[dependencies]
# T-006 has no runtime deps — pure Rust types and constants only.
# serde added here when T-046+ needs to serialize model inputs across the IPC boundary.
```

---

## File Structure After This Task

```
crates/synesthesia-core/src/
├── lib.rs       ← module declarations + pub use re-exports
├── audio.rs     ← SAMPLE_RATE, CHANNELS, PPQN; AudioConstants
├── music.rs     ← NoteEvent, PitchClass, PITCH_CLASS_NAMES, pitch_to_freq()
└── models.rs    ← LatentVector, MoodDescriptor, GenerationMode
```

---

## Implementation

### `crates/synesthesia-core/src/lib.rs`

```rust
//! # synesthesia-core
//!
//! Shared constants, types, and traits used by both the Tauri standalone app
//! (`kansas/`) and the nih-plug VST3/CLAP plugin (`plugin/`).
//!
//! ## Module layout
//!
//! - [`audio`]  — sample rate, channel count, buffer constants
//! - [`music`]  — note events, pitch classes, music theory primitives
//! - [`models`] — shared types for ML model inputs and outputs

pub mod audio;
pub mod music;
pub mod models;

// ── Convenience re-exports from crate root ────────────────────────────────────
// Allows `synesthesia_core::SAMPLE_RATE` without importing submodules.

pub use audio::{CHANNELS, PPQN, SAMPLE_RATE};
pub use models::{GenerationMode, LatentVector, MoodDescriptor};
pub use music::{PitchClass, NoteEvent, PITCH_CLASS_NAMES};
```

### `crates/synesthesia-core/src/audio.rs`

```rust
//! Audio system constants shared between the standalone app and the plugin.
//!
//! These values are the single source of truth. Do not redeclare them locally
//! in `kansas/` or `plugin/` — import from here.

/// System sample rate in Hz. Chosen to match Magenta RT's SpectroStream codec (48 kHz stereo).
/// All CPAL streams (T-004) and nih-plug processing (T-005+) target this rate.
pub const SAMPLE_RATE: u32 = 48_000;

/// Number of audio channels. Stereo throughout the system.
pub const CHANNELS: u16 = 2;

/// Pulses per quarter note — MIDI standard resolution used by the transport clock (T-003)
/// and Perf RNN tick scheduling (T-063).
pub const PPQN: u32 = 96;

/// Default audio output buffer size in frames at SAMPLE_RATE.
/// 512 frames @ 48 kHz ≈ 10.67 ms. The VST plugin inherits the DAW's buffer size instead.
pub const DEFAULT_BUFFER_FRAMES: u32 = 512;

/// Minimum and maximum supported BPM for the transport clock.
pub const BPM_MIN: f32 = 20.0;
pub const BPM_MAX: f32 = 300.0;

/// Compute tick interval in microseconds for a given BPM.
///
/// # Example
/// ```
/// use synesthesia_core::audio::tick_interval_us;
/// let us = tick_interval_us(128.0); // ≈ 4883 μs at 128 BPM
/// assert!((us - 4882.8).abs() < 1.0);
/// ```
#[inline]
pub fn tick_interval_us(bpm: f32) -> f32 {
    60_000_000.0 / bpm / PPQN as f32
}
```

### `crates/synesthesia-core/src/music.rs`

```rust
//! Music theory primitives shared between the generative engines and the UI.

/// A single MIDI-style note event produced by Performance RNN (T-062) and
/// consumed by the MIDI output (T-068) and nih-plug plugin (T-069).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct NoteEvent {
    /// MIDI pitch 0–127. Middle C = 60.
    pub pitch: u8,
    /// MIDI velocity 1–127. 0 = note-off.
    pub velocity: u8,
    /// MIDI channel 0–15.
    pub channel: u8,
    /// Position in transport ticks (PPQN resolution) from session start.
    pub tick: u64,
    /// Duration in ticks. 0 = instantaneous (gate closed immediately).
    pub duration_ticks: u32,
}

impl NoteEvent {
    /// True if this is a note-off event (velocity == 0).
    #[inline]
    pub fn is_note_off(&self) -> bool {
        self.velocity == 0
    }

    /// Convert pitch to frequency in Hz using equal temperament (A4 = 440 Hz).
    #[inline]
    pub fn frequency(&self) -> f32 {
        pitch_to_freq(self.pitch)
    }
}

/// The 12 pitch classes of the chromatic scale.
/// Used by the 12-dimensional pitch intelligence radar (UI, T-058)
/// and Steam Audio spatial positioning (T-100).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
#[repr(u8)]
pub enum PitchClass {
    C = 0, Cs, D, Ds, E, F, Fs, G, Gs, A, As, B,
}

impl PitchClass {
    /// Extract pitch class from a MIDI pitch number.
    #[inline]
    pub fn from_midi(pitch: u8) -> Self {
        match pitch % 12 {
            0  => Self::C,  1  => Self::Cs, 2  => Self::D,
            3  => Self::Ds, 4  => Self::E,  5  => Self::F,
            6  => Self::Fs, 7  => Self::G,  8  => Self::Gs,
            9  => Self::A,  10 => Self::As, _  => Self::B,
        }
    }

    /// Index 0–11.
    #[inline]
    pub fn index(self) -> usize {
        self as usize
    }
}

/// Human-readable pitch class names aligned with `PitchClass` index.
/// `PITCH_CLASS_NAMES[PitchClass::Fs as usize]` == `"F#"`.
pub const PITCH_CLASS_NAMES: [&str; 12] = [
    "C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B",
];

/// Convert MIDI pitch to frequency in Hz (equal temperament, A4 = 440 Hz).
#[inline]
pub fn pitch_to_freq(pitch: u8) -> f32 {
    440.0 * 2.0_f32.powf((pitch as f32 - 69.0) / 12.0)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn middle_c_freq() {
        let f = pitch_to_freq(60);
        assert!((f - 261.63).abs() < 0.1, "Middle C should be ~261.63 Hz, got {f}");
    }

    #[test]
    fn a4_freq() {
        let f = pitch_to_freq(69);
        assert!((f - 440.0).abs() < 0.01, "A4 should be 440 Hz, got {f}");
    }

    #[test]
    fn pitch_class_from_midi() {
        assert_eq!(PitchClass::from_midi(60), PitchClass::C);
        assert_eq!(PitchClass::from_midi(69), PitchClass::A);
        assert_eq!(PitchClass::from_midi(127), PitchClass::G);
    }
}
```

### `crates/synesthesia-core/src/models.rs`

```rust
//! Shared types for ML model inputs and outputs.
//!
//! These are the Rust-native equivalents of the IPC types in `kansas/src/ipc/types.rs`.
//! The IPC types are serialized to JSON for the Tauri frontend.
//! These types are used directly in Rust — no serialization overhead.

/// MusicVAE 512-dimensional latent vector.
/// Produced by the encoder (T-054) and consumed by the decoder (T-055)
/// and Track Mixer latent navigation (T-057).
#[derive(Debug, Clone, PartialEq)]
pub struct LatentVector(pub [f32; 512]);

impl LatentVector {
    /// The zero vector — corresponds to the VAE prior mean (silence/root).
    pub fn zero() -> Self {
        Self([0.0; 512])
    }

    /// Linear interpolation between two latent vectors.
    /// `t = 0.0` returns `self`, `t = 1.0` returns `other`.
    pub fn lerp(&self, other: &Self, t: f32) -> Self {
        let t = t.clamp(0.0, 1.0);
        let mut out = [0.0f32; 512];
        for i in 0..512 {
            out[i] = self.0[i] * (1.0 - t) + other.0[i] * t;
        }
        Self(out)
    }
}

/// Semantic mood state output from Gemma-3N visual analysis (T-052).
/// Rust-native mirror of `MoodState` in `kansas/src/ipc/types.rs`.
#[derive(Debug, Clone, PartialEq)]
pub struct MoodDescriptor {
    /// Natural language mood label e.g. "mysterious", "energetic", "serene".
    pub mood: String,
    /// Energy level 0.0–1.0.
    pub energy: f32,
    /// Timbral texture label e.g. "dense", "sparse", "glitchy".
    pub texture: String,
}

impl Default for MoodDescriptor {
    fn default() -> Self {
        Self {
            mood: "neutral".into(),
            energy: 0.5,
            texture: "balanced".into(),
        }
    }
}

/// Which generation engine is active on a channel.
/// Used by the plugin's process() loop and the standalone app's mix bus router.
/// T-032 (Perf RNN VST), T-065 (standalone), T-089 (Magenta RT) set this per channel.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum GenerationMode {
    /// No generation — channel produces silence.
    #[default]
    Silent,
    /// Performance RNN MIDI generation (T-062).
    PerfRnn,
    /// Magenta RT continuous audio generation (T-081).
    MagentaRt,
    /// DDSP timbre transfer on Perf RNN MIDI output (T-092).
    Ddsp,
    /// Audio passthrough — used by the T-005 scaffold and test paths.
    Passthrough,
}
```

---

## Update `kansas/src/audio/engine.rs`

Once T-006 is merged, replace the local constants in `engine.rs` (added by T-004) with imports:

```rust
// In kansas/src/audio/engine.rs — REPLACE these local definitions:
// const SAMPLE_RATE: u32 = 48_000;
// const CHANNELS: u16 = 2;
// const BUFFER_FRAMES: u32 = 512;

// WITH:
use synesthesia_core::{SAMPLE_RATE, CHANNELS};
use synesthesia_core::audio::{DEFAULT_BUFFER_FRAMES as BUFFER_FRAMES};
```

If T-004 and T-006 are merged in the same PR or T-006 merges first, apply this change directly. If T-004 merges before T-006, open a follow-up commit on `main` after T-006 merges.

Similarly, replace the local `PPQN` constant in `kansas/src/commands/transport.rs` (T-003):

```rust
// REPLACE:
// const PPQN: u32 = 96;

// WITH:
use synesthesia_core::PPQN;
```

---

## Testing

### Automated
- [ ] `cargo check --workspace` — zero errors
- [ ] `cargo clippy --workspace -- -D warnings` — zero warnings
- [ ] `cargo test -p synesthesia-core` — `middle_c_freq`, `a4_freq`, `pitch_class_from_midi` all pass

### Manual
```bash
# Verify both consumers compile against it
cargo check -p kansas
cargo check -p synesthesia-plugin
```

---

## GitHub CLI

```bash
gh issue create \
  --title "T-006: synesthesia-core crate scaffold — shared constants, types, module structure" \
  --label "type:task,stack:rust,agent:autonomous,priority:high,status:ready,day:1" \
  --body-file T-006.md
```

---

**Parent:** GENESIS  
**Blocks:** T-003 constant migration (`PPQN`), T-004 constant migration (`SAMPLE_RATE`, `CHANNELS`), T-057 (`LatentVector` lerp), T-062 (`NoteEvent` from Perf RNN), T-068 (`NoteEvent` to MIDI out), T-100 (`PitchClass` icosahedron positions)  
**Blocked By:** T-001  
**Version:** v0.1 · **Iteration:** iter-1 · **Effort:** XS (1 hour)