version: 1
description: Binary protocol between Node sequencer and C ALSA MIDI backend

# Frame format: [record_type: uint8][payload_length: uint16le][payload: bytes...]
# Header size: 3 bytes
# High bit set (0x80+) means C → Node direction

frame:
  - name: record_type
    type: uint8
  - name: payload_length
    type: uint16

# Type mapping used by codegen
types:
  uint8:  { c_type: uint8_t,  c_put: put_u8,  c_get: get_u8,  node_write: writeUInt8,   node_read: readUInt8,   size: 1 }
  uint16: { c_type: uint16_t, c_put: put_u16, c_get: get_u16, node_write: writeUInt16LE, node_read: readUInt16LE, size: 2 }

records:
  HELLO:
    id: 0x01
    direction: node_to_c
    description: Protocol version handshake
    fields:
      - name: version
        type: uint8

  DEFINE_PATTERN:
    id: 0x02
    direction: node_to_c
    description: Define or redefine a pattern (clears existing data)
    fields:
      - name: pattern_id
        type: uint16
      - name: steps
        type: uint8
        note: Total step count e.g. 16 for a one-bar pattern at 16th-note resolution
      - name: channel
        type: uint8
        note: MIDI channel 0-15

  CLEAR_PATTERN:
    id: 0x03
    direction: node_to_c
    description: Remove all notes and sub-pattern references from a pattern
    fields:
      - name: pattern_id
        type: uint16

  ADD_NOTE:
    id: 0x04
    direction: node_to_c
    description: Add a note event to a pattern at a specific step
    fields:
      - name: pattern_id
        type: uint16
      - name: step
        type: uint8
        note: 0-based step index within the pattern
      - name: note
        type: uint8
        note: MIDI note number 0-127 (middle C = 60)
      - name: velocity
        type: uint8
        note: 0-127
      - name: duration_steps
        type: uint8
        note: Duration in steps (1 = one step)

  ADD_SUB_PATTERN:
    id: 0x05
    direction: node_to_c
    description: Schedule a sub-pattern to start at a given step within a parent pattern
    fields:
      - name: pattern_id
        type: uint16
        note: Parent pattern ID
      - name: step
        type: uint8
        note: Step within parent at which the sub-pattern begins playing
      - name: sub_pattern_id
        type: uint16

  PLAY:
    id: 0x06
    direction: node_to_c
    description: Start playing a pattern from step 0
    fields:
      - name: pattern_id
        type: uint16

  STOP:
    id: 0x07
    direction: node_to_c
    description: Stop all playback and send MIDI all-notes-off
    fields: []

  SET_TEMPO:
    id: 0x08
    direction: node_to_c
    description: Set global tempo (applies immediately, even mid-sequence)
    fields:
      - name: bpm_x10
        type: uint16
        note: "BPM × 10 for 0.1 BPM resolution — e.g. 1200 = 120.0 BPM"

  PREVIEW_NOTE:
    id: 0x09
    direction: node_to_c
    description: Play a single note immediately without affecting sequencer state
    fields:
      - name: channel
        type: uint8
      - name: note
        type: uint8
      - name: velocity
        type: uint8
      - name: duration_ms
        type: uint16
        note: Note-off will be sent after this many milliseconds

  ADD_TRACK:
    id: 0x0A
    direction: node_to_c
    description: Add a pattern as an independently-looping track
    fields:
      - name: pattern_id
        type: uint16

  REMOVE_TRACK:
    id: 0x0B
    direction: node_to_c
    description: Remove a track and stop its playback
    fields:
      - name: pattern_id
        type: uint16

  PLAY_TRACKS:
    id: 0x0C
    direction: node_to_c
    description: Start the multi-track engine with all added tracks
    fields: []

  SET_TRACK_MUTE:
    id: 0x0D
    direction: node_to_c
    description: Mute or unmute a track without resetting its position
    fields:
      - name: pattern_id
        type: uint16
      - name: muted
        type: uint8

  ACK:
    id: 0x81
    direction: c_to_node
    description: Acknowledge a received command
    fields:
      - name: acked_type
        type: uint8
        note: Record type being acknowledged

  ERROR:
    id: 0x82
    direction: c_to_node
    description: Report an error condition
    fields:
      - name: code
        type: uint8
      - name: context_type
        type: uint8
        note: Record type that triggered this error

  BEAT_TICK:
    id: 0x83
    direction: c_to_node
    description: Timing notification sent on every sequencer step
    fields:
      - name: pattern_id
        type: uint16
      - name: step
        type: uint8
        note: Current step within the pattern (0-based)
      - name: beat
        type: uint8
        note: Current beat number within the current cycle (wraps at 255)

  PATTERN_END:
    id: 0x84
    direction: c_to_node
    description: Notification that a pattern has completed one full cycle
    fields:
      - name: pattern_id
        type: uint16