"""
FL-MCP Protocol — SysEx encoding/decoding.

Protocol: F0 7D [nibble-encoded UTF-8 JSON] F7
"""

from __future__ import annotations

SYSEX_ID = 0x7D


def nibble_encode(data: bytes) -> list[int]:
    """Split each byte into two nibbles (high, low) for MIDI data byte compliance (< 0x80).

    Each byte 0x00-0xFF is split into two MIDI-safe bytes:
    - high nibble: (byte >> 4) & 0x0F
    - low nibble:  byte & 0x0F
    """
    result: list[int] = []
    for byte in data:
        result.append((byte >> 4) & 0x0F)
        result.append(byte & 0x0F)
    return result


def nibble_decode(nibbles: list[int]) -> bytes:
    """Reconstruct bytes from nibble pairs.

    Each pair (high, low) reconstructs one byte:
    byte = (high << 4) | low
    """
    result = bytearray()
    for i in range(0, len(nibbles), 2):
        if i + 1 < len(nibbles):
            result.append(((nibbles[i] & 0x0F) << 4) | (nibbles[i + 1] & 0x0F))
    return bytes(result)


def encode_command(cmd: str, params: dict | None = None) -> list[int]:
    """Encode a JSON command as a complete SysEx message: F0 7D [nibbles...] F7."""
    import json

    payload = json.dumps({"cmd": cmd, "params": params or {}})
    nibbles = nibble_encode(payload.encode("utf-8"))
    return [0xF0, SYSEX_ID] + nibbles + [0xF7]


def decode_command(data: list[int]) -> dict | None:
    """Decode a SysEx message back to JSON dict.

    Returns None if the message is not a valid FL-MCP SysEx message.
    """
    if not data or len(data) < 4 or data[0] != 0xF0 or data[-1] != 0xF7:
        return None
    if data[1] != SYSEX_ID:
        return None
    import json

    nibbles = data[2:-1]
    if not nibbles:
        return None
    raw = nibble_decode(nibbles)
    if not raw:
        return None
    return json.loads(raw.decode("utf-8"))