/**
 * Ti-Pote — Binary UART protocol, TypeScript side.
 *
 * This file MUST stay byte-for-byte compatible with the C++ reference
 * at `apps/robot-hardware/include/protocol_types.h` and the decoder
 * implementation at `apps/robot-hardware/lib/Protocol/`.
 *
 * Frame layout:
 *
 *   ┌────────┬──────┬──────────┬──────────┬─────────────┬──────┐
 *   │ START  │ TYPE │ LENGTH_H │ LENGTH_L │  PAYLOAD    │ CRC8 │
 *   │ 0xAA   │ 1B   │ 1B       │ 1B       │ 0..65535 B  │ 1B   │
 *   └────────┴──────┴──────────┴──────────┴─────────────┴──────┘
 *
 * CRC8: poly=0x07, init=0x00, no reflection, no final XOR.
 * Computed over TYPE + LENGTH_H + LENGTH_L + PAYLOAD.
 */

export const FRAME_START = 0xaa;
export const FRAME_HEADER_SIZE = 4; // START + TYPE + LEN_H + LEN_L
export const FRAME_OVERHEAD = FRAME_HEADER_SIZE + 1; // + CRC
export const MAX_PAYLOAD_SIZE = 1024;

/**
 * Message type codes — keep in sync with `MsgType` in protocol_types.h.
 */
export enum MsgType {
  // Reserved for Phase 2 (not yet implemented)
  AUDIO_UP = 0x01,
  AUDIO_DOWN = 0x02,
  SERVO_CMD = 0x03,
  LED_CMD = 0x04,
  STATUS = 0x05,
  SENSOR_DATA = 0x06,
  CONFIG = 0x07,
  ACK = 0x08,
  IDLE_MODE = 0x09,

  // v0 — display / eyes
  DISPLAY_EMOTION = 0x20,
  DISPLAY_CLEAR = 0x21,

  // v0 — bring-up / diagnostics
  PING = 0xf0,
  PONG = 0xf1,
  LOG = 0xfd,
  ERROR = 0xfe,
}

/**
 * Emotion catalogue — keep in sync with `Emotion` in protocol_types.h
 * and the switch in `lib/Eyes/src/Eyes.cpp`.
 */
export enum Emotion {
  NEUTRAL = 0,
  HAPPY = 1,
  SAD = 2,
  ANGRY = 3,
  SURPRISED = 4,
  SLEEPY = 5,
  WINK = 6,
  LOVE = 7,
  DIZZY = 8,
  DEAD = 9,
}

export interface DecodedFrame {
  type: MsgType;
  payload: Buffer;
}

/**
 * CRC-8 with polynomial 0x07, init 0x00, no reflection, no final XOR.
 */
export function crc8(data: Uint8Array | Buffer, start = 0, end = data.length): number {
  let crc = 0x00;
  for (let i = start; i < end; i++) {
    crc ^= data[i]!;
    for (let b = 0; b < 8; b++) {
      crc = (crc & 0x80) !== 0 ? ((crc << 1) ^ 0x07) & 0xff : (crc << 1) & 0xff;
    }
  }
  return crc;
}

/**
 * Build a complete framed message ready to write to the serial port.
 */
export function encodeFrame(type: MsgType, payload: Buffer = Buffer.alloc(0)): Buffer {
  if (payload.length > MAX_PAYLOAD_SIZE) {
    throw new Error(
      `Payload too large: ${payload.length} > MAX_PAYLOAD_SIZE (${MAX_PAYLOAD_SIZE})`,
    );
  }
  const total = FRAME_OVERHEAD + payload.length;
  const out = Buffer.alloc(total);
  out[0] = FRAME_START;
  out[1] = type;
  out[2] = (payload.length >> 8) & 0xff;
  out[3] = payload.length & 0xff;
  payload.copy(out, FRAME_HEADER_SIZE);
  // CRC over TYPE + LEN + PAYLOAD (skip the START byte).
  out[FRAME_HEADER_SIZE + payload.length] = crc8(out, 1, FRAME_HEADER_SIZE + payload.length);
  return out;
}

/**
 * Streaming decoder: feed bytes as they come off the wire and the
 * decoder will emit complete, CRC-validated frames via the handler.
 *
 * Matches the state machine in `apps/robot-hardware/lib/Protocol/src/Protocol.cpp`.
 */
export class FrameDecoder {
  private state:
    | 'WAIT_START'
    | 'READ_TYPE'
    | 'READ_LEN_H'
    | 'READ_LEN_L'
    | 'READ_PAYLOAD'
    | 'READ_CRC' = 'WAIT_START';

  private type = 0;
  private length = 0;
  private payload: Buffer = Buffer.alloc(0);
  private payloadIdx = 0;

  private _ok = 0;
  private _dropped = 0;

  constructor(private readonly onFrame: (frame: DecodedFrame) => void) {}

  get framesOk(): number {
    return this._ok;
  }
  get framesDropped(): number {
    return this._dropped;
  }

  feed(chunk: Buffer): void {
    for (const byte of chunk) {
      this.feedByte(byte);
    }
  }

  private reset(): void {
    this.state = 'WAIT_START';
    this.type = 0;
    this.length = 0;
    this.payloadIdx = 0;
  }

  private feedByte(byte: number): void {
    switch (this.state) {
      case 'WAIT_START':
        if (byte === FRAME_START) this.state = 'READ_TYPE';
        return;

      case 'READ_TYPE':
        this.type = byte;
        this.state = 'READ_LEN_H';
        return;

      case 'READ_LEN_H':
        this.length = (byte & 0xff) << 8;
        this.state = 'READ_LEN_L';
        return;

      case 'READ_LEN_L':
        this.length |= byte & 0xff;
        if (this.length > MAX_PAYLOAD_SIZE) {
          this._dropped++;
          this.reset();
          return;
        }
        this.payload = Buffer.alloc(this.length);
        this.payloadIdx = 0;
        this.state = this.length === 0 ? 'READ_CRC' : 'READ_PAYLOAD';
        return;

      case 'READ_PAYLOAD':
        this.payload[this.payloadIdx++] = byte;
        if (this.payloadIdx === this.length) this.state = 'READ_CRC';
        return;

      case 'READ_CRC': {
        // Re-materialise the header to CRC over it.
        const header = Buffer.from([
          this.type,
          (this.length >> 8) & 0xff,
          this.length & 0xff,
        ]);
        let crc = crc8(header);
        // Fold the payload in (matches the C++ impl exactly).
        for (let i = 0; i < this.length; i++) {
          crc ^= this.payload[i]!;
          for (let b = 0; b < 8; b++) {
            crc = (crc & 0x80) !== 0 ? ((crc << 1) ^ 0x07) & 0xff : (crc << 1) & 0xff;
          }
        }

        if (crc === byte) {
          this._ok++;
          this.onFrame({ type: this.type as MsgType, payload: this.payload });
        } else {
          this._dropped++;
        }
        this.reset();
        return;
      }
    }
  }
}