export const controllerDevToolsCommandContentType = 'application/vnd.agl.devtools-commands.v1' as const;
export const controllerDevToolsEventContentType = 'application/vnd.agl.devtools-events.v1' as const;
export const controllerDevToolsCommandBytes = 1024 * 1024;
export const controllerDevToolsEventBytes = 8 * 1024 * 1024;
export const controllerDevToolsWriteBytes = 1024 * 1024;

/** CDP messages are length-prefixed because they can exceed a logical stream write. */
export const encodeControllerDevToolsMessage = (message: string, maximumBytes: number): Uint8Array => {
  const body = new TextEncoder().encode(message);
  if (!body.length || body.length > maximumBytes) throw new Error('DevTools message exceeds the size limit.');
  const packet = new Uint8Array(body.length + 4);
  new DataView(packet.buffer).setUint32(0, body.length);
  packet.set(body, 4);
  return packet;
};

/** Allocates once per bounded message and accepts arbitrarily fragmented stream chunks. */
export class ControllerDevToolsMessageDecoder {
  private header = new Uint8Array(4);
  private headerBytes = 0;
  private body?: Uint8Array;
  private bodyBytes = 0;
  private decoder = new TextDecoder('utf-8', { fatal: true });
  constructor(private maximumBytes: number) {}

  public *push(chunk: Uint8Array): Generator<string> {
    let offset = 0;
    while (offset < chunk.length) {
      if (!this.body) {
        const count = Math.min(4 - this.headerBytes, chunk.length - offset);
        this.header.set(chunk.subarray(offset, offset + count), this.headerBytes);
        offset += count;
        this.headerBytes += count;
        if (this.headerBytes !== 4) continue;
        const length = new DataView(this.header.buffer).getUint32(0);
        if (!length || length > this.maximumBytes) throw new Error('Invalid DevTools message length.');
        this.body = new Uint8Array(length);
      }
      const count = Math.min(this.body.length - this.bodyBytes, chunk.length - offset);
      this.body.set(chunk.subarray(offset, offset + count), this.bodyBytes);
      offset += count;
      this.bodyBytes += count;
      if (this.bodyBytes === this.body.length) {
        const message = this.decoder.decode(this.body);
        this.body = undefined;
        this.bodyBytes = 0;
        this.headerBytes = 0;
        yield message;
      }
    }
  }

  public finish(): void {
    if (this.headerBytes || this.body) throw new Error('Incomplete DevTools message.');
  }
}