/**
 * Length-Prefixed-Message → Compressed-Flag Message-Length Message
 * Compressed-Flag → 0 / 1 # encoded as 1 byte unsigned integer
 * Message-Length → {length of Message} # encoded as 4 byte unsigned integer (big endian)
 * Message → *{binary octet}
 *
 * https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md
 *
 * For gRPC-Web the 8th (MSB) bit of the 1st gRPC frame byte also indicates
 * whether the message represents metadata.
 *
 * https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-WEB.md
 *
 * @internal
 */
export const LPM_HEADER_LENGTH = 5;

/** @internal */
export type ParsedLpmHeader = {
  compressed: boolean;
  isMetadata: boolean;
  length: number;
};

/** @internal */
export function parseLpmHeader(data: Uint8Array): ParsedLpmHeader {
  if (data.length !== LPM_HEADER_LENGTH) {
    throw new Error(`Invalid LPM header length: ${data.length}`);
  }

  const view = new DataView(data.buffer, data.byteOffset, data.byteLength);

  const compressed = (view.getUint8(0) & 1) !== 0;
  const isMetadata = (view.getUint8(0) & 0x80) !== 0;
  const length = view.getUint32(1);

  return {
    compressed,
    isMetadata,
    length,
  };
}

/** @internal */
export function encodeFrame(data: Uint8Array): Uint8Array {
  const messageBytes = new Uint8Array(LPM_HEADER_LENGTH + data.length);

  new DataView(messageBytes.buffer, 1, 4).setUint32(0, data.length, false);
  messageBytes.set(data, LPM_HEADER_LENGTH);

  return messageBytes;
}