/**
 * A range's chunks on the wire (REBUILD.md, prefetch): one streaming
 * response, a frame per chunk: from (8 bytes), to (8 bytes), length
 * (4 bytes), then the chunk as the news server sent it (compressed XOVER).
 */
export interface Frame {
  from: number
  to: number
  data: Buffer
}

const HEADER = 20

export function encodeFrame(frame: Frame): Buffer {
  const head = Buffer.alloc(HEADER)
  head.writeBigUInt64BE(BigInt(frame.from), 0)
  head.writeBigUInt64BE(BigInt(frame.to), 8)
  head.writeUInt32BE(frame.data.length, 16)
  return Buffer.concat([head, frame.data])
}

/** The frames of a byte stream, as they complete. */
export async function* decodeFrames(stream: AsyncIterable<Uint8Array> | Iterable<Uint8Array>): AsyncGenerator<Frame> {
  let buffer = Buffer.alloc(0)
  for await (const piece of stream) {
    buffer = buffer.length === 0 ? Buffer.from(piece) : Buffer.concat([buffer, piece])
    while (buffer.length >= HEADER) {
      const length = buffer.readUInt32BE(16)
      if (buffer.length < HEADER + length) break
      yield { from: Number(buffer.readBigUInt64BE(0)), to: Number(buffer.readBigUInt64BE(8)), data: buffer.subarray(HEADER, HEADER + length) }
      buffer = buffer.subarray(HEADER + length)
    }
  }
  if (buffer.length > 0) throw new Error(`Stream ended inside a frame (${buffer.length} bytes left)`)
}