export function decodeFrameBodyHeader()

in modules/serialize/src/decode_body_header.ts [64:121]

• 37 lines of code
• 4 McCabe index (conditional complexity)

export function decodeFrameBodyHeader(
  buffer: Uint8Array,
  headerInfo: HeaderInfo,
  readPos: number
): FrameBodyHeader | false {
  /* Precondition: The contentType must be FRAMED_DATA. */
  needs(
    ContentType.FRAMED_DATA === headerInfo.messageHeader.contentType,
    'Unknown contentType'
  )

  const { frameLength } = headerInfo.messageHeader
  const { ivLength, tagLength } = headerInfo.algorithmSuite

  /* Uint8Array is a view on top of the underlying ArrayBuffer.
   * This means that raw underlying memory stored in the ArrayBuffer
   * may be larger than the Uint8Array.  This is especially true of
   * the Node.js Buffer object.  The offset and length *must* be
   * passed to the DataView otherwise I will get unexpected results.
   */
  const dataView = new DataView(
    buffer.buffer,
    buffer.byteOffset,
    buffer.byteLength
  )

  /* Precondition: decodeFrameBodyHeader readPos must be within the byte length of the buffer given. */
  needs(
    dataView.byteLength >= readPos && readPos >= 0,
    'readPos out of bounds.'
  )

  /* Check for early return (Postcondition): There must be enough data to decodeFrameBodyHeader.
   * The format expressed here is
   * SequenceIdentifier: Uint32
   * IVLength: Uint8
   * There is a special case where the SequenceIdentifier is the Final Frame.
   */
  if (4 + ivLength + readPos > dataView.byteLength) return false

  const sequenceNumber = dataView.getUint32(readPos)
  /* Postcondition: decodeFrameBodyHeader sequenceNumber must be greater than 0. */
  needs(sequenceNumber > 0, 'Malformed sequenceNumber.')
  if (sequenceNumber === SequenceIdentifier.SEQUENCE_NUMBER_END) {
    return decodeFinalFrameBodyHeader(buffer, headerInfo, readPos)
  }

  const iv = buffer.slice((readPos += 4), (readPos += ivLength))
  return {
    sequenceNumber,
    iv,
    contentLength: frameLength,
    readPos,
    tagLength,
    isFinalFrame: false,
    contentType: ContentType.FRAMED_DATA,
  }
}