webm.ts

import makeDebug from 'debug'

const debug = makeDebug('webm')

// Parse EBML variable-length integer
function parseVInt(buffer: Uint8Array, start: number): { value: number; length: number } {
  let firstByte = buffer[start]
  let numBytes = 0

  for (let mask = 0x80; mask >= 0x08; mask >>= 1) {
    if ((firstByte & mask) !== 0) {
      numBytes = 8 - Math.log2(mask)
      break
    }
  }

  if (numBytes === 0) {
    throw new TypeError('Invalid VInt')
  }

  let value = firstByte & (0xff >>> numBytes)

  for (let i = 1; i < numBytes; i++) {
    value = (value << 8) | buffer[start + i]
  }

  return { value, length: numBytes }
}

// Remove WebM header from the beginning of the chunk
export function removeWebmHeader(webmData: Uint8Array): Uint8Array {
  if (
    webmData[0] === 0x1a &&
    webmData[1] === 0x45 &&
    webmData[2] === 0xdf &&
    webmData[3] === 0xa3
  ) {
    let { value, length } = parseVInt(webmData, 4)

    const totalHeaderSize = 4 + length + value

    debug('Skipping header with size %d', value)

    return webmData.slice(totalHeaderSize)
  }

  return webmData
}