bedrock-protocol/src/datatypes/varlong.js

Minecraft Bedrock Edition protocol library

function sizeOfVarLong (value) {
  if (typeof value.valueOf() === 'object') {
    value = (BigInt(value[0]) << 32n) | BigInt(value[1])
  } else if (typeof value !== 'bigint') value = BigInt(value)

  let cursor = 0
  while (value > 127n) {
    value >>= 7n
    cursor++
  }
  return cursor + 1
}

/**
 * Reads a 64-bit VarInt as a BigInt
 */
function readVarLong (buffer, offset) {
  let result = BigInt(0)
  let shift = 0n
  let cursor = offset
  let size = 0

  while (true) {
    if (cursor + 1 > buffer.length) { throw new Error('unexpected buffer end') }
    const b = buffer.readUInt8(cursor)
    result |= (BigInt(b) & 0x7fn) << shift // Add the bits to our number, except MSB
    cursor++
    if (!(b & 0x80)) { // If the MSB is not set, we return the number
      size = cursor - offset
      break
    }
    shift += 7n // we only have 7 bits, MSB being the return-trigger
    if (shift > 63n) throw new Error(`varint is too big: ${shift}`)
  }

  return { value: result, size }
}

/**
 * Writes a zigzag encoded 64-bit VarInt as a BigInt
 */
function writeVarLong (value, buffer, offset) {
  // if an array, turn it into a BigInt
  if (typeof value.valueOf() === 'object') {
    value = BigInt.asIntN(64, (BigInt(value[0]) << 32n)) | BigInt(value[1])
  } else if (typeof value !== 'bigint') value = BigInt(value)

  let cursor = 0
  while (value > 127n) { // keep writing in 7 bit slices
    const num = Number(value & 0xFFn)
    buffer.writeUInt8(num | 0x80, offset + cursor)
    cursor++
    value >>= 7n
  }
  buffer.writeUInt8(Number(value), offset + cursor)
  return offset + cursor + 1
}

module.exports = {
  Read: { varint64: ['native', readVarLong] },
  Write: { varint64: ['native', writeVarLong] },
  SizeOf: { varint64: ['native', sizeOfVarLong] }
}