bitcoinjs-lib/src/script_number.js

function decode (buffer, maxLength, minimal) {
  maxLength = maxLength || 4
  minimal = minimal === undefined ? true : minimal

  var length = buffer.length
  if (length === 0) return 0
  if (length > maxLength) throw new TypeError('Script number overflow')
  if (minimal) {
    if ((buffer[length - 1] & 0x7f) === 0) {
      if (length <= 1 || (buffer[length - 2] & 0x80) === 0) throw new Error('Non-minimally encoded script number')
    }
  }

  var result

  // 40-bit
  if (length === 5) {
    var a = buffer.readUInt32LE(0)
    var b = buffer.readUInt8(4)

    if (b & 0x80) return -((b & ~0x80) * 0x100000000 + a)
    return b * 0x100000000 + a
  }

  // 32-bit / 24-bit / 16-bit / 8-bit
  for (var i = 0; i < length; ++i) {
    result |= buffer[i] << (8 * i)
  }

  if (buffer[length - 1] & 0x80) return -(result & ~(0x80 << (8 * (length - 1))))
  return result
}

function scriptNumSize (i) {
  return i > 0x7fffffff ? 5
  : i > 0x7fffff ? 4
  : i > 0x7fff ? 3
  : i > 0x7f ? 2
  : i > 0x00 ? 1
  : 0
}

function encode (number) {
  var value = Math.abs(number)
  var size = scriptNumSize(value)
  var buffer = new Buffer(size)
  var negative = number < 0

  for (var i = 0; i < size; ++i) {
    buffer.writeUInt8(value & 0xff, i)
    value >>= 8
  }

  if (buffer[size - 1] & 0x80) {
    buffer.writeUInt8(negative ? 0x80 : 0x00, size - 1)
  } else if (negative) {
    buffer[size - 1] |= 0x80
  }

  return buffer
}

module.exports = {
  decode: decode,
  encode: encode
}