varstruct/lib/varbuffer.js

encode/decode variable binary structures

'use strict'
const Buffer = require('safe-buffer').Buffer
const util = require('./util')

module.exports = function (lengthType) {
  if (!util.isAbstractCodec(lengthType)) throw new TypeError('lengthType is invalid codec')

  function _length (value) {
    if (!Buffer.isBuffer(value)) throw new TypeError('value must be a Buffer instance')

    return lengthType.encodingLength(value.length) + value.length
  }

  function encode (value, buffer, offset) {
    if (!offset) offset = 0

    const bytes = _length(value)
    if (!buffer) buffer = Buffer.allocUnsafe(bytes)
    else if ((buffer.length - offset) < bytes) throw new RangeError('destination buffer is too small')

    lengthType.encode(value.length, buffer, offset)
    offset += lengthType.encode.bytes

    value.copy(buffer, offset)
    encode.bytes = bytes

    return buffer
  }

  function decode (buffer, offset, end) {
    if (!offset) offset = 0
    if (!end) end = buffer.length
    const start = offset

    const length = lengthType.decode(buffer, offset, end)
    offset += lengthType.decode.bytes

    if (offset + length > end) throw new RangeError('not enough data for decode')

    decode.bytes = (offset + length) - start
    return Buffer.from(buffer.slice(offset, offset + length))
  }

  function encodingLength (value) {
    return _length(value)
  }

  return { encode, decode, encodingLength }
}