bitPackingInJavascript.js

// Apache2.0 license
// Copyright (c) 2016 Zeleke Snyder

import { chunk } from 'lodash'

export const packIntoStr = function ([...bytes]) {
  // Pack bytes into string

  var bytesLen = bytes.length
  var m = 4 - (bytesLen % 4)    // remaining to make it fit
  if (m !== 0) {
    // append to bytes to make it fit
    bytes = bytes.concat(Array(m).fill(bytesLen))     // last byte is array len
  } else {
    bytes = bytes.concat(Array(4).fill(bytesLen))     // ensure last byte is array len
  }

  let chunks = chunk(bytes, 4)    // split into 4 bytes each

  // do the packing
  return chunks.map(fourBytes => {
    return fourBytes.reduce((x, y, i) => {
      x |= (y & 0xff) << (8 * i)
      return x
    })
  })
  .map(packed => packed.toString(36))   // store as base-36 strings
  .join('#')                            // finally store as one string
}

export const unpackFromStr = function (str) {
  // Unpack bytes as array from base-36 string

  return str.split('#')
  .map(s => {
    let v = parseInt(s, 36)
    return [
      v & 0xff,
      (v >> 8) & 0xff,
      (v >> 16) & 0xff,
      (v >> 24) & 0xff
    ]
  })
  .reduce((x, y) => x.concat(y))
  .filter((z, i, a) => {
    const arrLen = a[a.length - 1]  // actual array length is stored as last value
    return i < arrLen
  })
}

/*
 * Example:
 * var a = packIntoStr([1,2,3,4,5,6,7,8])   // '142lmp#286m85#28816w'
 * var b = unpackFromStr(a)   // [ 1, 2, 3, 4, 5, 6, 7, 8 ]
 */