//= ===============================================================
// SHA1.js
//
// Module that replicates the SHA-1 Cryptographic Hash
// function in Javascript.
//= ===============================================================

// main variables
const CHAR_SIZE = 8

/**
 * Adds padding to binary/hex string representation
 *
 * @param {string} str - string representation (binary/hex)
 * @param {int} bits - total number of bits wanted
 * @return {string} - string representation padding with empty (0) bits
 *
 * @example
 *      pad("10011", 8); // "00010011"
 */
function pad(str, bits) {
  let res = str
  while (res.length % bits !== 0) {
    res = '0' + res
  }
  return res
}

/**
 * Separates string into chunks of the same size
 *
 * @param {string} str - string to separate into chunks
 * @param {int} size - number of characters wanted in each chunk
 * @return {array} - array of original string split into chunks
 *
 * @example
 *      chunkify("this is a test", 2)
 */
function chunkify(str, size) {
  const chunks = []
  for (let i = 0; i < str.length; i += size) {
    chunks.push(str.slice(i, i + size))
  }
  return chunks
}

/**
 * Rotates string representation of bits to the left
 *
 * @param {string} bits - string representation of bits
 * @param {int} turns - number of rotations to make
 * @return {string} - string representation of bits after rotation
 *
 * @example
 *      rotateLeft("1011", 3); // "1101"
 */
function rotateLeft(bits, turns) {
  return bits.substr(turns) + bits.substr(0, turns)
}

/**
 * Pre-processes message to feed the algorithm loop
 *
 * @param {string} message - message to pre-process
 * @return {string} - processed message
 */
function preProcess(message) {
  // convert message to binary representation padded to
  // 8 bits, and add 1
  let m =
    message
      .split('')
      .map((e) => e.charCodeAt(0))
      .map((e) => e.toString(2))
      .map((e) => pad(e, 8))
      .join('') + '1'

  // extend message by adding empty bits (0)
  while (m.length % 512 !== 448) {
    m += '0'
  }

  // length of message in binary, padded, and extended
  // to a 64 bit representation
  let ml = (message.length * CHAR_SIZE).toString(2)
  ml = pad(ml, 8)
  ml = '0'.repeat(64 - ml.length) + ml

  return m + ml
}

/**
 * Hashes message using SHA-1 Cryptographic Hash Function
 *
 * @param {string} message - message to hash
 * @return {string} - message digest (hash value)
 */
function SHA1(message) {
  // main variables
  let H0 = 0x67452301
  let H1 = 0xefcdab89
  let H2 = 0x98badcfe
  let H3 = 0x10325476
  let H4 = 0xc3d2e1f0

  // pre-process message and split into 512 bit chunks
  const bits = preProcess(message)
  const chunks = chunkify(bits, 512)

  chunks.forEach(function (chunk, i) {
    // break each chunk into 16 32-bit words
    const words = chunkify(chunk, 32)

    // extend 16 32-bit words to 80 32-bit words
    for (let i = 16; i < 80; i++) {
      const val = [words[i - 3], words[i - 8], words[i - 14], words[i - 16]]
        .map((e) => parseInt(e, 2))
        .reduce((acc, curr) => curr ^ acc, 0)
      const bin = (val >>> 0).toString(2)
      const paddedBin = pad(bin, 32)
      const word = rotateLeft(paddedBin, 1)
      words.push(word)
    }

    // initialize variables for this chunk
    let [a, b, c, d, e] = [H0, H1, H2, H3, H4]

    for (let i = 0; i < 80; i++) {
      let f, k
      if (i < 20) {
        f = (b & c) | (~b & d)
        k = 0x5a827999
      } else if (i < 40) {
        f = b ^ c ^ d
        k = 0x6ed9eba1
      } else if (i < 60) {
        f = (b & c) | (b & d) | (c & d)
        k = 0x8f1bbcdc
      } else {
        f = b ^ c ^ d
        k = 0xca62c1d6
      }
      // make sure f is unsigned
      f >>>= 0

      const aRot = rotateLeft(pad(a.toString(2), 32), 5)
      const aInt = parseInt(aRot, 2) >>> 0
      const wordInt = parseInt(words[i], 2) >>> 0
      const t = aInt + f + e + k + wordInt
      e = d >>> 0
      d = c >>> 0
      const bRot = rotateLeft(pad(b.toString(2), 32), 30)
      c = parseInt(bRot, 2) >>> 0
      b = a >>> 0
      a = t >>> 0
    }

    // add values for this chunk to main hash variables (unsigned)
    H0 = (H0 + a) >>> 0
    H1 = (H1 + b) >>> 0
    H2 = (H2 + c) >>> 0
    H3 = (H3 + d) >>> 0
    H4 = (H4 + e) >>> 0
  })

  // combine hash values of main hash variables and return
  const HH = [H0, H1, H2, H3, H4]
    .map((e) => e.toString(16))
    .map((e) => pad(e, 8))
    .join('')

  return HH
}

// export SHA1 function
export { SHA1 }