# Array sum helper function.
sum = (array) ->
  array.reduce (x, y) -> x + y

md5 = do ->
  # Per-round shift amounts.
  s = [738695, 669989, 770404, 703814]
  s = (s[i >> 4] >> i % 4 * 5 & 31 for i in [0..63])

  # Constants cache generated by sine.
  K = (Math.floor 2**32 * Math.abs Math.sin i for i in [1..64])

  # Bitwise left rotate helper function.
  lrot = (x, y) ->
    x << y | x >>> 32 - y;

  (input) ->
    # Initialize values.
    d0 = 0x10325476;
    a0 = 0x67452301;
    b0 = ~d0
    c0 = ~a0;

    # Convert the message to 32-bit words, little-endian.
    M =
      for i in [0...input.length] by 4
        sum (input.charCodeAt(i + j) << j*8 for j in [0..3])

    # Pre-processing: append a 1 bit, then message length % 2^64.
    len = input.length * 8
    M[len >> 5] |= 128 << len % 32
    M[(len + 64 >>> 9 << 4) + 14] = len

    # Process the message in chunks of 16 32-bit words.
    for x in [0...M.length] by 16
      [A, B, C, D] = [a0, b0, c0, d0]

      # Main loop.
      for i in [0..63]
        if i < 16
          F = B & C | ~B & D
          g = i
        else if i < 32
          F = B & D | C & ~D
          g = i * 5 + 1
        else if i < 48
          F = B ^ C ^ D
          g = i * 3 + 5
        else
          F = C ^ (B | ~D)
          g = i * 7

        [A, B, C, D] =
          [D, B + lrot(A + F + K[i] + (M[x + g % 16] ? 0), s[i]), B, C]

      a0 += A
      b0 += B
      c0 += C
      d0 += D

    # Convert the four words back to a string.
    return (
      for x in [a0, b0, c0, d0]
        (String.fromCharCode x >>> 8 * y & 255 for y in [0..3]).join ''
    ).join ''