///|
/// `@crc32.Digest` implements the 32-bit cyclic redundancy check, or CRC-32,
/// checksum. See https://en.wikipedia.org/wiki/Cyclic_redundancy_check for
/// information.
/// This package is based on the Go Rosetta Stone implementation found here:
/// https://rosettacode.org/wiki/CRC-32
pub(all) struct Digest {
  mut crc : UInt
  name : String
}

///|
/// `Digest::new` returns a new, reset Digest, ready to sum.
pub fn Digest::new() -> Digest {
  { crc: 0U.lnot(), name: "crc32" }
}

///|
let _trait : &HashFunc = Digest::new()

///|
pub impl HashFunc for Digest with fn name(self) {
  self.name
}

///|
/// `digest` returns the final crc32 as a UInt.
pub fn Digest::digest(self : Digest) -> UInt {
  self.crc.lnot()
}

///|
/// `check_sum` returns the final crc32 as a hex string.
pub impl HashFunc for Digest with fn check_sum(self) {
  let digest = self.crc.lnot()
  let result = Buffer(size_hint=8)
  result.write_char_utf16le(to_hex((digest >> 28) & 0xf))
  result.write_char_utf16le(to_hex((digest >> 24) & 0xf))
  result.write_char_utf16le(to_hex((digest >> 20) & 0xf))
  result.write_char_utf16le(to_hex((digest >> 16) & 0xf))
  result.write_char_utf16le(to_hex((digest >> 12) & 0xf))
  result.write_char_utf16le(to_hex((digest >> 8) & 0xf))
  result.write_char_utf16le(to_hex((digest >> 4) & 0xf))
  result.write_char_utf16le(to_hex(digest & 0xf))
  result.contents().to_unchecked_string()
}

///|
fn to_hex(v : UInt) -> Char {
  let v = v.reinterpret_as_int()
  if v < 10 {
    Int::unsafe_to_char(v + b'0'.to_int())
  } else {
    Int::unsafe_to_char(v - 10 + b'a'.to_int())
  }
}

///|
/// `write` writes a byte to the digest.
pub impl HashFunc for Digest with fn write(self, b) {
  self.crc = table[(self.crc.to_byte() & b'\xff').lxor(b).to_int()] ^
    (self.crc >> 8)
}

///|
let table : FixedArray[UInt] = [
  0, 1996959894, 3993919788, 2567524794, 124634137, 1886057615, 3915621685, 2657392035,
  249268274, 2044508324, 3772115230, 2547177864, 162941995, 2125561021, 3887607047,
  2428444049, 498536548, 1789927666, 4089016648, 2227061214, 450548861, 1843258603,
  4107580753, 2211677639, 325883990, 1684777152, 4251122042, 2321926636, 335633487,
  1661365465, 4195302755, 2366115317, 997073096, 1281953886, 3579855332, 2724688242,
  1006888145, 1258607687, 3524101629, 2768942443, 901097722, 1119000684, 3686517206,
  2898065728, 853044451, 1172266101, 3705015759, 2882616665, 651767980, 1373503546,
  3369554304, 3218104598, 565507253, 1454621731, 3485111705, 3099436303, 671266974,
  1594198024, 3322730930, 2970347812, 795835527, 1483230225, 3244367275, 3060149565,
  1994146192, 31158534, 2563907772, 4023717930, 1907459465, 112637215, 2680153253,
  3904427059, 2013776290, 251722036, 2517215374, 3775830040, 2137656763, 141376813,
  2439277719, 3865271297, 1802195444, 476864866, 2238001368, 4066508878, 1812370925,
  453092731, 2181625025, 4111451223, 1706088902, 314042704, 2344532202, 4240017532,
  1658658271, 366619977, 2362670323, 4224994405, 1303535960, 984961486, 2747007092,
  3569037538, 1256170817, 1037604311, 2765210733, 3554079995, 1131014506, 879679996,
  2909243462, 3663771856, 1141124467, 855842277, 2852801631, 3708648649, 1342533948,
  654459306, 3188396048, 3373015174, 1466479909, 544179635, 3110523913, 3462522015,
  1591671054, 702138776, 2966460450, 3352799412, 1504918807, 783551873, 3082640443,
  3233442989, 3988292384, 2596254646, 62317068, 1957810842, 3939845945, 2647816111,
  81470997, 1943803523, 3814918930, 2489596804, 225274430, 2053790376, 3826175755,
  2466906013, 167816743, 2097651377, 4027552580, 2265490386, 503444072, 1762050814,
  4150417245, 2154129355, 426522225, 1852507879, 4275313526, 2312317920, 282753626,
  1742555852, 4189708143, 2394877945, 397917763, 1622183637, 3604390888, 2714866558,
  953729732, 1340076626, 3518719985, 2797360999, 1068828381, 1219638859, 3624741850,
  2936675148, 906185462, 1090812512, 3747672003, 2825379669, 829329135, 1181335161,
  3412177804, 3160834842, 628085408, 1382605366, 3423369109, 3138078467, 570562233,
  1426400815, 3317316542, 2998733608, 733239954, 1555261956, 3268935591, 3050360625,
  752459403, 1541320221, 2607071920, 3965973030, 1969922972, 40735498, 2617837225,
  3943577151, 1913087877, 83908371, 2512341634, 3803740692, 2075208622, 213261112,
  2463272603, 3855990285, 2094854071, 198958881, 2262029012, 4057260610, 1759359992,
  534414190, 2176718541, 4139329115, 1873836001, 414664567, 2282248934, 4279200368,
  1711684554, 285281116, 2405801727, 4167216745, 1634467795, 376229701, 2685067896,
  3608007406, 1308918612, 956543938, 2808555105, 3495958263, 1231636301, 1047427035,
  2932959818, 3654703836, 1088359270, 936918000, 2847714899, 3736837829, 1202900863,
  817233897, 3183342108, 3401237130, 1404277552, 615818150, 3134207493, 3453421203,
  1423857449, 601450431, 3009837614, 3294710456, 1567103746, 711928724, 3020668471,
  3272380065, 1510334235, 755167117,
]