How to correctly translate this block CRC32 from Go to JavaScript?

Question

I have this function in Go:

package main

import (
    "fmt"
    "github.com/snksoft/crc"
)

var crcTable *crc.Table

func init() {
        params := crc.CRC32
        params.FinalXor = 0
        params.ReflectOut = false
        crcTable = crc.NewTable(params)
}

func crcCalculateBlock(data []byte) uint32 {

    if len(data)%4 > 0 {
        panic("block size needs to be a multiple of 4")
    }

    h := crc.NewHashWithTable(crcTable)

    var buf [4]byte
    for i := 0; i < len(data); i += 4 {
        buf[0] = data[i+3]
        buf[1] = data[i+2]
        buf[2] = data[i+1]
        buf[3] = data[i+0]
        h.Update(buf[:])
    }

    return h.CRC32()
}

func main() {
  data := []byte{1, 2, 3, 4, 5, 6, 7, 8}
    crc := crcCalculateBlock([]byte(data))
    fmt.Printf("CRC is 0x%04X\n", crc)
}

The result is: 0x948B389D

I am trying to translate it to JavaScript but I am missing something:

var makeCRCTable = function(){
    var c;
    var crcTable = [];
    for(var n =0; n < 256; n++){
        c = n;
        for(var k =0; k < 8; k++){
            c = ((c&1) ? (0xEDB88320 ^ (c >>> 1)) : (c >>> 1));
        }
        crcTable[n] = c;
    }
    return crcTable;
}

var crc32 = function(u8array) {
    var crcTable = window.crcTable || (window.crcTable = makeCRCTable());
    var crc = 0 ^ (-1);

    for (var i = 0; i < u8array.length; i+=4 ) {
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i+3]) & 0xFF];
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i+2]) & 0xFF];
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i+1]) & 0xFF];
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i]) & 0xFF];
    }

    return (crc ^ (-1)) >>> 0;
};

console.log(crc32(Uint8Array.from([1,2,3,4,5,6,7,8])).toString(16))

but the result is different. ( 46e32ed6 )
even without the final xor I get b91cd129

Can anyone explain to me how to correct that and why is that wrong?

Answer 1

There are two differences:

1. the Go implementation has called reflect (see https://github.com/snksoft/crc/blob/03404db21ad4e7182edf4843b51f6252799f7140/crc.go#L168-L170):

   if t.crcParams.ReflectOut != t.crcParams.ReflectIn {
       ret = reflect(ret, t.crcParams.Width)
   }
   

2. the FinalXor in Go is 0 (params.FinalXor = 0) while in js it's -1 (return (crc ^ (-1)) >>> 0;)

Here is the updated js implementation that generates the same hash value.

var makeCRCTable = function () {
  var c;
  var crcTable = [];
  for (var n = 0; n < 256; n++) {
    c = n;
    for (var k = 0; k < 8; k++) {
      c = c & 1 ? 0xedb88320 ^ (c >>> 1) : c >>> 1;
    }
    crcTable[n] = c;
  }
  return crcTable;
};

var crc32 = function (u8array) {
  var crcTable = window.crcTable || (window.crcTable = makeCRCTable());
  var crc = 0 ^ -1;

  for (var i = 0; i < u8array.length; i += 4) {
    crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i + 3]) & 0xff];
    crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i + 2]) & 0xff];
    crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i + 1]) & 0xff];
    crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i]) & 0xff];
  }

  crc = reverseBits(crc, 32);

  return (crc ^ 0) >>> 0;
};

function reverseBits(integer, bitLength) {
  if (bitLength > 32) {
    throw Error(
      'Bit manipulation is limited to <= 32 bit numbers in JavaScript.'
    );
  }

  let result = 0;
  for (let i = 0; i < bitLength; i++) {
    result |= ((integer >> i) & 1) << (bitLength - 1 - i);
  }

  return result >>> 0; // >>> 0 makes it unsigned even if bit 32 (the sign bit) was set
}

console.log(crc32(Uint8Array.from([1, 2, 3, 4, 5, 6, 7, 8])).toString(16));

Note: the reverseBits function is copied from this answer: https://stackoverflow.com/a/67064710/1369400

Answer 2

Thanks to Zeke Lu, I prefer this code.

function rev(x) {
    x = ((x >> 1) & 0x55555555) | ((x & 0x55555555) << 1);
    x = ((x >> 2) & 0x33333333) | ((x & 0x33333333) << 2);
    x = ((x >> 4) & 0x0F0F0F0F) | ((x & 0x0F0F0F0F) << 4);
    x = ((x >> 8) & 0x00FF00FF) | ((x & 0x00FF00FF) << 8);
    x = (x >>> 16) | (x << 16);

    return x >>> 0;
}
var makeCRCTable = function(){
    var c;
    var crcTable = [];
    for(var n =0; n < 256; n++){
        c = n;
        for(var k =0; k < 8; k++){
            c = ((c&1) ? (0xEDB88320 ^ (c >>> 1)) : (c >>> 1));
        }
        crcTable[n] = c;
    }
    return crcTable;
}

var crc32 = function(u8array) {
    var crcTable = window.crcTable || (window.crcTable = makeCRCTable());
    var crc = 0 ^ (-1);

    for (var i = 0; i < u8array.length; i+=4 ) {
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i+3]) & 0xFF];
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i+2]) & 0xFF];
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i+1]) & 0xFF];
        crc = (crc >>> 8) ^ crcTable[(crc ^ u8array[i]) & 0xFF];
    }

    return rev(crc);
};

console.log(crc32(Uint8Array.from([1,2,3,4,5,6,7,8])).toString(16))