Qfyilyi

From this answer I adapted the code below:

function _makeCRCTable() {

    const CRCTable = new Uint32Array(256);

    for (let i = 256; i--;) {

        let char = i;

        for (let j = 8; j--;) {

            char = char & 1 ? 3988292384 ^ char >>> 1 : char >>> 1;

        }

        CRCTable[i] = char;

    }

    return CRCTable;

}

This code generates table as here, but for Ogg I need another table - as here.

From Ogg documentation:

32 bit CRC value (direct algorithm, initial val and final XOR = 0,
generator polynomial=0x04c11db7)

parseInt('04c11db7', 16)

return 79764919 - I tried this polynomial but resulting table is not correct.

I am new to the CRC field, as I found there are a few variations of CRC32 algorithm.

I'm not sure of javascript precedence, but the xor needs to occur after the shift:

char = char & 1 ? 3988292384 ^ (char >>> 1) : char >>> 1;

However the first table you show seems correct, as table[128] = table[0x80] = 3988292384 = 0xEDB88320 which is 0x104c11db7 bit reversed, then shifted right one bit.

The second table you have is for a left shifting CRC, where table[1] = x04c11db7. In this case the inner loop would include something like this:

let char = i << 24;

for (let j = 8; j--;) {

    char = char & 0x80000000 ? 0x04c11db7 ^ char << 1 : char << 1;

}

Example C code for comparison, generates crc for the patterns {0x01}, {0x01,0x00}, {0x01,0x00,0x00}, {0x01,0x00,0x00,0x00}.

#include <stdio.h>



typedef unsigned char uint8_t;

typedef unsigned int  uint32_t;



uint32_t crctbl[256];



void gentbl(void)

{

uint32_t crc;

uint32_t b;

uint32_t c;

uint32_t i;

    for(c = 0; c < 0x100; c++){

        crc = c<<24;

        for(i = 0; i < 8; i++){

            b = crc>>31;

            crc <<= 1;

            crc ^= (0 - b) & 0x04c11db7;

        }

        crctbl[c] = crc;

    }

}



uint32_t crc32(uint8_t * bfr, size_t size)

{

uint32_t crc = 0;

    while(size--)

        crc = (crc << 8) ^ crctbl[(crc >> 24)^*bfr++];

    return(crc);

}



int main(int argc, char** argv)

{

    uint32_t crc;

    uint8_t bfr[4] = {0x01,0x00,0x00,0x00};

    gentbl();

    crc = crc32(bfr, 1);        /* 0x04c11db7 */

    printf("%08xn", crc);

    crc = crc32(bfr, 2);        /* 0xd219c1dc */

    printf("%08xn", crc);

    crc = crc32(bfr, 3);        /* 0x01d8ac87 */

    printf("%08xn", crc);

    crc = crc32(bfr, 4);        /* 0xdc6d9ab7 */

    printf("%08xn", crc);

    return(0);

}

For JS:

function _makeCRC32Table() {

    const polynomial = 79764919;

    const mask = 2147483648;

    const CRCTable = new Uint32Array(256);

    for (let i = 256; i--;) {

        let char = i << 24;

        for (let j = 8; j--;) {

            char = char & mask ? polynomial ^ char << 1 : char << 1;

        }

        CRCTable[i] = char;

    }

    return CRCTable;

}

How to use this table:

[1, 0].reduce((crc, byte) => crc << 8 >>> 0 ^ CRCTable[crc >>> 24 ^ byte], 0) >>> 0

Here we added >>> 0 that takes the module of the number - because there is no unsigned int in JS - JavaScript doesn't have integers. It only has double precision floating-point numbers.

Note that for Ogg you must set generated CRC in the reverse order.