How to build CRC32 table for Ogg?

问题

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.

回答1:

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("%08x\n", crc);
    crc = crc32(bfr, 2);        /* 0xd219c1dc */
    printf("%08x\n", crc);
    crc = crc32(bfr, 3);        /* 0x01d8ac87 */
    printf("%08x\n", crc);
    crc = crc32(bfr, 4);        /* 0xdc6d9ab7 */
    printf("%08x\n", 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.

来源：https://stackoverflow.com/questions/53438815/how-to-build-crc32-table-for-ogg

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javascript

crc

ogg

crc32