如何为Ogg建立CRC32表？ [英] How to build CRC32 table for Ogg?

问题描述

来自此答案我修改了以下代码：

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;
}

此代码生成表格如此处，但对于Ogg，我需要另一个表格-如此处。

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

来自 Ogg文档：

32位CRC值（直接算法，初始val和最终XOR = 0，
生成多项式= 0x04c11db7）

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

parseInt('04c11db7', 16)

返回 79764919 -我尝试过此多项式，但结果表不正确。

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

我是CRC字段的新手，如我发现的 CRC32算法有一些变体。 / p>

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

解决方案

我不确定javascript的优先级，但是异或需要在转换后发生：

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

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

但是您显示的第一个表似乎正确，因为table [128] = table [0x80] = 3988292384 = 0xEDB88320，即0x104c11db7位反转，然后右移一位。

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.

第二个表用于左移CRC，其中table [1] = x04c11db7。在这种情况下，内部循环将包括以下内容：

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;
}

---

示例C代码进行比较，为模式{0x01}，{0x01,0x00}，{0x01,0x00,0x00}，{0x01,0x00,0x00,0x00}生成crc。

---

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);
}

对于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;
}

如何使用此表：

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

在这里，我们添加了>> 0 接受数字模块-因为JS中没有unsigned int- JavaScript没有整数。它只有双精度浮点数。

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.

请注意，对于Ogg，您必须以相反的顺序设置生成的CRC。

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

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