crc.c

ffmpeg源码分析

#include "common.h"
#include "crc.h"

AVCRC *av_crcEDB88320;
AVCRC *av_crc04C11DB7;
AVCRC *av_crc8005    ;
AVCRC *av_crc07      ;

/**
 * Inits a crc table.
 * @param ctx must be an array of sizeof(AVCRC)*257 or sizeof(AVCRC)*1024
 * @param cts_size size of ctx in bytes
 * @return <0 on failure
 */
int av_crc_init(AVCRC *ctx, int le, int bits, uint32_t poly, int ctx_size){
    int i, j;
    uint32_t c;

    if (bits < 8 || bits > 32 || poly >= (1LL<<bits))
        return -1;
    if (ctx_size != sizeof(AVCRC)*257 && ctx_size != sizeof(AVCRC)*1024)
        return -1;

    for (i = 0; i < 256; i++) {
        if (le) {
            for (c = i, j = 0; j < 8; j++)
                c = (c>>1)^(poly & (-(c&1)));
            ctx[i] = c;
        } else {
            for (c = i << 24, j = 0; j < 8; j++)
                c = (c<<1) ^ ((poly<<(32-bits)) & (((int32_t)c)>>31) );
            ctx[i] = bswap_32(c);
        }
    }
    ctx[256]=1;
    if(ctx_size >= sizeof(AVCRC)*1024)
        for (i = 0; i < 256; i++)
            for(j=0; j<3; j++)
                ctx[256*(j+1) + i]= (ctx[256*j + i]>>8) ^ ctx[ ctx[256*j + i]&0xFF ];

    return 0;
}

uint32_t av_crc(const AVCRC *ctx, uint32_t crc, const uint8_t *buffer, size_t length){
    const uint8_t *end= buffer+length;

    if(!ctx[256])
        while(buffer<end-3){
            crc ^= le2me_32(*(uint32_t*)buffer); buffer+=4;
            crc =  ctx[3*256 + ( crc     &0xFF)]
                  ^ctx[2*256 + ((crc>>8 )&0xFF)]
                  ^ctx[1*256 + ((crc>>16)&0xFF)]
                  ^ctx[0*256 + ((crc>>24)     )];
        }
    while(buffer<end)
        crc = ctx[((uint8_t)crc) ^ *buffer++] ^ (crc >> 8);

    return crc;
}

#ifdef TEST
#undef printf
main(){
    uint8_t buf[1999];
    int i;
    int p[4][4]={{1, 32, 0xedb88320L, 0x3D5CDD04},
                 {0, 32, 0x04c11db7L, 0xC0F5BAE0},
                 {0, 16, 0x8005     , 0x1FBB    },
                 {0,  8, 0x07       , 0xE3      },};
    AVCRC ctx[1 ? 1024:257];

    for(i=0; i<sizeof(buf); i++)
        buf[i]= i+i*i;

    for(i=0; i<4; i++){
        av_crc_init(ctx, p[i][0], p[i][1], p[i][2], sizeof(ctx));
        printf("crc %08X =%X\n", p[i][2], av_crc(ctx, 0, buf, sizeof(buf)));
    }
}
#endif