用C语言实现PHP的base64_decode函数

有base64_encode自然有base64_decode，也是定义在PHP源码的ext\standard\base64.c文件中，仍然需要注意内存泄露。

unsigned char *base64_decode(const unsigned char *str, int strict)
{
    static const char base64_pad = '=';
    static const short base64_reverse_table[256] = {
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -1, -1, -2, -2, -1, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -1, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, 62, -2, -2, -2, 63,
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -2, -2, -2, -2, -2, -2,
        -2,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
        15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -2, -2, -2, -2, -2,
        -2, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
        41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2,
        -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2, -2
    };
    const unsigned char *current = str;
    int length = strlen(str);
    int ch, i = 0, j = 0, k;
    
    unsigned char *result;
    
    result = (unsigned char *)malloc(length + 1);

    
    while ((ch = *current++) != '\0' && length-- > 0) {
        if (ch == base64_pad) {
            if (*current != '=' && (i % 4) == 1) {
                free(result);
                return NULL;
            }
            continue;
        }

        ch = base64_reverse_table[ch];
        if ((!strict && ch < 0) || ch == -1) { 
            continue;
        } else if (ch == -2) {
            free(result);
            return NULL;
        }

        switch(i % 4) {
        case 0:
            result[j] = ch << 2;
            break;
        case 1:
            result[j++] |= ch >> 4;
            result[j] = (ch & 0x0f) << 4;
            break;
        case 2:
            result[j++] |= ch >>2;
            result[j] = (ch & 0x03) << 6;
            break;
        case 3:
            result[j++] |= ch;
            break;
        }
        i++;
    }

    k = j;
    
    if (ch == base64_pad) {
        switch(i % 4) {
        case 1:
            free(result);
            return NULL;
        case 2:
            k++;
        case 3:
            result[k++] = 0;
        }
    }

    result[j] = '\0';
    return result;
}

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