función hex2bin en C, implementación en PHP

hex2bin.c

static char *php_hex2bin(const unsigned char *old, const size_t oldlen, size_t *newlen)
{
    size_t target_length = oldlen >> 1;
    register unsigned char *str = (unsigned char *)safe_emalloc(target_length, sizeof(char), 1);
    size_t i, j;
    for (i = j = 0; i < target_length; i++) {
        char c = old[j++];
        if (c >= '0' && c <= '9') {
            str[i] = (c - '0') << 4;
        } else if (c >= 'a' && c <= 'f') {
            str[i] = (c - 'a' + 10) << 4;
        } else if (c >= 'A' && c <= 'F') {
            str[i] = (c - 'A' + 10) << 4;
        } else {
            efree(str);
            return NULL;
        }
        c = old[j++];
        if (c >= '0' && c <= '9') {
            str[i] |= c - '0';
        } else if (c >= 'a' && c <= 'f') {
            str[i] |= c - 'a' + 10;
        } else if (c >= 'A' && c <= 'F') {
            str[i] |= c - 'A' + 10;
        } else {
            efree(str);
            return NULL;
        }
    }
    str[target_length] = '\0';

    if (newlen)
        *newlen = target_length;

    return (char *)str;
}