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/*
 *  from --> to  %e8%8b%a6 --> 苦
 */
void strdecode(char *to, char *from)
{
    for ( ; *from != '\0'; ++to, ++from) {

        if (from[0] == '%' && isxdigit(from[1]) && isxdigit(from[2])) { //依次判断from中 %20 三个字符

            *to = hexit(from[1])*16 + hexit(from[2]);
            from += 2;                      //移过已经处理的两个字符(%21指针指向1),表达式3的++from还会再向后移一个字符
        } else
            *to = *from;
    }
    *to = '\0';
}

//16进制数转化为10进制, return 0不会出现
int hexit(char c)
{
    if (c >= '0' && c <= '9')
        return c - '0';
    if (c >= 'a' && c <= 'f')
        return c - 'a' + 10;
    if (c >= 'A' && c <= 'F')
        return c - 'A' + 10;

    return 0;
}

//"编码"，用作回写浏览器的时候，将除字母数字及/_.-~以外的字符转义后回写。
//苦 --> %e8%8b%a6
void strencode(char* to, size_t tosize, const char* from)
{
    int tolen;

    for (tolen = 0; *from != '\0' && tolen + 4 < tosize; ++from) {
        if (isalnum(*from) || strchr("/_.-~", *from) != (char*)0) {
            *to = *from;
            ++to;
            ++tolen;
        } else {
            sprintf(to, "%%%02x", (int) *from & 0xff);
            to += 3;
            tolen += 3;
        }
    }
    *to = '\0';
}