跨平台 C URL 编解码

char dec2hexChar(short int n) {
 if ( 0 <= n && n <= 9 ) {
  return char( short('0') + n );
 } else if ( 10 <= n && n <= 15 ) {
  return char( short('A') + n - 10 );
 } else {
  return char(0);
 }
}
short int hexChar2dec(char c) {
 if ( '0'<=c && c<='9' ) {
  return short(c-'0');
 } else if ( 'a'<=c && c<='f' ) {
  return ( short(c-'a') + 10 );
 } else if ( 'A'<=c && c<='F' ) {
  return ( short(c-'A') + 10 );
 } else {
  return -1;
 }
}
string escapeURL(const string &URL)
{
 string result = "";
 for ( unsigned int i=0; i<URL.size(); i++ ) {
  char c = URL[i];
  if (
   ( '0'<=c && c<='9' ) ||
   ( 'a'<=c && c<='z' ) ||
   ( 'A'<=c && c<='Z' ) ||
   c=='/' || c=='.'
   ) {
   result += c;
  } else {
   int j = (short int)c;
   if ( j < 0 ) {
    j += 256;
   }
   int i1, i0;
   i1 = j / 16;
   i0 = j - i1*16;
   result += '%';
   result += dec2hexChar(i1);
   result += dec2hexChar(i0);
  }
 }
 return result;
}
std::string deescapeURL(const std::string &URL) {
 string result = "";
 for ( unsigned int i=0; i<URL.size(); i++ ) {
  char c = URL[i];
  if ( c != '%' ) {
   result += c;
  } else {
   char c1 = URL[++i];
   char c0 = URL[++i];
   int num = 0;
   num += hexChar2dec(c1) * 16 + hexChar2dec(c0);
   result += char(num);