/* URL encoding C version
*       Lin Jensen
*       Feb 21, 1999
*       A new string is produced, URL encoded. Rules are
*               Alphanumeric chars unchanged
*               Space --> '+'
*               Special chars (the rest) --> "%hh" where hh is hexadecimal
************************************************************************/
#include <stdio.h>

char bin2hex (char ch){         /* return ascii char for hex value of
                                    rightmost 4 bits of input */
    ch = ch & 0x0f;            /* mask off right nibble  - & bitwise AND*/
    ch += '0';                  /* make ascii '0' - '9'  */
    if (ch > '9')
        ch += 7;                /* account for 7 chars between '9' and 'A' */
    return (ch);
}

int AlphaNumeric (char ch){
    return ((ch >='a') && (ch <= 'z') ||   /* logical AND &&, OR || */
            (ch >='A') && (ch <= 'Z') ||
            (ch >='0') && (ch <= '9') );
}


int URLencode (const char * plain, char * encode, int maxlen){
    char ch;            /* each char, use $t2 */
    char * limit;       /* point to last available location in encode */
    char * start;       /* save start of encode for length calculation */

    limit = encode + maxlen - 4;    /* need to store 3 chars and a zero */
    ch = *plain ++;    /* get first character */
    while (ch != 0) {       /* end of string, asciiz */
        if (ch == ' ')
            * encode ++ = '+';
        else if (AlphaNumeric (ch))
            * encode ++ = ch;
        else {
            * encode ++ = '%';
            * encode ++ = bin2hex (ch >> 4); /*shift right for left nibble*/
            * encode ++ = bin2hex (ch);      /* right nibble */
        }
        ch = *plain ++;               /* ready for next character */
        if (encode > limit){
            *encode = 0;        /* still room to terminate string */
            return (-1);        /* get out with error indication */
        }
    }
    * encode = 0;               /* store zero byte to terminate string */
    return (encode - start);    /* done, return count of characters */
}   /* URLencode*/

void main() {
   char * sample = "This is my string, $2.95!";
   char result[50];
   URLencode (sample, result, 50);
   printf ("%s\n%s\n", sample, result);
}