// ShowVars
// A demonstration of CGI written in C
// This program shows all environment variables
// and POST data (when invoked with the POST method)

#include <windows.h>  // only required for Windows
#include <stdio.h>
#include <stdlib.h>

// SwapChar:  This routine swaps one character for another
void SwapChar(char * pOriginal, char cBad, char cGood) {
    int i;    // generic counter variable

    // Loop through the input string (cOriginal), character by
    // character, replacing each instance of cBad with cGood

    i = 0;
    while (pOriginal[i]) {
        if (pOriginal[i] == cBad) pOriginal[i] = cGood;
        i++;
    }
}

// IntFromHex:  A subroutine to unescape escaped characters.
static int IntFromHex(char *pChars) {
    int Hi;        // holds high byte
    int Lo;        // holds low byte
    int Result;    // holds result

    // Get the value of the first byte to Hi

    Hi = pChars[0];
    if ('0' <= Hi && Hi <= '9') {
        Hi -= '0';
    } else
    if ('a' <= Hi && Hi <= 'f') {
        Hi -= ('a'-10);
    } else
    if ('A' <= Hi && Hi <= 'F') {
        Hi -= ('A'-10);
    }

    // Get the value of the second byte to Lo

    Lo = pChars[1];
    if ('0' <= Lo && Lo <= '9') {
        Lo -= '0';
    } else
    if ('a' <= Lo && Lo <= 'f') {
        Lo -= ('a'-10);
    } else
    if ('A' <= Lo && Lo <= 'F') {
        Lo -= ('A'-10);
    }
    Result = Lo + (16 * Hi);
    return (Result);
}

// URLDecode: This routine loops through the string pEncoded
// (passed as a parameter), and decodes it in place. It checks for
// escaped values, and changes all plus signs to spaces. The result
// is a normalized string. It calls the two subroutines directly
// above in this listing, IntFromHex() and SwapChar().

void URLDecode(unsigned char *pEncoded) {
    char *pDecoded;          // generic pointer

    // First, change those pesky plusses to spaces
    SwapChar (pEncoded, '+', ' ');

    // Now, loop through looking for escapes
    pDecoded = pEncoded;
    while (*pEncoded) {
        if (*pEncoded=='%') {
            // A percent sign followed by two hex digits means
            // that the digits represent an escaped character. We
            // must decode it.

            pEncoded++;
            if (isxdigit(pEncoded[0]) && isxdigit(pEncoded[1])) {
                *pDecoded++ = (char) IntFromHex(pEncoded);
                pEncoded += 2;
            }
        } else {
            *pDecoded ++ = *pEncoded++;
        }
    }
    *pDecoded = '\0';
}