/* The C Programming Language
   my answer to exercise 2-3

   Convert a hexadecimal string to an integer.
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

int htoi(char h[]);

int main(int argc, char* argv[])
{
    char hex[] = "0xDEAD";
    printf("%s in hex is equivalent to %d",
           hex, htoi(hex) );
    return 0;
}

/* Accepts a string of hexadecimal characters
   and returns its equivalent integral value. */

int htoi(char h[])
{
    int dec, n, i;
    dec = n = 0;

    for (i = strlen(h)-1; i >= 0; --i, ++n)
        if (h[i] == '0')
            continue;
        else if (h[i] >= '0' && h[i] <= '9')
            dec += (h[i] - '0') * pow(16,n);
        else if(h[i] >= 'A' && h[i] <= 'F')
            dec += (h[i] - 'A' + 10) * pow(16,n);
        else
            if(i == 1 && h[i] == 'x' && h[i-1] == '0')
                return dec;
            else
                return 0;

    return dec;
}