C_PRG_LANG_EX_2.3

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

int power(int base, int p)
{
    int result = 1;

    while(p != 0)
    {
        result *= base;
        p--;
    }

    return result;
}

int htoi(char *hex)
{
    int i = 0;
    while (hex[i] != '\0')
        i++;

    i--;
    int dec = 0;
    int degree = 0;
    while(i >= 0 && hex[i] != 'x' && hex[i] != 'X')
    {
        if (isdigit(hex[i]))
            dec += (hex[i] - '0') * power(16, degree);
        else
        {
            if (hex[i] == 'a' || hex[i] == 'A')
                dec += (10) * power(16, degree);
            else if (hex[i] == 'b' || hex[i] == 'B')
                dec += (11) * power(16, degree);
            else if (hex[i] == 'c' || hex[i] == 'C')
                dec += (12) * power(16, degree);
            else if (hex[i] == 'd' || hex[i] == 'D')
                dec += (13) * power(16, degree);
            else if (hex[i] == 'e' || hex[i] == 'E')
                dec += (14) * power(16, degree);
            else if (hex[i] == 'f' || hex[i] == 'F')
                dec += (15) * power(16, degree);
        }
        i--;
        degree++;
    }

    return dec;
}

int main(void)
{
    char *hex = malloc(sizeof(char) * 256);
    strcpy(hex, "0XAB");
    printf("%s in hex is %i in dec\n", hex, htoi(hex));
    strcpy(hex, "0xab");
    printf("%s in hex is %i in dec\n", hex, htoi(hex));
    strcpy(hex, "ab");
    printf("%s in hex is %i in dec\n", hex, htoi(hex));
    strcpy(hex, "BBB");
    printf("%s in hex is %i in dec\n", hex, htoi(hex));
    strcpy(hex, "CCC");
    printf("%s in hex is %i in dec\n", hex, htoi(hex));
    strcpy(hex, "F23F");
    printf("%s in hex is %i in dec\n", hex, htoi(hex));
    free(hex);
    return 0;
}