/******************************************************************************
 *  Base to Decimal (hex to int)
 * 
 *  Solution to K&$ 2-3 (supposed to be called htoi.c for hex to int)
 *  But this version is generalized through #defines to sort of work
 *  with any base (11 and up), so I left out the leading 0x--actually
 *  I forgot about it.
 *  At first I was annoyed by how hard this seemed, coming so early 
 *  in the book. But after I worked it out it made sense. 
 *
 *  program: base_to_dec
 *  by.....: yetimach
 *  date...: 25 October 2024
 *
 *****************************************************************************/

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

#define MAX 7 
#define BASE 16
#define BASE_NAME "hex"
#define END_LETTER 'f'

void read_line(char string[], int max){
    int c, i = 0;
    while (i < max && (c = getchar()) != '\n' && c != EOF){
        string[i] = c;
        ++i;
    }
}

int power(int base, int pow){
    int result = 1;
    if (pow == 0)
        return result;
    while (pow-- > 0)
        result *= base;
    return result;
}

int base_to_dec(char string[]){
    int i = 0, result = 0, single_value;
    int degree = 0;
    while (string[i] != '\0')
        ++i;
    --i;
    while (i >= 0){
        single_value = tolower(string[i]);
        if (string[i] >= '0' && string[i] <= '9'){
            result += ((power(BASE, degree)) * (string[i] - '0'));   
        }else if (single_value >= 'a' && single_value <= END_LETTER){
            result += ((power(BASE, degree) * (single_value - 'a' + 10)));
        }else{
            printf("\nInvalid %s value entered.\n", BASE_NAME);
            return -1; 
        }
        ++degree;
        --i;
    }
    return result;
}

int main(void)
{
    char string[MAX+1];
    printf("\nEnter a %s value:", BASE_NAME);
    read_line(string, MAX);
    int result = base_to_dec(string);
    if (result != -1)
        printf("\nThat is %d in decimal.\n", result);
    return 0;
}