Exercise 2-3. Write a function htoi(s), which converts a string of hexadecimal digits (including an optional 0x or 0X) into its equivalent integer value. The allowable digits are 0 through 9, a through f, and A through F.

/* Exercise 2-3. Write a function htoi(s), which converts a string of hexadecimal digits (including an optional 0x or 0X) into its equivalent integer value. 
   The allowable digits are 0 through 9, a through f, and A through F. */

#include <stdio.h>
#include <ctype.h>
#include <stdbool.h>
#define LIMIT 8     /* Maximum no. of Hexadecimal bits */

int get_hex(char line[], int lim);
unsigned long int hex2int(char s[], int length);
unsigned long int char2int(char c);

int main()
{
    int len = 0;
    unsigned long int integer = 0;
    char inp[LIMIT + 2];

    printf("Enter the hexadecimal no (Max allowed no. of bits is %d):\n", LIMIT);
    len = get_hex(inp, (LIMIT + 2));
    integer = hex2int(inp, len);
    printf("\nHexadecimal number (Input): %s\nEquivalent integer number (Output): %lu\n", inp, integer);

    return 0;
}

/* getline: read a line (UPPERCASE) into s, return length */
int get_hex(char s[],int lim)
{
    int c, i;

    for (i = 0; i < lim && (c=getchar()) != EOF && c != '\n'; ++i)
        s[i] = toupper(c);
    s[i] = '\0';

    return i;
}

/* hex2int: read array containing heaxadecimal no. and return decimal integer value. */
unsigned long int hex2int(char s[], int length)
{
    int bit_pos;
    unsigned long int a = 0;
    bool cond = false;
    cond = (s[0] == '0' && s[1] == 'X');

    for (bit_pos = (length-1); (bit_pos >= (cond ? 2 : 0)); --bit_pos)      /* cond is used to exclude 0X prefix if present in the input*/
        a += (char2int(s[bit_pos])) << (4 * (length -1 - bit_pos));

    return a;
}

/* convert given character back to its integer form */
unsigned long int char2int(char c)
{
    unsigned long int i = 0;

    if (c >= '0' && c <= '9')
        i = (unsigned long int) (c - '0');
    else
    {
        switch(c)
        {
            case 'A':
                i = 10;
                break;
            case 'B':
                i = 11;
                break;
            case 'C':
                i = 12;
                break;
            case 'D':
                i = 13;
                break;
            case 'E':
                i = 14;
                break;
            case 'F':
                i = 15;
                break;
            default:
                i = 0;
                break;
        }   
    }

    return i;
}