/** Exercise 3-3 from K&R The C programming language Book */

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

#define MAX_LENGTH 1000

void expand(char s1[], char s2[]);

int main()
{
    char to[MAX_LENGTH];
    for (int i = 0; i < MAX_LENGTH; i++)
    {
        to[i] = 0;
    }
    char from[] = "---a-b-z-A-Z0-9-----";
    expand(from, to);
    printf("%s\n", to);
}

void expand(char s1[], char s2[])
{
    char previous_char = '\0';
    int j = 0;
    for (int i = 0; s1[i] != '\0'; i++)
    {
        char next_char = s1[i + 1];
        if (s1[i] == '-' && previous_char != '\0' && next_char != '\0' && previous_char < next_char && isalnum(previous_char) && isalnum(next_char))
        {
            while(previous_char < next_char - 1)
            {
                s2[j++] = ++previous_char;
            }
            previous_char = '\0';
        } else {
            s2[j++] = s1[i];
            previous_char = s1[i];
        }
    }
    s2[j] = '\0';
}