/*
 * Program to check whether line of text is palindrome -- the same
 * backward as forward, ignoring case and characters other than letters
 * and digits.
 *
 * Also prints error message if line is too long for program.
 */
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include <ctype.h>

#define LINESIZE 100

bool is_palindrome(char * start, char * end);

int main(void) {
    printf("enter a line of text:\n");
    char line[LINESIZE];
    fgets(line, sizeof(line), stdin);
    char *endptr = strchr(line, '\n');
    if (endptr == NULL) {
        printf("line too long, maximum %zd\n", sizeof(line));
        return 1;
    }
    *endptr = '\0';
    printf("input '%s'\n", line);
    if (is_palindrome(line, line+strlen(line)-1)) {
        printf("a palindrome\n");
    } 
    else {
        printf("not a palindrome\n");
    }
}

bool is_palindrome(char * start, char * end) {
    if (end <= start) {
        /* nothing more to check */
        return true; 
    } 
    else if (!isalnum(*start)) {
        /* first character not a letter or digit -- skip */
        return is_palindrome(start+1, end);
    } 
    else if (!isalnum(*end)) {
        /* last character not a letter or digit -- skip */
        return is_palindrome(start, end-1);
    } 
    else if (tolower(*start) == tolower(*end)) {
        /* first, last characters match, ignoring case */
        return is_palindrome(start+1, end-1);
    }
    else {
        /* first, last characters don't match, ignoring case */
        return false;
    }
}