/*
 * 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 * s);

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)) {
        printf("a palindrome\n");
    } 
    else {
        printf("not a palindrome\n");
    }
}

bool is_palindrome(char * s) {
    char *p1 = s;
    char *p2 = s + strlen(s) - 1;
    while (p1 < p2) {
        while (!isalnum(*p1) && (p1 < p1+strlen(s))) ++p1;
        while (!isalnum(*p2) && (p2 >= s)) --p2;
        if (p1 >= p2) {
            return true;
        }
        if (tolower(*p1) != tolower(*p2)) {
            return false;
        }
        ++p1; --p2;
    }
    return true;
}