/*
Programmer:   Greg Perkins
Program:      pal.c
Class:        CS 73  C/UNIX
Due:          March 10, 1998

This program will identify whether or not a string of characters (line)
entered by the user is a palindrome.  The program will accept a maximum
of 80 characters to check.
*/

#include <stdio.h>

#define SIZE 81

void get_string (char *str);
/*
PRE:  str must be declared does not have to be initialized
		SIZE (global variable) declared and initialized to max value desired
POST: str contains user entered string of length SIZE or less
*/

int clean (char *str);
/*
PRE:  str must be declared and initialized
POST: str contains all alnum chars from original str (all in lowercase)
		fctval = number of chars in str minus '\0'
*/

void get_mid (const int len, int *low, int *high);
/*
PRE:  len must represent the number of elements in an array
		low must be declared
		high must be declared
POST: low holds position of center element
		high holds position of center element
		low == high, when number of elements is odd
		low == high + 1, when number of elements is even
*/

int is_palindrome (const char *str, const int len, int low, int high);
/*
PRE:  str must contain ONLY lowercase alnum chars
		len must represent the number of alnum chars in str
		low must represent the center element of str
		high must represent the center element of str
		low == high, when number of elements is odd
		low == high + 1, when number of elements is even
POST: fctval == 1, if no chars, or only one char, in str
		fctval == 1, if 2 or more chars in str are a palindrome
		fctval == 0, if chars in str are not a palindrome
*/

int main ()
{
	char line[SIZE];
	int length;
	int low_mid, high_mid;
	int *l_ptr, *h_ptr;

	l_ptr = &low_mid;
	h_ptr = &high_mid;

	get_string(line);                    /* get test string from user */

	printf("You entered:\n%s\n", line);

   /* remove all non-alnum chars and make remaining chars lowercase */
	length = clean (line);               /* get num of chars in line */

	printf("Clean line (length = %d):\n%s\n", length, line);

	get_mid (length, l_ptr, h_ptr);      /* find middle element(s) */

	/* test line and display the results */
	if (is_palindrome (line, length, low_mid, high_mid)) 
		printf("\nYour string is a palindrome.\n");
	else
		printf("\nYour string is NOT a palindrome.\n");
		
	return (0);
}

void get_string(char *str)
{
	char ch;
	int i = 0;

	while ((i < SIZE) && ((ch = getchar()) != EOF) && (ch != '\n'))
		str[i++] = ch;

	if (ch == '\n')
		str[i++] = ch;

	str[i] = '\0';

	return;
}

int clean (char *str)
{
	int i, j = 0;

	/* examine str[] - keep only alnum chars and convert them to lowercase */
	for (i = 0; str[i] != '\0'; i++)
		if (isalnum(str[i]))
			str[j++] = tolower(str[i]);

	str[j] = '\0';

	return (j);            /* return num of chars in str not including '\0' */
}

void get_mid (const int len, int *low, int *high)
{
	*low = (len - 1) / 2;

	if (((len - 1) % 2) == 0)
		*high = *low;
	else
		*high = *low + 1;
}

int is_palindrome (const char *str, const int len, int low, int high)
{
	if ((len == 0) || (len == 1))
		return (1);

	if (str[low] == str[high])
		if (low == 0)
			return (1);
		else
			return (is_palindrome (str, len, --low, ++high));
	else
		return (0);
}