palindromic calculation

palindromic.c

//
// A palindromic number reads the same both ways. The largest palindrome
// made from the product of two 2-digit numbers is 9009 = 91 х 99.
//
// Find the largest palindrome made from the product of two 3-digit numbers.
//

#include <stdio.h>
#include <stdlib.h>
#include <math.h>  // !!! don't forget to link against math library: -lm !!!


// ok, seems I need this function now()
// although it would be more «obvious» to do this working with number as a
// string let's do numbers
int is_palindrome(size_t number) {
#define DIGIT(number, power) ((size_t) number / ((size_t) pow(10, power))) % 10

	// figuring out digits numbers in the number
	ssize_t first = 0, last = 0, digits = 0;
	for (ssize_t d = number; d > 1E+1; d *= 1E-1) {
		digits++;
	}

	// also, not the best way to do this, it works twice!!!
	for (first = digits, last = 0; first != -1; first -= 1, last += 1) {
		if (DIGIT(number, first) != DIGIT(number, last))
			return 0;
	}

	return 1;
}

int main(int argc, char *argv[])
{

	// there can be deffinitely more productive ways to produce input for
	// iteration which would cut their domain by the power of ten

	#define NUM 1E+3
	size_t palindrome = 0, first, last;

	for (size_t i = 0; i < NUM; i++) {
		for (size_t j = 0; j  <= NUM; j++) {
			if (is_palindrome(i*j) && (i*j) > palindrome) {
				first = i;
				last = j;
				palindrome = first * last;

			}
		}
	}
	printf("%zu * %zu gives the largest palindrome:%zu\n",
			first, last, palindrome);
	return EXIT_SUCCESS;
}