55.c

/**
 * If we take 47, reverse and add, 47 + 74 = 121, which is palindromic.
 * 
 * Not all numbers produce palindromes so quickly. For example,
 * 
 * 349 + 943 = 1292,
 * 1292 + 2921 = 4213
 * 4213 + 3124 = 7337
 * 
 * That is, 349 took three iterations to arrive at a palindrome.
 * 
 * Although no one has proved it yet, it is thought that some numbers, like 196,
 * never produce a palindrome. A number that never forms a palindrome through
 * the reverse and add process is called a Lychrel number. Due to the
 * theoretical nature of these numbers, and for the purpose of this problem, we
 * shall assume that a number is Lychrel until proven otherwise. In addition you
 * are given that for every number below ten-thousand, it will either (i) become
 * a palindrome in less than fifty iterations, or, (ii) no one, with all the
 * computing power that exists, has managed so far to map it to a palindrome. In
 * fact, 10677 is the first number to be shown to require over fifty iterations
 * before producing a palindrome: 4668731596684224866951378664 (53 iterations,
 * 28-digits).
 * 
 * Surprisingly, there are palindromic numbers that are themselves Lychrel
 * numbers; the first example is 4994.
 * 
 * How many Lychrel numbers are there below ten-thousand?
 * 
 * NOTE: Wording was modified slightly on 24 April 2007 to emphasise the
 * theoretical nature of Lychrel numbers.
 */

#include <stdio.h>

#include "helpers.h"

#define LIMIT 10000

int lychrel(int n)
{
  long long unsigned int tmp = n, sum, rev;
  int i;

  for (i = 0; i < 50; i++) {
    rev = reverse(tmp);
    sum = tmp + rev;

    if (palindromic10(sum)) {
      return 0;
    } else {
      tmp = sum;
    }
  }

  return 1;
}

int main(int argc, char const *argv[])
{
  int result = 0, i;

  for (i = 0; i < LIMIT; i++) {
    if (lychrel(i))
      result++;
  }

  printf("\n\n");
  printf("Answer: %d\n", result);

  return 0;
}