/**
 * puzzle.c
 *   A brute force approach to a simple puzzle.
 */

#include <stdio.h>

/**
 * Determine if X is a solution to the puzzle.
 */
int is_solution (int X)
{
  int Y = 2*X;                  // Y from the puzzle
  unsigned int digits = 1;      // A bit set to represent the digits seen
  unsigned int bit;             // One bit in the set
  int i;                        // Generic counter variable

  // Process the digits in X
  for (i = 0; i < 4; i++)
    {
      bit = 0x1 << (X % 10);
      if (digits & bit) return 0;
      digits |= bit;
      X = X / 10;
    }

  // Process the digits in Y
  for (i = 0; i < 5; i++)
    {
      bit = 0x1 << (Y % 10);
      if (digits & bit) return 0;
      digits |= bit;
      Y = Y / 10;
    }

  // If we've made it this far, we've got a solution.
  return 1;
} // is_solution

int
main ()
{
  int X;
  for (X = 0; X <= 9999; X++)
    if (is_solution (X))
      printf ("X = %d, Y = %d\n", X, X*2);
  return 0;
} // main