lychrel number

lychrel.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <gmp.h>

bool check_symmetry(mpz_t i)
{
  char* s = mpz_get_str(NULL, 10, i);
  size_t len = strlen(s);
  for (size_t j = 0; j < len / 2 ; ++j) {
    if (s[j] != s[len - j - 1]) {
      free(s);
      return false;
    }
  }
  free(s);
  return true;
}

char *strrev(char *str)
{
  char *p1, *p2;

  if (! str || ! *str)
    return str;
  for (p1 = str, p2 = str + strlen(str) - 1; p2 > p1; ++p1, --p2)
  {
    *p1 ^= *p2;
    *p2 ^= *p1;
    *p1 ^= *p2;

  }
  return str;
}

void reverse(mpz_t r, mpz_t i)
{
  char* rs = strrev(mpz_get_str(NULL, 10, i));
  mpz_set_str(r, rs, 10);
  free(rs);
}

int main (int argc, char **argv)
{
  mpz_t i;

  if (argc != 2)
  {
    printf ("Usage: %s <number>\n", argv[0]);
    return EXIT_FAILURE;
  }

  mpz_init_set_str(i, argv[1], 10);
  mpz_t r;
  mpz_init(r);

  int c = 1;
  for (;;) {
    if (c % 1000 == 0) {
      printf("%d\n", c);
    }
    if (check_symmetry(i)) {
      gmp_printf ("%Zd\n", i);
      return EXIT_SUCCESS;
    }
    reverse(r, i);
    mpz_add(i, i, r);
    ++c;
  }
}