/* Simple Sieve of Eratosthenes microbenchmark. */
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

void
mark_off(char *is_composite, int p, int max)
{
  for (int i = p * p; i < max; i += p) is_composite[i] = 1;
}

int 
main(int argc, char **argv)
{
  int max;
  if (argc != 2 || (max = atoi(argv[1])) == 0) {
    fprintf(stderr, "Usage: %s 53\n"
            "Prints the last prime number before 53\n",
            argv[0]);
    return -1;
  }

  char *is_composite = malloc(max);
  if (!is_composite) {
    perror("malloc");
    return 1;
  }

  memset(is_composite, 0, max);

  mark_off(is_composite, 2, max);
  for (int i = 3; i * i < max; i += 2) {
    if (!is_composite[i]) mark_off(is_composite, i, max);
  }

  for (int p = max-1; p > 1; p--) {
    if (is_composite[p]) continue;
    printf("%d\n", p);
    break;
  }

  return 0;
}