sieve.c

/*
emcc -O3 sieve.c -s EXPORTED_FUNCTIONS='["_pi"]' -s
EXPORTED_RUNTIME_METHODS='["ccall","cwrap"]'

Then:

> pi2 = require('./a.out.js').cwrap('pi', 'number', ['number']); {pi} =
require('./sieve.js') > t = new Date(); for(i=0;i<10;i++){pi2(1000000+i);} new
Date() - t; 71 > t = new Date(); for(i=0;i<10;i++){pi(1000000+i);} new Date() -
t; 559
*/

#include <stdio.h>
#include <malloc.h>

int pi(int number) {
  int i, j;

  int* primes = (int*)malloc(sizeof(int) * (number + 1));

  for (i = 2; i <= number; i++) primes[i] = i;

  i = 2;
  while ((i * i) <= number) {
    if (primes[i] != 0) {
      for (j = 2; j < number; j++) {
        if (primes[i] * j > number)
          break;
        else
          // Instead of deleting , making elements 0
          primes[primes[i] * j] = 0;
      }
    }
    i++;
  }

  int cnt = 0;
  for (i = 2; i <= number; i++) {
    // If number is not 0 then it is prime
    if (primes[i] != 0) {
      cnt += 1;
    }
  }

  free(primes);

  return cnt;
}

int main() {
  int j = 0;
  for (int i = 0; i < 100; i++) {
    j += pi(3 * 1000000);
    printf('%d\n', j);
    fflush(stdout);
  }
  printf("%d\n", j);
}