//-------------------------------------------------------------------
//	sieve.cpp
//
//	This program implements a version of the famous Prime Number
//	Sieve algorithm, credited to the ancient Greek mathematician
//	Erastosthenes (circa 250 B.C.).  We will use some variations
//	of this program to study data-access timings and the effects
//	of caches used by the processor and by the operating system.
//
//	programmer: ALLAN CRUSE
//	written on: 12 SEP 2004
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <stdlib.h>	// for atoi(), malloc(), exit(), free()  

int main( int argc, char **argv )
{
	int	max = 400000;	// default array-size	
	if ( argc > 1 ) max = atoi( argv[ 1 ] );
	printf( "\ncounting prime numbers below %d \n", max );

	// allocate array-storage 
	char	*array = (char*)malloc( max );
	if ( !array ) { perror( "malloc" ); exit(1); }

	// initialize the array
	for (int i = 0; i < max; i++) array[ i ] = ( i > 1 ) ? 1 : 0;

	// perform 'sieving' of non-primes
	for (int p = 0; p < max; p++)
		{
		if ( array[ p ] == 0 ) continue;
		for (int i = p+p; i < max; i += p) array[ i ] = 0;
		}
	
	// count those integers remaining
	int	count = 0;
	for (int i = 0; i < max; i++) if ( array[ i ] ) ++count;
	printf( "\nNumber of primes below %d is %d \n\n", max, count );

	// release allocated array-storage
	free( array );
}