//-------------------------------------------------------------------
//	fsieve.cpp
//
//	This is a modification of out 'sieve.cpp' program which uses
//	a disk-file (instead of a memory-array) as its storage area.
//
//	programmer: ALLAN CRUSE
//	written on: 12 SEP 2004
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror(), remove() 
#include <stdlib.h>	// for atoi(), malloc(), exit(), free()  
#include <unistd.h>	// for write(), lseek(), read(), close()
#include <fcntl.h>	// for open()

#define FILEFLAGS ( O_RDWR | O_CREAT | O_TRUNC | O_SYNC )

char filename[] = "sieve.dat";

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 ", max );

	int	m = 512;	// default index-multiplier
	if ( argc > 2 ) m = atoi( argv[ 2 ] );
	printf( "(index-multiplier=%d) \n", m );
	
	// create a new disk-file for storage
	int	fd = open( filename, FILEFLAGS, 0666 );
	if ( fd < 0 ) { perror( filename ); exit(1); }
	
	// initialize the storage-area
	for (int i = 0; i < max; i++) 
		{
		char	val = ( i > 1 ) ? 1 : 0;
		lseek( fd, i*m, SEEK_SET );
		write( fd, &val, 1 );
		}

	// performing 'sieving' of non-primes
	for (int p = 0; p < max; p++)
		{
		char	val;
		lseek( fd, p*m, SEEK_SET );
		read( fd, &val, 1 );	
		if ( val == 0 ) continue;
		val = 0;
		for (int i = p+p; i < max; i += p) 
			{
			lseek( fd, i*m, SEEK_SET );
			write( fd, &val, 1 );
			}
		}

	// count those integers remaining	
	int	count = 0;
	for (int i = 0; i < max; i++) 
		{
		char	val;
		lseek( fd, i*m, SEEK_SET );
		read( fd, &val, 1 );
		if ( val ) ++count;
		}
	printf( "\nNumber of primes below %d is %d \n\n", max, count );

	// close the file and release its disk-space  
	close( fd );
	remove( filename );
}