Usage: visprim <width> <height> <output-filename> [number-of-threads]

Makefile

CPP = g++
LIB = `Magick++-config --cppflags --cxxflags --ldflags --libs` -lboost_thread-mt
CPPFLAGS = -Wall -O3 `Magick++-config --cppflags --cxxflags`
LDFLAGS = $(LIB) -Wall -O3
BIN = visprim

.PHONY: all zip clean

all: $(BIN)

$(BIN): visprim.cpp
	$(CPP) $< -o $@ $(LDFLAGS)

zip: visprim.zip

visprim.zip:
	zip -r9 visprim.zip visprim.cpp Makefile

clean:
	rm -f $(BIN)

visprim.cpp

#include <cmath>
#include <iostream>
#include <iomanip>
#include <boost/lexical_cast.hpp>
#include <boost/thread.hpp>
#include <Magick++.h>

using namespace std;
using namespace boost;
using namespace Magick;

void usage(const char * appname) {
	cout << "Usage: " << appname << " <width> <height> <output-filename> [number-of-threads]" << endl;
}

bool inline isprime(unsigned int n) {
	if (n == 2) {
		return true;
	}
	else if (!(n & 1) || n < 2) {
		return false;
	}
	else {
		unsigned int stop = (unsigned int)ceil(sqrt((double)n));

		for (unsigned int p = 3; p <= stop; p += 2) {
			if (n % p == 0) {
				return false;
			}
		}

		return true;
	}
}

class Worker {
	private:
		PixelPacket *const pixel_cache;
		const unsigned int begin;
		const unsigned int end;

	public:
		Worker(PixelPacket *pixel_cache, unsigned int begin, unsigned int end) :
			pixel_cache(pixel_cache), begin(begin), end(end) {}

		void operator () () const {
			const ColorRGB red( 1, 0, 0 );

			for (unsigned int i = begin; i < end; ++ i) {
				if (isprime(i)) {
					pixel_cache[i] = red;
				}
			}
		}
};

int main(int argc, char * argv[]) {
	if (argc < 4 || argc > 5) {
		cerr << "*** illegal number of arguments" << endl;
		if (argc < 1) {
			usage("visprim");
		}
		else {
			usage(*argv);
		}
		return 1;
	}

	unsigned int width;
	unsigned int height;
	unsigned int threadcount = 1;
	const char * filename = argv[3];

	try {
		width  = lexical_cast<unsigned int>(argv[1]);
		height = lexical_cast<unsigned int>(argv[2]);

		if (argc == 5) {
			threadcount = lexical_cast<unsigned int>(argv[4]);
		}
	}
	catch (bad_lexical_cast &) {
		cerr << "*** illegal argument" << endl;
		usage(*argv);
		return 1;
	}

	if (threadcount == 0) {
		cerr << "*** illegal argument" << endl;
		usage(*argv);
		return 1;
	}

	Image image( Geometry( width, height ), ColorRGB( 1, 1, 1 ) );
	
	// Ensure that there are no other references to this image
	image.modifyImage();
	image.type(TrueColorType);
	
	PixelPacket *pixel_cache = image.getPixels(0,0,width,height);
	unsigned int n = width * height;
	
	if (threadcount == 1) {
		Worker(pixel_cache, 0, n)();
	}
	else {
		thread_group threads;
		unsigned int begin = 0;
		double segment = ((double)n * (double)n) / (threadcount * 2.0l);
		double area    = 0;
		double x       = 0;
		
		for (unsigned int i = 0; i < threadcount - 1; ++ i) {
			area += segment;
			x = sqrt(2.0l * area);

			unsigned int end = (unsigned int) x;
			
			threads.create_thread( Worker(pixel_cache, begin, end) );
			
			begin = end;
		}
		threads.create_thread( Worker(pixel_cache, begin, n) );
		
		threads.join_all();
	}
	// Save changes to underlying image
	image.syncPixels();
	image.write( filename );

	return 0;
}