http://www.numberspiral.com

numberspiral.go

package main

import (
	"image"
	"image/color"
	"image/draw"
	"image/png"
	"log"
	"math"
	"os"
)

type NumberSpiral struct {
	NIntegers  int
	Resolution float64 // Pixel per unit
	image      draw.Image
	nonprimes  []bool
}

func (ns *NumberSpiral) prepare() {
	// Calculate image bounds and initialize image
	max := int(math.Ceil(math.Sqrt(float64(ns.NIntegers)) * ns.Resolution))
	rect := image.Rect(
		-max, -max, max, max,
	)
	ns.image = image.NewGray(rect)

	// Sieve primes
	ns.nonprimes = make([]bool, ns.NIntegers)
	ns.nonprimes[0] = true
	ns.nonprimes[1] = true
	sr := int(math.Sqrt(float64(ns.NIntegers)))
	for i := 2; i <= sr; i++ {
		if ns.nonprimes[i] {
			continue
		}
		for n := i * 2; n < ns.NIntegers; n += i {
			ns.nonprimes[n] = true
		}
	}
}

func (ns *NumberSpiral) Draw() {
	ns.prepare()
	// Fill with white color
	draw.Draw(ns.image, ns.image.Bounds(), image.White, image.ZP, draw.Src)
	// Draw numbers.
	for i := 0; i < ns.NIntegers; i++ {
		r := math.Sqrt(float64(i))
		theta := r * 2 * math.Pi
		x := int(r * math.Cos(theta) * ns.Resolution)
		y := int(-r * math.Sin(theta) * ns.Resolution)
		var c color.Color
		if ns.nonprimes[i] {
			c = color.Gray{192}
		} else {
			c = color.Black
		}
		ns.image.Set(x, y, c)
	}
}

func (ns *NumberSpiral) Image() image.Image {
	return ns.image
}

func main() {
	ns := NumberSpiral{NIntegers: 1000, Resolution: 5.0}
	ns.Draw()

	f, err := os.Create("output.png")
	if err != nil {
		log.Fatal("Could not open output file.")
	}
	png.Encode(f, ns.Image())
	f.Close()
}