main.go

package main

import (
	"flag"
	"fmt"
	"image"
	"image/color"
	"os"

	"github.com/kloster/ulam/prime"
	"golang.org/x/image/bmp"
)

func main() {
	var dim int
	flag.IntVar(&dim, "dim", 100, "The dimension of the resulting image (it will be square)")
	flag.Parse()

	outputFile := "./UlamSpiral.bmp"
	if len(flag.Args()) > 0 {
		outputFile = flag.Arg(0)
	}

	// Force the dimension to be odd to avoid black lines in the image
	if dim%2 == 0 {
		dim++
	}

	// Create the image
	img := image.NewRGBA(image.Rect(0, 0, dim, dim))

	// Fill it with the Ulam spiral
	fillWithUlamSpiral(img, color.RGBA{255, 255, 255, 255}, color.RGBA{0, 150, 0, 0})

	// Create the ouput file
	imgFile, err := os.Create(outputFile)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer imgFile.Close()

	// Encode the image as a bmp file
	if err = bmp.Encode(imgFile, img); err != nil {
		fmt.Println(err)
	}
}

// A straightforward implementation
func fillWithUlamSpiral(img *image.RGBA, compositeColor, primeColor color.RGBA) {
	bounds := img.Bounds()
	dim := bounds.Dx()                   // One of the dimensions of the image (it is square)
	x, y := dim/2, dim/2                 // The points to be painted. We start from the center
	dx, dy := 1, -1                      // The increment direction
	alreadyMovedX, alreadyMovedY := 0, 0 // The points we already have moved in the current row/column
	rowSize, columSize := 1, 1           // The current row/column size

	totalPoints := dim * dim
	for i := 0; i < totalPoints; i++ {
		numToCheck := i + 1
		if prime.Is(numToCheck) {
			img.Set(x, y, primeColor)
		} else {
			img.Set(x, y, compositeColor)
		}

		if alreadyMovedX < rowSize {
			x += dx
			alreadyMovedX++
		} else {
			if alreadyMovedY < columSize {
				y += dy
				alreadyMovedY++
			}
			if alreadyMovedY >= columSize {
				dx *= -1
				dy *= -1
				alreadyMovedX, alreadyMovedY = 0, 0
				rowSize++
				columSize++
			}
		}
	}
}