// +build example

package main

import (
        "log"
        "math"

        "github.com/hajimehoshi/ebiten"
)

const (
        screenWidth  = 640
        screenHeight = 640
        maxIt        = 128
)

var (
        offscreen    *ebiten.Image
        offscreenPix []byte
        palette      [maxIt]byte
)

func init() {
        offscreen, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterDefault)
        offscreenPix = make([]byte, screenWidth*screenHeight*4)
        for i := range palette {
                palette[i] = byte(math.Sqrt(float64(i)/float64(len(palette))) * 0x80)
        }
}

func color(it int) (r, g, b byte) {
        if it == maxIt {
                return 0xff, 0xff, 0xff
        }
        c := palette[it]
        return c, c, c
}

func updateOffscreen(centerX, centerY, size float64) {
        for j := 0; j < screenHeight; j++ {
                for i := 0; i < screenHeight; i++ {
                        x := float64(i)*size/screenWidth - size/2 + centerX
                        y := (screenHeight-float64(j))*size/screenHeight - size/2 + centerY
                        c := complex(x, y)
                        z := complex(0, 0)
                        it := 0
                        for ; it < maxIt; it++ {
                                z = z*z + c
                                if real(z)*real(z)+imag(z)*imag(z) > 4 {
                                        break
                                }
                        }
                        r, g, b := color(it)
                        p := 4 * (i + j*screenWidth)
                        offscreenPix[p] = r
                        offscreenPix[p+1] = g
                        offscreenPix[p+2] = b
                        offscreenPix[p+3] = 0xff
                }
        }
        offscreen.ReplacePixels(offscreenPix)
}

func init() {
        // Now it is not feasible to call updateOffscreen every frame due to performance.
        updateOffscreen(-0.75, 0.25, 2)
}

func update(screen *ebiten.Image) error {
        if ebiten.IsRunningSlowly() {
                return nil
        }

        screen.DrawImage(offscreen, nil)
        return nil
}

func main() {
        if err := ebiten.Run(update, screenWidth, screenHeight, 1, "Mandelbrot (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}