Ebiten example - sprites

package main

import (
    "fmt"
    _ "image/png"
    "log"
    "math/rand"

    "github.com/hajimehoshi/ebiten"
    "github.com/hajimehoshi/ebiten/ebitenutil"
)

const (
    screenWidth  = 320
    screenHeight = 240
)

var (
    ebitenImage       *ebiten.Image
    ebitenImageWidth  = 0
    ebitenImageHeight = 0
)

type Sprite struct {
    image *ebiten.Image
    x     int
    y     int
    vx    int
    vy    int
}

func (s *Sprite) Update() {
    s.x += s.vx
    s.y += s.vy
    if s.x < 0 {
        s.x = -s.x
        s.vx = -s.vx
    }
    if s.y < 0 {
        s.y = -s.y
        s.vy = -s.vy
    }
    w, h := s.image.Size()
    if screenWidth <= s.x+w {
        s.x = 2*(screenWidth-w) - s.x
        s.vx = -s.vx
    }
    if screenHeight <= s.y+h {
        s.y = 2*(screenHeight-h) - s.y
        s.vy = -s.vy
    }
}

type Sprites struct {
    sprites []*Sprite
    num     int
}

func (s Sprites) Update() {
    for _, sprite := range s.sprites {
        sprite.Update()
    }
}

func (s Sprites) Len() int {
    return s.num
}

func (s Sprites) Dst(i int) (x0, y0, x1, y1 int) {
    if s.num <= i {
        return 0, 0, 0, 0
    }
    ss := s.sprites[i]
    return ss.x, ss.y, ss.x + ebitenImageWidth, ss.y + ebitenImageHeight
}

func (s Sprites) Src(i int) (x0, y0, x1, y1 int) {
    if s.num <= i {
        return 0, 0, 0, 0
    }
    return 0, 0, ebitenImageWidth, ebitenImageHeight
}

const (
    MinSprites = 0
    MaxSprites = 10000
)

var sprites = &Sprites{make([]*Sprite, MaxSprites), 500}

func update(screen *ebiten.Image) error {
    if ebiten.IsKeyPressed(ebiten.KeyLeft) {
        sprites.num -= 20
        if sprites.num < MinSprites {
            sprites.num = MinSprites
        }
    }
    if ebiten.IsKeyPressed(ebiten.KeyRight) {
        sprites.num += 20
        if MaxSprites < sprites.num {
            sprites.num = MaxSprites
        }
    }
    sprites.Update()

    if ebiten.IsRunningSlowly() {
        return nil
    }
    op := &ebiten.DrawImageOptions{
        ImageParts: sprites,
    }
    op.ColorM.Scale(1.0, 1.0, 1.0, 0.5)
    if err := screen.DrawImage(ebitenImage, op); err != nil {
        return err
    }
    msg := fmt.Sprintf(`FPS: %0.2f
Num of sprites: %d
Press <- or -> to change the number of sprites`, ebiten.CurrentFPS(), sprites.Len())
    ebitenutil.DebugPrint(screen, msg)
    return nil
}

func main() {
    var err error
    ebitenImage, _, err = ebitenutil.NewImageFromFile("_resources/images/ebiten.png", ebiten.FilterNearest)
    if err != nil {
        log.Fatal(err)
    }
    ebitenImageWidth, ebitenImageHeight = ebitenImage.Size()
    for i, _ := range sprites.sprites {
        w, h := ebitenImage.Size()
        x, y := rand.Intn(screenWidth-w), rand.Intn(screenHeight-h)
        vx, vy := 2*rand.Intn(2)-1, 2*rand.Intn(2)-1
        sprites.sprites[i] = &Sprite{
            image: ebitenImage,
            x:     x,
            y:     y,
            vx:    vx,
            vy:    vy,
        }
    }
    if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Sprites (Ebiten Demo)"); err != nil {
        log.Fatal(err)
    }
}