// +build example

package main

import (
        "log"
        "math/rand"
        "time"

        "github.com/hajimehoshi/ebiten"
)

// World represents the game state.
type World struct {
        area [][]bool
}

func newArea(width, height int) [][]bool {
        a := make([][]bool, height)
        for i := 0; i < height; i++ {
                a[i] = make([]bool, width)
        }
        return a
}

// NewWorld creates a new world.
func NewWorld(width, height int, maxInitLiveCells int) *World {
        w := &World{
                area: newArea(width, height),
        }
        w.init(maxInitLiveCells)
        return w
}

func init() {
        rand.Seed(time.Now().UnixNano())
}

// init inits world with a random state.
func (w *World) init(maxLiveCells int) {
        height := len(w.area)
        width := len(w.area[0])
        for i := 0; i < maxLiveCells; i++ {
                x := rand.Intn(width)
                y := rand.Intn(height)
                w.area[y][x] = true
        }
}

// Update game state by one tick.
func (w *World) Update() {
        height := len(w.area)
        width := len(w.area[0])
        next := newArea(width, height)
        for y := 0; y < height; y++ {
                for x := 0; x < width; x++ {
                        pop := neighbourCount(w.area, x, y)
                        switch {
                        case pop < 2:
                                // rule 1. Any live cell with fewer than two live neighbours
                                // dies, as if caused by under-population.
                                next[y][x] = false

                        case (pop == 2 || pop == 3) && w.area[y][x]:
                                // rule 2. Any live cell with two or three live neighbours
                                // lives on to the next generation.
                                next[y][x] = true

                        case pop > 3:
                                // rule 3. Any live cell with more than three live neighbours
                                // dies, as if by over-population.
                                next[y][x] = false

                        case pop == 3:
                                // rule 4. Any dead cell with exactly three live neighbours
                                // becomes a live cell, as if by reproduction.
                                next[y][x] = true
                        }
                }
        }
        w.area = next
}

// Draw paints current game state.
func (w *World) Draw(pix []byte) {
        height := len(w.area)
        width := len(w.area[0])
        for y := 0; y < height; y++ {
                for x := 0; x < width; x++ {
                        idx := 4*y*width + 4*x
                        if w.area[y][x] {
                                pix[idx] = 0xff
                                pix[idx+1] = 0xff
                                pix[idx+2] = 0xff
                                pix[idx+3] = 0xff
                        } else {
                                pix[idx] = 0
                                pix[idx+1] = 0
                                pix[idx+2] = 0
                                pix[idx+3] = 0
                        }
                }
        }
}

func max(a, b int) int {
        if a < b {
                return b
        }
        return a
}

func min(a, b int) int {
        if a < b {
                return a
        }
        return b
}

// neighbourCount calculates the Moore neighborhood of (x, y).
func neighbourCount(a [][]bool, x, y int) int {
        w := len(a[0])
        h := len(a)
        minI := max(x-1, 0)
        minJ := max(y-1, 0)
        maxI := min(x+1, w-1)
        maxJ := min(y+1, h-1)

        c := 0
        for j := minJ; j <= maxJ; j++ {
                for i := minI; i <= maxI; i++ {
                        if i == x && j == y {
                                continue
                        }
                        if a[j][i] {
                                c++
                        }
                }
        }
        return c
}

const (
        screenWidth  = 320
        screenHeight = 240
)

var (
        world  = NewWorld(screenWidth, screenHeight, int((screenWidth*screenHeight)/10))
        pixels = make([]byte, screenWidth*screenHeight*4)
)

func update(screen *ebiten.Image) error {
        world.Update()

        if ebiten.IsRunningSlowly() {
                return nil
        }

        world.Draw(pixels)
        screen.ReplacePixels(pixels)
        return nil
}

func main() {
        if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Game of Life (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}