// +build example

package main

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

        "github.com/hajimehoshi/ebiten"
)

// World represents the game state
type World struct {
        area [][]bool
        rnd  *rand.Rand
}

// NewWorld creates a new world
func NewWorld(width, height int) *World {
        world := World{
                area: makeArea(width, height),
                rnd:  rand.New(rand.NewSource(time.Now().UnixNano())),
        }
        return &world
}

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

// Progress game state by one tick
func (w *World) Progress() {
        height := len(w.area)
        width := len(w.area[0])
        next := makeArea(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
}

// DrawImage paints current game state
func (w *World) DrawImage(pix []uint8) {
        height := len(w.area)
        width := len(w.area[0])
        for y := 0; y < height; y++ {
                for x := 0; x < width; x++ {
                        pos := 4*y*width + 4*x
                        if w.area[y][x] {
                                pix[pos] = 0xff
                                pix[pos+1] = 0xff
                                pix[pos+2] = 0xff
                                pix[pos+3] = 0xff
                        } else {
                                pix[pos] = 0
                                pix[pos+1] = 0
                                pix[pos+2] = 0
                                pix[pos+3] = 0
                        }
                }
        }
}

// neighbourCount calculates the Moore neighborhood of x, y
func neighbourCount(a [][]bool, x, y int) int {
        height := len(a)
        width := len(a[0])
        lowX := 0
        if x > 0 {
                lowX = x - 1
        }
        lowY := 0
        if y > 0 {
                lowY = y - 1
        }
        highX := width - 1
        if x < width-1 {
                highX = x + 1
        }
        highY := height - 1
        if y < height-1 {
                highY = y + 1
        }
        near := 0
        for pY := lowY; pY <= highY; pY++ {
                for pX := lowX; pX <= highX; pX++ {
                        if !(pX == x && pY == y) && a[pY][pX] {
                                near++
                        }
                }
        }

        return near
}

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

const (
        screenWidth  = 320
        screenHeight = 240
)

var (
        world  = NewWorld(screenWidth, screenHeight)
        pixels = make([]uint8, screenWidth*screenHeight*4)
)

func update(screen *ebiten.Image) error {
        world.Progress()
        if ebiten.IsRunningSlowly() {
                return nil
        }
        world.DrawImage(pixels)
        screen.ReplacePixels(pixels)
        return nil
}

func main() {
        world.RandomSeed(int((screenWidth * screenHeight) / 10))
        if err := ebiten.Run(update, screenWidth, screenHeight, 2.0, "Game of Life (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}