// Copyright 2020 The Ebiten Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package main

import (
	"fmt"
	"image/color"
	"log"
	"math/rand/v2"

	"github.com/hajimehoshi/ebiten/v2"
	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
	"github.com/hajimehoshi/ebiten/v2/inpututil"
	"github.com/hajimehoshi/ebiten/v2/vector"
)

const (
	screenWidth        = 640
	screenHeight       = 480
	gridSize           = 10
	xGridCountInScreen = screenWidth / gridSize
	yGridCountInScreen = screenHeight / gridSize
)

const (
	dirNone = iota
	dirLeft
	dirRight
	dirDown
	dirUp
)

type Position struct {
	X int
	Y int
}

type Game struct {
	moveDirection int
	snakeBody     []Position
	apple         Position
	timer         int
	moveTime      int
	score         int
	bestScore     int
	level         int
}

func (g *Game) collidesWithApple() bool {
	return g.snakeBody[0].X == g.apple.X &&
		g.snakeBody[0].Y == g.apple.Y
}

func (g *Game) collidesWithSelf() bool {
	for _, v := range g.snakeBody[1:] {
		if g.snakeBody[0].X == v.X &&
			g.snakeBody[0].Y == v.Y {
			return true
		}
	}
	return false
}

func (g *Game) collidesWithWall() bool {
	return g.snakeBody[0].X < 0 ||
		g.snakeBody[0].Y < 0 ||
		g.snakeBody[0].X >= xGridCountInScreen ||
		g.snakeBody[0].Y >= yGridCountInScreen
}

func (g *Game) needsToMoveSnake() bool {
	return g.timer%g.moveTime == 0
}

func (g *Game) reset() {
	g.apple.X = xGridCountInScreen / 3
	g.apple.Y = yGridCountInScreen / 3
	g.moveTime = 4
	g.snakeBody = make([]Position, 1)
	g.snakeBody[0].X = xGridCountInScreen / 2
	g.snakeBody[0].Y = yGridCountInScreen / 2
	g.score = 0
	g.level = 1
	g.moveDirection = dirNone
}

func (g *Game) Update() error {
	// Decide the snake's direction along with the user input.
	// A U-turn is forbidden here (e.g. if the snake is moving in the left direction, the snake cannot go to the right direction immediately).
	if inpututil.IsKeyJustPressed(ebiten.KeyArrowLeft) || inpututil.IsKeyJustPressed(ebiten.KeyA) {
		if g.moveDirection != dirRight {
			g.moveDirection = dirLeft
		}
	} else if inpututil.IsKeyJustPressed(ebiten.KeyArrowRight) || inpututil.IsKeyJustPressed(ebiten.KeyD) {
		if g.moveDirection != dirLeft {
			g.moveDirection = dirRight
		}
	} else if inpututil.IsKeyJustPressed(ebiten.KeyArrowDown) || inpututil.IsKeyJustPressed(ebiten.KeyS) {
		if g.moveDirection != dirUp {
			g.moveDirection = dirDown
		}
	} else if inpututil.IsKeyJustPressed(ebiten.KeyArrowUp) || inpututil.IsKeyJustPressed(ebiten.KeyW) {
		if g.moveDirection != dirDown {
			g.moveDirection = dirUp
		}
	} else if inpututil.IsKeyJustPressed(ebiten.KeyEscape) {
		g.reset()
	}

	if g.needsToMoveSnake() {
		if g.collidesWithWall() || g.collidesWithSelf() {
			g.reset()
		}

		if g.collidesWithApple() {
			g.apple.X = rand.IntN(xGridCountInScreen - 1)
			g.apple.Y = rand.IntN(yGridCountInScreen - 1)
			g.snakeBody = append(g.snakeBody, Position{
				X: g.snakeBody[len(g.snakeBody)-1].X,
				Y: g.snakeBody[len(g.snakeBody)-1].Y,
			})
			if len(g.snakeBody) > 10 && len(g.snakeBody) < 20 {
				g.level = 2
				g.moveTime = 3
			} else if len(g.snakeBody) > 20 {
				g.level = 3
				g.moveTime = 2
			} else {
				g.level = 1
			}
			g.score++
			if g.bestScore < g.score {
				g.bestScore = g.score
			}
		}

		for i := int64(len(g.snakeBody)) - 1; i > 0; i-- {
			g.snakeBody[i].X = g.snakeBody[i-1].X
			g.snakeBody[i].Y = g.snakeBody[i-1].Y
		}
		switch g.moveDirection {
		case dirLeft:
			g.snakeBody[0].X--
		case dirRight:
			g.snakeBody[0].X++
		case dirDown:
			g.snakeBody[0].Y++
		case dirUp:
			g.snakeBody[0].Y--
		}
	}

	g.timer++

	return nil
}

func (g *Game) Draw(screen *ebiten.Image) {
	for _, v := range g.snakeBody {
		vector.FillRect(screen, float32(v.X*gridSize), float32(v.Y*gridSize), gridSize, gridSize, color.RGBA{0x80, 0xa0, 0xc0, 0xff}, false)
	}
	vector.FillRect(screen, float32(g.apple.X*gridSize), float32(g.apple.Y*gridSize), gridSize, gridSize, color.RGBA{0xFF, 0x00, 0x00, 0xff}, false)

	if g.moveDirection == dirNone {
		ebitenutil.DebugPrint(screen, fmt.Sprintf("Press up/down/left/right to start"))
	} else {
		ebitenutil.DebugPrint(screen, fmt.Sprintf("FPS: %0.2f Level: %d Score: %d Best Score: %d", ebiten.ActualFPS(), g.level, g.score, g.bestScore))
	}
}

func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
	return screenWidth, screenHeight
}

func newGame() *Game {
	g := &Game{}
	g.reset()
	return g
}

func main() {
	ebiten.SetWindowSize(screenWidth, screenHeight)
	ebiten.SetWindowTitle("Snake (Ebitengine Demo)")
	if err := ebiten.RunGame(newGame()); err != nil {
		log.Fatal(err)
	}
}
