// Copyright 2014 Hajime Hoshi
//
// 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 ebiten

import (
	"fmt"
	"image"
	"math"
	"sync/atomic"

	"github.com/hajimehoshi/ebiten/v2/internal/atlas"
	"github.com/hajimehoshi/ebiten/v2/internal/ui"
)

const screenShaderSrc = `package main

func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
	// TODO: Calculate the scale in the shader after pixels become the main unit in shaders (#1431)
	_, dr := imageDstRegionOnTexture()
	_, sr := imageSrcRegionOnTexture()
	scale := (imageDstTextureSize() * dr) / (imageSrcTextureSize() * sr)

	sourceSize := imageSrcTextureSize()
	// texelSize is one pixel size in texel sizes.
	texelSize := 1 / sourceSize
	halfScaledTexelSize := texelSize / 2 / scale

	// Shift 1/512 [texel] to avoid the tie-breaking issue.
	pos := texCoord
	p0 := pos - halfScaledTexelSize + (texelSize / 512)
	p1 := pos + halfScaledTexelSize + (texelSize / 512)

	// Texels must be in the source rect, so it is not necessary to check.
	c0 := imageSrc0UnsafeAt(p0)
	c1 := imageSrc0UnsafeAt(vec2(p1.x, p0.y))
	c2 := imageSrc0UnsafeAt(vec2(p0.x, p1.y))
	c3 := imageSrc0UnsafeAt(p1)

	// p is the p1 value in one pixel assuming that the pixel's upper-left is (0, 0) and the lower-right is (1, 1).
	p := fract(p1 * sourceSize)

	// rate indicates how much the 4 colors are mixed. rate is in between [0, 1].
	//
	//     0 <= p <= 1/Scale: The rate is in between [0, 1]
	//     1/Scale < p:       Don't care. Adjacent colors (e.g. c0 vs c1 in an X direction) should be the same.
	rate := clamp(p*scale, 0, 1)
	return mix(mix(c0, c1, rate.x), mix(c2, c3, rate.x), rate.y)
}
`

var screenFilterEnabled = int32(1)

func isScreenFilterEnabled() bool {
	return atomic.LoadInt32(&screenFilterEnabled) != 0
}

func setScreenFilterEnabled(enabled bool) {
	v := int32(0)
	if enabled {
		v = 1
	}
	atomic.StoreInt32(&screenFilterEnabled, v)
}

type gameForUI struct {
	game         Game
	offscreen    *Image
	screen       *Image
	screenShader *Shader
}

func newGameForUI(game Game) *gameForUI {
	g := &gameForUI{
		game: game,
	}

	s, err := NewShader([]byte(screenShaderSrc))
	if err != nil {
		panic(fmt.Sprintf("ebiten: compiling the screen shader failed: %v", err))
	}
	g.screenShader = s

	return g
}

func (c *gameForUI) NewOffscreenImage(width, height int) *ui.Image {
	if c.offscreen != nil {
		c.offscreen.Dispose()
		c.offscreen = nil
	}

	// Keep the offscreen an unmanaged image that is always isolated from an atlas (#1938).
	// The shader program for the screen is special and doesn't work well with an image on an atlas.
	// An image on an atlas is surrounded by a transparent edge,
	// and the shader program unexpectedly picks the pixel on the edges.
	imageType := atlas.ImageTypeUnmanaged
	if ui.IsScreenClearedEveryFrame() {
		// A violatile image is also always isolated.
		imageType = atlas.ImageTypeVolatile
	}
	c.offscreen = newImage(image.Rect(0, 0, width, height), imageType)
	return c.offscreen.image
}

func (c *gameForUI) NewScreenImage(width, height int) *ui.Image {
	if c.screen != nil {
		c.screen.Dispose()
		c.screen = nil
	}

	c.screen = newImage(image.Rect(0, 0, width, height), atlas.ImageTypeScreen)
	return c.screen.image
}

func (c *gameForUI) Layout(outsideWidth, outsideHeight int) (int, int) {
	return c.game.Layout(outsideWidth, outsideHeight)
}

func (c *gameForUI) Update() error {
	return c.game.Update()
}

func (c *gameForUI) DrawOffscreen() {
	c.game.Draw(c.offscreen)
}

func (g *gameForUI) DrawScreen(scale, offsetX, offsetY float64) {
	switch {
	case !isScreenFilterEnabled(), math.Floor(scale) == scale:
		op := &DrawImageOptions{}
		op.GeoM.Scale(scale, scale)
		op.GeoM.Translate(offsetX, offsetY)
		g.screen.DrawImage(g.offscreen, op)
	case scale < 1:
		op := &DrawImageOptions{}
		op.GeoM.Scale(scale, scale)
		op.GeoM.Translate(offsetX, offsetY)
		op.Filter = FilterLinear
		g.screen.DrawImage(g.offscreen, op)
	default:
		op := &DrawRectShaderOptions{}
		op.Images[0] = g.offscreen
		op.GeoM.Scale(scale, scale)
		op.GeoM.Translate(offsetX, offsetY)
		w, h := g.offscreen.Size()
		g.screen.DrawRectShader(w, h, g.screenShader, op)
	}
}