package main

import (
	"bytes"
	"errors"
	"fmt"
	gl "github.com/chsc/gogl/gl21"
	"github.com/jteeuwen/glfw"
	"image"
	"image/png"
	"io"
	"os"
)

const (
	Title  = "Spinning Gopher"
	Width  = 640
	Height = 480
)

var (
	texture     gl.Uint
	rotx, roty  gl.Float
	ambient     []gl.Float  = []gl.Float{0.5, 0.5, 0.5, 1}
	diffuse     []gl.Float  = []gl.Float{1, 1, 1, 1}
	lightpos    []gl.Float  = []gl.Float{-5, 5, 10, 0}
)

func main() {
	if err := glfw.Init(); err != nil {
		fmt.Fprintf(os.Stderr, "glfw: %s\n", err)
		return
	}
	defer glfw.Terminate()

	glfw.OpenWindowHint(glfw.WindowNoResize, 1)

	if err := glfw.OpenWindow(Width, Height, 0, 0, 0, 0, 16, 0, glfw.Windowed); err != nil {
		fmt.Fprintf(os.Stderr, "glfw: %s\n", err)
		return
	}
	defer glfw.CloseWindow()

	glfw.SetSwapInterval(1)
	glfw.SetWindowTitle(Title)

	if err := gl.Init(); err != nil {
		fmt.Fprintf(os.Stderr, "gl: %s\n", err)
	}

	if err := initScene(); err != nil {
		fmt.Fprintf(os.Stderr, "init: %s\n", err)
		return
	}
	defer destroyScene()

	for glfw.WindowParam(glfw.Opened) == 1 {
		drawScene()
		glfw.SwapBuffers()
	}
}

func createTexture(r io.Reader) (textureId gl.Uint, err error) {
	img, err := png.Decode(r)
	if err != nil {
		return 0, err
	}

	rgbaImg, ok := img.(*image.NRGBA)
	if !ok {
		return 0, errors.New("texture must be an NRGBA image")
	}

	gl.GenTextures(1, &textureId)
	gl.BindTexture(gl.TEXTURE_2D, textureId)
	gl.TexParameterf(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR)
	gl.TexParameterf(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR)

	// flip image: first pixel is lower left corner
	imgWidth, imgHeight := img.Bounds().Dx(), img.Bounds().Dy()
	data := make([]byte, imgWidth * imgHeight * 4)
	lineLen := imgWidth * 4
	dest := len(data)-lineLen
	for src := 0; src < len(rgbaImg.Pix); src+=rgbaImg.Stride {
		copy(data[dest:dest+lineLen], rgbaImg.Pix[src:src+rgbaImg.Stride])
		dest-=lineLen
	}
	gl.TexImage2D(gl.TEXTURE_2D, 0, 4, gl.Sizei(imgWidth), gl.Sizei(imgHeight), 0, gl.RGBA, gl.UNSIGNED_BYTE, gl.Pointer(&data[0]))

	return textureId, nil
}

func createTextureFromBytes(data []byte) (gl.Uint, error) {
	r := bytes.NewBuffer(data)
	return createTexture(r)
}

func initScene() (err error) {
	gl.Enable(gl.TEXTURE_2D)
	gl.Enable(gl.DEPTH_TEST)
	gl.Enable(gl.LIGHTING)

	gl.ClearColor(0.5, 0.5, 0.5, 0.0)
	gl.ClearDepth(1)
	gl.DepthFunc(gl.LEQUAL)

	gl.Lightfv(gl.LIGHT0, gl.AMBIENT, &ambient[0])
	gl.Lightfv(gl.LIGHT0, gl.DIFFUSE, &diffuse[0])
	gl.Lightfv(gl.LIGHT0, gl.POSITION, &lightpos[0])
	gl.Enable(gl.LIGHT0)

	gl.Viewport(0, 0, Width, Height)
	gl.MatrixMode(gl.PROJECTION)
	gl.LoadIdentity()
	gl.Frustum(-1, 1, -1, 1, 1.0, 10.0)
	gl.MatrixMode(gl.MODELVIEW)
	gl.LoadIdentity()

	texture, err = createTextureFromBytes(gopher_png[:])
	return
}

func destroyScene() {
	gl.DeleteTextures(1, &texture)
}

func drawScene() {
	gl.Clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT)

	gl.MatrixMode(gl.MODELVIEW)
	gl.LoadIdentity()
	gl.Translatef(0, 0, -3.0)
	gl.Rotatef(rotx, 1, 0, 0)
	gl.Rotatef(roty, 0, 1, 0)

	rotx += 0.5
	roty += 0.5

	gl.BindTexture(gl.TEXTURE_2D, texture)

	gl.Color4f(1, 1, 1, 1)

	gl.Begin(gl.QUADS)

	gl.Normal3f(0, 0, 1)
	gl.TexCoord2f(0, 0)
	gl.Vertex3f(-1, -1, 1)
	gl.TexCoord2f(1, 0)
	gl.Vertex3f(1, -1, 1)
	gl.TexCoord2f(1, 1)
	gl.Vertex3f(1, 1, 1)
	gl.TexCoord2f(0, 1)
	gl.Vertex3f(-1, 1, 1)

	gl.Normal3f(0, 0, -1)
	gl.TexCoord2f(1, 0)
	gl.Vertex3f(-1, -1, -1)
	gl.TexCoord2f(1, 1)
	gl.Vertex3f(-1, 1, -1)
	gl.TexCoord2f(0, 1)
	gl.Vertex3f(1, 1, -1)
	gl.TexCoord2f(0, 0)
	gl.Vertex3f(1, -1, -1)

	gl.Normal3f(0, 1, 0)
	gl.TexCoord2f(0, 1)
	gl.Vertex3f(-1, 1, -1)
	gl.TexCoord2f(0, 0)
	gl.Vertex3f(-1, 1, 1)
	gl.TexCoord2f(1, 0)
	gl.Vertex3f(1, 1, 1)
	gl.TexCoord2f(1, 1)
	gl.Vertex3f(1, 1, -1)

	gl.Normal3f(0, -1, 0)
	gl.TexCoord2f(1, 1)
	gl.Vertex3f(-1, -1, -1)
	gl.TexCoord2f(0, 1)
	gl.Vertex3f(1, -1, -1)
	gl.TexCoord2f(0, 0)
	gl.Vertex3f(1, -1, 1)
	gl.TexCoord2f(1, 0)
	gl.Vertex3f(-1, -1, 1)

	gl.Normal3f(1, 0, 0)
	gl.TexCoord2f(1, 0)
	gl.Vertex3f(1, -1, -1)
	gl.TexCoord2f(1, 1)
	gl.Vertex3f(1, 1, -1)
	gl.TexCoord2f(0, 1)
	gl.Vertex3f(1, 1, 1)
	gl.TexCoord2f(0, 0)
	gl.Vertex3f(1, -1, 1)

	gl.Normal3f(-1, 0, 0)
	gl.TexCoord2f(0, 0)
	gl.Vertex3f(-1, -1, -1)
	gl.TexCoord2f(1, 0)
	gl.Vertex3f(-1, -1, 1)
	gl.TexCoord2f(1, 1)
	gl.Vertex3f(-1, 1, 1)
	gl.TexCoord2f(0, 1)
	gl.Vertex3f(-1, 1, -1)

	gl.End()
}
