package main

import (
	"runtime"

	"github.com/go-gl/gl"
	"github.com/veandco/go-sdl2/sdl"
)

func main() {
	winTitle := "OpenGL Demo"
	winX, winY := sdl.WINDOWPOS_UNDEFINED, sdl.WINDOWPOS_UNDEFINED
	winWidth, winHeight := 800, 600

	runtime.LockOSThread()

	sdl.Init(sdl.INIT_VIDEO)
	defer sdl.Quit()

	sdl.GL_SetAttribute(sdl.GL_CONTEXT_PROFILE_MASK, sdl.GL_CONTEXT_PROFILE_CORE)
	sdl.GL_SetAttribute(sdl.GL_CONTEXT_MAJOR_VERSION, 3)
	sdl.GL_SetAttribute(sdl.GL_CONTEXT_MINOR_VERSION, 2)

	window, _ := sdl.CreateWindow(winTitle, winX, winY, winWidth, winHeight, sdl.WINDOW_OPENGL)
	defer window.Destroy()

	context := sdl.GL_CreateContext(window)
	defer sdl.GL_DeleteContext(context)

	gl.Init()

	vertSource := `
	#version 150 core

    in vec2 position;
    void main() {
       gl_Position = vec4(position, 0.0, 1.0);
    }`

	fragSource := `
	#version 150 core

    out vec4 outColor;
    void main() {
       outColor = vec4(1.0, 1.0, 1.0, 1.0);
    }`

	vao := gl.GenVertexArray()
	vao.Bind()
	defer vao.Delete()

	vbo := gl.GenBuffer()
	vbo.Bind(gl.ARRAY_BUFFER)
	defer vbo.Delete()

	verts := []float32{
		0.0, 0.5,
		0.5, -0.5,
		-0.5, -0.5,
	}
	gl.BufferData(gl.ARRAY_BUFFER, len(verts)*4, verts, gl.STATIC_DRAW)

	vertShader := gl.CreateShader(gl.VERTEX_SHADER)
	vertShader.Source(vertSource)
	vertShader.Compile()
	defer vertShader.Delete()

	fragShader := gl.CreateShader(gl.FRAGMENT_SHADER)
	fragShader.Source(fragSource)
	fragShader.Compile()
	defer fragShader.Delete()

	shaderProgram := gl.CreateProgram()
	shaderProgram.AttachShader(vertShader)
	shaderProgram.AttachShader(fragShader)
	shaderProgram.BindFragDataLocation(0, "outColor")
	shaderProgram.Link()
	shaderProgram.Use()
	defer shaderProgram.Delete()

	posAttrib := shaderProgram.GetAttribLocation("position")
	posAttrib.EnableArray()
	posAttrib.AttribPointer(2, gl.FLOAT, false, 0, nil)

	for running := true; running; {
		for e := sdl.PollEvent(); e != nil; e = sdl.PollEvent() {
			switch e.(type) {
			case *sdl.QuitEvent:
				running = false
			}
		}

		gl.ClearColor(0, 0, 0, 1)
		gl.Clear(gl.COLOR_BUFFER_BIT)

		gl.DrawArrays(gl.TRIANGLES, 0, 3)

		sdl.GL_SwapWindow(window)
	}
}