package main

import (
    "fmt"
    "log"
    "runtime"
    "strings"

    "github.com/go-gl/gl/v4.1-core/gl" // OR: github.com/go-gl/gl/v2.1/gl
    "github.com/go-gl/glfw/v3.2/glfw"
)

const (
    width  = 500
    height = 500

    vertexShaderSource = `
        #version 410
        in vec3 vp;
        void main() {
            gl_Position = vec4(vp, 1.0);
        }
    ` + "\x00"

    fragmentShaderSource = `
        #version 410
        out vec4 frag_colour;
        void main() {
            frag_colour = vec4(1, 1, 1, 1.0);
        }
    ` + "\x00"
)

var (
    triangle = []float32{
        0, 0.5, 0,
        -0.5, -0.5, 0,
        0.5, -0.5, 0,
    }
)

func main() {
    runtime.LockOSThread()

    window := initGlfw()
    defer glfw.Terminate()
    program := initOpenGL()

    vao := makeVao(triangle)
    for !window.ShouldClose() {
        draw(vao, window, program)
    }
}

func draw(vao uint32, window *glfw.Window, program uint32) {
    gl.Clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT)
    gl.UseProgram(program)

    gl.BindVertexArray(vao)
    gl.DrawArrays(gl.TRIANGLES, 0, int32(len(triangle)/3))

    glfw.PollEvents()
    window.SwapBuffers()
}

// initGlfw initializes glfw and returns a Window to use.
func initGlfw() *glfw.Window {
    if err := glfw.Init(); err != nil {
        panic(err)
    }
    glfw.WindowHint(glfw.Resizable, glfw.False)
    glfw.WindowHint(glfw.ContextVersionMajor, 4)
    glfw.WindowHint(glfw.ContextVersionMinor, 1)
    glfw.WindowHint(glfw.OpenGLProfile, glfw.OpenGLCoreProfile)
    glfw.WindowHint(glfw.OpenGLForwardCompatible, glfw.True)

    window, err := glfw.CreateWindow(width, height, "Conway's Game of Life", nil, nil)
    if err != nil {
        panic(err)
    }
    window.MakeContextCurrent()

    return window
}

// initOpenGL initializes OpenGL and returns an intiialized program.
func initOpenGL() uint32 {
    if err := gl.Init(); err != nil {
        panic(err)
    }
    version := gl.GoStr(gl.GetString(gl.VERSION))
    log.Println("OpenGL version", version)

    vertexShader, err := compileShader(vertexShaderSource, gl.VERTEX_SHADER)
    if err != nil {
        panic(err)
    }

    fragmentShader, err := compileShader(fragmentShaderSource, gl.FRAGMENT_SHADER)
    if err != nil {
        panic(err)
    }

    prog := gl.CreateProgram()
    gl.AttachShader(prog, vertexShader)
    gl.AttachShader(prog, fragmentShader)
    gl.LinkProgram(prog)
    return prog
}

// makeVao initializes and returns a vertex array from the points provided.
func makeVao(points []float32) uint32 {
    var vbo uint32
    gl.GenBuffers(1, &vbo)
    gl.BindBuffer(gl.ARRAY_BUFFER, vbo)
    gl.BufferData(gl.ARRAY_BUFFER, 4*len(points), gl.Ptr(points), gl.STATIC_DRAW)

    var vao uint32
    gl.GenVertexArrays(1, &vao)
    gl.BindVertexArray(vao)
    gl.EnableVertexAttribArray(0)
    gl.BindBuffer(gl.ARRAY_BUFFER, vbo)
    gl.VertexAttribPointer(0, 3, gl.FLOAT, false, 0, nil)

    return vao
}

func compileShader(source string, shaderType uint32) (uint32, error) {
    shader := gl.CreateShader(shaderType)

    csources, free := gl.Strs(source)
    gl.ShaderSource(shader, 1, csources, nil)
    free()
    gl.CompileShader(shader)

    var status int32
    gl.GetShaderiv(shader, gl.COMPILE_STATUS, &status)
    if status == gl.FALSE {
        var logLength int32
        gl.GetShaderiv(shader, gl.INFO_LOG_LENGTH, &logLength)

        log := strings.Repeat("\x00", int(logLength+1))
        gl.GetShaderInfoLog(shader, logLength, nil, gl.Str(log))

        return 0, fmt.Errorf("failed to compile %v: %v", source, log)
    }

    return shader, nil
}