// vim: shiftwidth=4 tabstop=4 noexpandtab

package main

import (
	"fmt"
	"log"
	"strings"

	"github.com/go-gl/gl/v4.6-core/gl"
)

var (
	// a single triangle
	triangle = []float32{
		 0.0,  0.5,  0.0,
		-0.5, -0.5,  0.0,
		 0.5, -0.5,  0.0,
	}
)

const (
	// vertex shader to draw points
	VERTEX_SHADER = `
        #version 410
        in vec3 vp;
        void main() {
            gl_Position = vec4(vp, 1.0);
        }
    ` + "\x00"

	// fragment shader to draw surfaces
	FRAGMENT_SHADER = `
        #version 410
        out vec4 frag_colour;
        void main() {
            frag_colour = vec4(0, 0, 1, 1);  // RGBA color format
        }
    ` + "\x00"
)

type Graphics struct {
	version string
	vao uint32
	vertexShader uint32
	fragmentShader uint32
	program uint32
}

func newGraphics() Graphics {

	var graphics Graphics = Graphics{}

	// init OpenGL and save/log version
	log.Println("OpenGL init")
	err := gl.Init()
	if err != nil {
		log.Fatal(err)
	}
	graphics.version = gl.GoStr(gl.GetString(gl.VERSION))
	log.Println("OpenGL version", graphics.version)

	// compile shaders
	graphics.vertexShader, err = compileShader(VERTEX_SHADER, gl.VERTEX_SHADER)
	if err != nil {
		log.Fatal(err)
	}
	graphics.fragmentShader, err = compileShader(FRAGMENT_SHADER, gl.FRAGMENT_SHADER)
	if err != nil {
		log.Fatal(err)
	}

	// create GL program
	graphics.program = gl.CreateProgram()
	gl.AttachShader(graphics.program, graphics.vertexShader)
	gl.AttachShader(graphics.program, graphics.fragmentShader)
	gl.LinkProgram(graphics.program)

	// create VAO
	graphics.vao = makeVao(triangle)

	return graphics
}

func (graphics Graphics) draw() {
	gl.Clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT)
	gl.UseProgram(graphics.program)

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

// makeVao initializes and returns a vertex array from the points provided.
func makeVao(points []float32) uint32 {
	log.Println("Creating VAO")

	// start with vertex buffer object (VBO):
	var vbo uint32  // VBO ID
	gl.GenBuffers(1, &vbo)
	gl.BindBuffer(gl.ARRAY_BUFFER, vbo)
	gl.BufferData(gl.ARRAY_BUFFER, 4*len(points), gl.Ptr(points), gl.STATIC_DRAW)

	// vertex array objects (VAO) are a feature of newer GL implementations:
	var vao uint32  // VAO ID
	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)  // tell GL to use 3D float vectors

	return vao
}

func compileShader(source string, shaderType uint32) (uint32, error) {
	log.Println("Compiling shader")

	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
}