package pixelgl

import (
	"errors"
	"runtime"

	"github.com/go-gl/gl/v3.3-core/gl"
)

// Due to the existance and usage of thread-local variables by OpenGL, it's recommended to
// execute all OpenGL calls from a single dedicated thread. This file defines functions to make
// it possible.

var callQueue = make(chan func(), 32)

func init() {
	go func() {
		runtime.LockOSThread()
		for f := range callQueue {
			f()
		}
	}()
}

// Init initializes OpenGL by loading the function pointers from the active OpenGL context.
// This function must be manually run inside the dedicated thread (Do, DoErr, DoVal, etc.).
//
// It must be called under the presence of an active OpenGL context, e.g., always after calling window.MakeContextCurrent().
// Also, always call this function when switching contexts.
func Init() {
	err := gl.Init()
	if err != nil {
		panic(err)
	}
}

// DoNoBlock executes a function inside a dedicated OpenGL thread.
// DoNoBlock does not wait until the function finishes.
func DoNoBlock(f func()) {
	callQueue <- f
}

// Do executes a function inside a dedicated OpenGL thread.
// Do blocks until the function finishes.
//
// All OpenGL calls must be done in the dedicated thread.
func Do(f func()) {
	done := make(chan bool)
	callQueue <- func() {
		f()
		done <- true
	}
	<-done
}

// DoErr executes a function inside a dedicated OpenGL thread and returns an error to the called.
// DoErr blocks until the function finishes.
//
// All OpenGL calls must be done in the dedicated thread.
func DoErr(f func() error) error {
	err := make(chan error)
	callQueue <- func() {
		err <- f()
	}
	return <-err
}

// DoVal executes a function inside a dedicated OpenGL thread and returns a value to the caller.
// DoVal blocks until the function finishes.
//
// All OpenGL calls must be done in the dedicated thread.
func DoVal(f func() interface{}) interface{} {
	val := make(chan interface{})
	callQueue <- func() {
		val <- f()
	}
	return <-val
}

// DoGLErr is same as Do, but also return an error generated by OpenGL.
func DoGLErr(f func()) (gl error) {
	glerr := make(chan error)
	callQueue <- func() {
		getLastGLErr() // swallow
		f()
		glerr <- getLastGLErr()
	}
	return <-glerr
}

// DoErrGLErr is same as DoErr, but also returns an error generated by OpenGL.
func DoErrGLErr(f func() error) (_, gl error) {
	err := make(chan error)
	glerr := make(chan error)
	callQueue <- func() {
		getLastGLErr() // swallow
		err <- f()
		glerr <- getLastGLErr()
	}
	return <-err, <-glerr
}

// DoValGLErr is same as DoVal, but also returns an error generated by OpenGL.
func DoValGLErr(f func() interface{}) (_ interface{}, gl error) {
	val := make(chan interface{})
	glerr := make(chan error)
	callQueue <- func() {
		getLastGLErr() // swallow
		val <- f()
		glerr <- getLastGLErr()
	}
	return <-val, <-glerr
}

// getLastGLErr returns (and consumes) the last error generated by OpenGL.
// Don't use outside DoGLErr, DoErrGLErr and DoValGLErr.
func getLastGLErr() error {
	err := uint32(gl.NO_ERROR)
	for e := gl.GetError(); e != gl.NO_ERROR; e = gl.GetError() {
		err = e
	}
	if err == gl.NO_ERROR {
		return nil
	}
	switch err {
	case gl.INVALID_ENUM:
		return errors.New("invalid enum")
	case gl.INVALID_VALUE:
		return errors.New("invalid value")
	case gl.INVALID_OPERATION:
		return errors.New("invalid operation")
	case gl.STACK_OVERFLOW:
		return errors.New("stack overflow")
	case gl.STACK_UNDERFLOW:
		return errors.New("stack underflow")
	case gl.OUT_OF_MEMORY:
		return errors.New("out of memory")
	case gl.INVALID_FRAMEBUFFER_OPERATION:
		return errors.New("invalid framebuffer operation")
	case gl.CONTEXT_LOST:
		return errors.New("context lost")
	default:
		return errors.New("unknown error")
	}
}