// Copyright 2016 Hajime Hoshi
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package loop

import (
	"errors"
	"sync"
	"time"

	"github.com/hajimehoshi/ebiten/internal/ui"
)

func CurrentFPS() float64 {
	return currentRunContext.currentFPS()
}

func IsRunning() bool {
	return currentRunContext.isRunning()
}

func IsRunningSlowly() bool {
	return currentRunContext.isRunningSlowly()
}

type runContext struct {
	running       bool
	fps           float64
	runningSlowly bool
	m             sync.RWMutex
}

var currentRunContext *runContext

func (c *runContext) startRunning() {
	c.m.Lock()
	defer c.m.Unlock()
	c.running = true
}

func (c *runContext) isRunning() bool {
	c.m.Lock()
	defer c.m.Unlock()
	return c.running
}

func (c *runContext) endRunning() {
	c.m.Lock()
	defer c.m.Unlock()
	c.running = false
}

func (c *runContext) currentFPS() float64 {
	c.m.RLock()
	defer c.m.RUnlock()
	if !c.running {
		// TODO: Should panic here?
		return 0
	}
	return c.fps
}

func (c *runContext) updateFPS(fps float64) {
	c.m.Lock()
	defer c.m.Unlock()
	c.fps = fps
}

func (c *runContext) isRunningSlowly() bool {
	c.m.RLock()
	defer c.m.RUnlock()
	if !c.running {
		// TODO: Should panic here?
		return false
	}
	return c.runningSlowly
}

func (c *runContext) setRunningSlowly(isRunningSlowly bool) {
	c.m.Lock()
	defer c.m.Unlock()
	c.runningSlowly = isRunningSlowly
}

type GraphicsContext interface {
	SetSize(width, height, scale int) error
	Update() error
	Resume() error
}

func Run(g GraphicsContext, width, height, scale int, title string, fps int) error {
	if currentRunContext != nil {
		return errors.New("loop: The game is already running")
	}
	currentRunContext = &runContext{}
	currentRunContext.startRunning()
	defer currentRunContext.endRunning()

	if err := ui.CurrentUI().Start(width, height, scale, title); err != nil {
		return err
	}
	// TODO: Use the error value
	defer ui.CurrentUI().Terminate()

	frames := 0
	n := now()
	beforeForUpdate := n
	beforeForFPS := n
	for {
		e, err := ui.CurrentUI().Update()
		if err != nil {
			return err
		}
		switch e := e.(type) {
		case ui.ScreenSizeEvent:
			if err := g.SetSize(e.Width, e.Height, e.ActualScale); err != nil {
				return err
			}
		case ui.CloseEvent:
			return nil
		case ui.RenderEvent:
			n2 := now()
			// If beforeForUpdate is too old, we assume that screen is not shown.
			if 5*int64(time.Second)/int64(fps) < n2-beforeForUpdate {
				currentRunContext.setRunningSlowly(false)
				beforeForUpdate = n2
			} else {
				// Note that generally t is a little different from 1/60[sec].
				t := n2 - beforeForUpdate
				tt := int(t * int64(fps) / int64(time.Second))
				// As t is not accurate 1/60[sec], errors are accumulated.
				// To make the FPS stable, set tt 1 if t is a little less than 1/60[sec].
				if tt == 0 && (int64(time.Second)/int64(fps)-int64(5*time.Millisecond)) < t {
					tt = 1
				}
				for i := 0; i < tt; i++ {
					slow := i < tt-1
					currentRunContext.setRunningSlowly(slow)
					if err := g.Update(); err != nil {
						return err
					}
				}
				if err := ui.CurrentUI().SwapBuffers(); err != nil {
					return err
				}
				beforeForUpdate += int64(tt) * int64(time.Second) / int64(fps)
				frames++
			}
			// Calc the current FPS.
			if time.Second <= time.Duration(n2-beforeForFPS) {
				fps := float64(frames) * float64(time.Second) / float64(n2-beforeForFPS)
				currentRunContext.updateFPS(fps)
				beforeForFPS = n2
				frames = 0
			}
			e.Done <- struct{}{}
		case ui.ResumeEvent:
			if err := g.Resume(); err != nil {
				return err
			}
			e.Done <- struct{}{}
		default:
			panic("not reach")
		}
	}
}