// Copyright 2017 The Ebiten Authors
//
// 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 clock manages game timers.
package clock

import (
	"sync"
	"time"
)

var (
	// lastSystemTime is the last system time in the previous Update.
	lastSystemTime int64

	currentFPS     float64
	lastFPSUpdated int64
	framesForFPS   int64

	started bool
	onStart func()

	m sync.Mutex
)

func CurrentFPS() float64 {
	m.Lock()
	v := currentFPS
	m.Unlock()
	return v
}

func OnStart(f func()) {
	m.Lock()
	onStart = f
	m.Unlock()
}

func calcCountFromTPS(tps int64, now int64) int {
	if tps == 0 {
		return 0
	}
	if tps < 0 {
		panic("clock: tps must >= 0")
	}

	// Initialize lastSystemTime if needed.
	if lastSystemTime == 0 {
		lastSystemTime = now
	}

	diff := now - lastSystemTime
	if diff < 0 {
		return 0
	}

	count := 0
	syncWithSystemClock := false

	if diff > int64(time.Second)*5/60 {
		// The previous time is too old.
		// Let's force to sync the game time with the system clock.
		syncWithSystemClock = true
	} else {
		count = int(diff * tps / int64(time.Second))
	}

	// Stabilize FPS.
	// Without this adjustment, count can be unstable like 0, 2, 0, 2, ...
	if count == 0 && (int64(time.Second)/tps/2) < diff {
		count = 1
	}
	if count == 2 && (int64(time.Second)/tps*3/2) > diff {
		count = 1
	}

	if syncWithSystemClock {
		lastSystemTime = now
	} else {
		lastSystemTime += int64(count) * int64(time.Second) / tps
	}

	return count
}

func updateFPS(now int64) {
	if lastFPSUpdated == 0 {
		lastFPSUpdated = now
	}
	framesForFPS++
	if time.Second > time.Duration(now-lastFPSUpdated) {
		return
	}
	currentFPS = float64(framesForFPS) * float64(time.Second) / float64(now-lastFPSUpdated)
	lastFPSUpdated = now
	framesForFPS = 0
}

// Update updates the inner clock state and returns an integer value
// indicating how many game frames the game should update based on given tps.
// tps represents TPS (ticks per second).
// If tps <= 0, Update always returns 0.
//
// Update is expected to be called per frame.
func Update(tps int) int {
	m.Lock()
	defer m.Unlock()

	if !started {
		if onStart != nil {
			onStart()
		}
		started = true
	}

	n := now()
	updateFPS(n)
	if tps > 0 {
		return calcCountFromTPS(int64(tps), n)
	}
	return 0
}