// Copyright (c) 2018 Shivaram Lingamneni <slingamn@cs.stanford.edu>
// released under the MIT license

package irc

import (
	"time"
)

// fakelag is a system for artificially delaying commands when a user issues
// them too rapidly

type FakelagState uint

const (
	// initially, the client is "bursting" and can send n commands without
	// encountering fakelag
	FakelagBursting FakelagState = iota
	// after that, they're "throttled" and we sleep in between commands until
	// they're spaced sufficiently far apart
	FakelagThrottled
)

// this is intentionally not threadsafe, because it should only be touched
// from the loop that accepts the client's input and runs commands
type Fakelag struct {
	window                    time.Duration
	burstLimit                uint
	throttleMessagesPerWindow uint
	nowFunc                   func() time.Time
	sleepFunc                 func(time.Duration)

	state      FakelagState
	burstCount uint // number of messages sent in the current burst
	lastTouch  time.Time
}

func NewFakelag(window time.Duration, burstLimit uint, throttleMessagesPerWindow uint) *Fakelag {
	return &Fakelag{
		window:                    window,
		burstLimit:                burstLimit,
		throttleMessagesPerWindow: throttleMessagesPerWindow,
		nowFunc:                   time.Now,
		sleepFunc:                 time.Sleep,
		state:                     FakelagBursting,
	}
}

// register a new command, sleep if necessary to delay it
func (fl *Fakelag) Touch() {
	if fl == nil {
		return
	}

	now := fl.nowFunc()
	// XXX if lastTouch.IsZero(), treat it as "very far in the past", which is fine
	elapsed := now.Sub(fl.lastTouch)
	fl.lastTouch = now

	if fl.state == FakelagBursting {
		// determine if the previous burst is over
		// (we could use 2*window instead)
		if elapsed > fl.window {
			fl.burstCount = 0
		}

		fl.burstCount++
		if fl.burstCount > fl.burstLimit {
			// reset burst window for next time
			fl.burstCount = 0
			// transition to throttling
			fl.state = FakelagThrottled
			// continue to throttling logic
		} else {
			return
		}
	}

	if fl.state == FakelagThrottled {
		if elapsed > fl.window {
			// let them burst again (as above, we could use 2*window instead)
			fl.state = FakelagBursting
			return
		}
		// space them out by at least window/messagesperwindow
		sleepDuration := time.Duration((int64(fl.window) / int64(fl.throttleMessagesPerWindow)) - int64(elapsed))
		if sleepDuration < 0 {
			sleepDuration = 0
		}
		fl.sleepFunc(sleepDuration)
	}
}