// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

// This is a modified, simplified version of code from golang.org/x/time/rate.

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package rate

import (
	"math"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	"tailscale.com/tstime/mono"
)

func closeEnough(a, b Limit) bool {
	return (math.Abs(float64(a)/float64(b)) - 1.0) < 1e-9
}

func TestEvery(t *testing.T) {
	cases := []struct {
		interval time.Duration
		lim      Limit
	}{
		{1 * time.Nanosecond, Limit(1e9)},
		{1 * time.Microsecond, Limit(1e6)},
		{1 * time.Millisecond, Limit(1e3)},
		{10 * time.Millisecond, Limit(100)},
		{100 * time.Millisecond, Limit(10)},
		{1 * time.Second, Limit(1)},
		{2 * time.Second, Limit(0.5)},
		{time.Duration(2.5 * float64(time.Second)), Limit(0.4)},
		{4 * time.Second, Limit(0.25)},
		{10 * time.Second, Limit(0.1)},
		{time.Duration(math.MaxInt64), Limit(1e9 / float64(math.MaxInt64))},
	}
	for _, tc := range cases {
		lim := Every(tc.interval)
		if !closeEnough(lim, tc.lim) {
			t.Errorf("Every(%v) = %v want %v", tc.interval, lim, tc.lim)
		}
	}
}

const (
	d = 100 * time.Millisecond
)

var (
	t0 = mono.Now()
	t1 = t0.Add(time.Duration(1) * d)
	t2 = t0.Add(time.Duration(2) * d)
)

type allow struct {
	t  mono.Time
	ok bool
}

func run(t *testing.T, lim *Limiter, allows []allow) {
	t.Helper()
	for i, allow := range allows {
		ok := lim.allow(allow.t)
		if ok != allow.ok {
			t.Errorf("step %d: lim.AllowN(%v) = %v want %v",
				i, allow.t, ok, allow.ok)
		}
	}
}

func TestLimiterBurst1(t *testing.T) {
	run(t, NewLimiter(10, 1), []allow{
		{t0, true},
		{t0, false},
		{t0, false},
		{t1, true},
		{t1, false},
		{t1, false},
		{t2, true},
		{t2, false},
	})
}

func TestLimiterJumpBackwards(t *testing.T) {
	run(t, NewLimiter(10, 3), []allow{
		{t1, true}, // start at t1
		{t0, true}, // jump back to t0, two tokens remain
		{t0, true},
		{t0, false},
		{t0, false},
		{t1, true}, // got a token
		{t1, false},
		{t1, false},
		{t2, true}, // got another token
		{t2, false},
		{t2, false},
	})
}

// Ensure that tokensFromDuration doesn't produce
// rounding errors by truncating nanoseconds.
// See golang.org/issues/34861.
func TestLimiter_noTruncationErrors(t *testing.T) {
	if !NewLimiter(0.7692307692307693, 1).Allow() {
		t.Fatal("expected true")
	}
}

func TestSimultaneousRequests(t *testing.T) {
	const (
		limit       = 1
		burst       = 5
		numRequests = 15
	)
	var (
		wg    sync.WaitGroup
		numOK = uint32(0)
	)

	// Very slow replenishing bucket.
	lim := NewLimiter(limit, burst)

	// Tries to take a token, atomically updates the counter and decreases the wait
	// group counter.
	f := func() {
		defer wg.Done()
		if ok := lim.Allow(); ok {
			atomic.AddUint32(&numOK, 1)
		}
	}

	wg.Add(numRequests)
	for range numRequests {
		go f()
	}
	wg.Wait()
	if numOK != burst {
		t.Errorf("numOK = %d, want %d", numOK, burst)
	}
}

func BenchmarkAllowN(b *testing.B) {
	lim := NewLimiter(Every(1*time.Second), 1)
	now := mono.Now()
	b.ReportAllocs()
	b.ResetTimer()
	b.RunParallel(func(pb *testing.PB) {
		for pb.Next() {
			lim.allow(now)
		}
	})
}