package routing

import (
	"errors"
	"sync"
	"sync/atomic"
	"testing"
	"time"
)

type fixedRandomSource struct {
	values []int
	next   int
}

func (s *fixedRandomSource) Intn(n int) int {
	value := s.values[s.next]
	s.next++
	return value % n
}

func TestSelectorsReturnStableErrorWhenNoCandidateIsEligible(t *testing.T) {
	selectors := map[string]Selector{
		"random":            NewRandom(),
		"round robin":       NewRoundRobin(),
		"weighted":          NewWeighted(),
		"least connections": NewLeastConnections(),
	}
	for name, selector := range selectors {
		t.Run(name, func(t *testing.T) {
			_, err := selector.Select([]Candidate{{Name: "disabled"}})
			if !errors.Is(err, ErrNoCandidate) {
				t.Fatalf("Select() error = %v, want ErrNoCandidate", err)
			}
		})
	}
}

func TestRandomSelectsOnlyFromEligibleCandidates(t *testing.T) {
	selector := NewRandom(&fixedRandomSource{values: []int{1}})
	candidates := []Candidate{
		{Name: "disabled", Eligible: false},
		{Name: "a", Eligible: true},
		{Name: "b", Eligible: true},
	}

	got, err := selector.Select(candidates)
	if err != nil {
		t.Fatalf("Select(): %v", err)
	}
	if got.Name != "b" {
		t.Fatalf("Select() = %q, want b", got.Name)
	}
}

func TestRoundRobinCyclesThroughEligibleCandidates(t *testing.T) {
	selector := NewRoundRobin()
	candidates := []Candidate{
		{Name: "disabled", Eligible: false},
		{Name: "a", Eligible: true},
		{Name: "b", Eligible: true},
	}

	for call, want := range []string{"a", "b", "a"} {
		got, err := selector.Select(candidates)
		if err != nil {
			t.Fatalf("Select() call %d: %v", call+1, err)
		}
		if got.Name != want {
			t.Fatalf("Select() call %d = %q, want %q", call+1, got.Name, want)
		}
	}
}

func TestRoundRobinKeepsInputOrderWhenEligibilityChanges(t *testing.T) {
	selector := NewRoundRobin()
	candidates := []Candidate{
		{Name: "a", Eligible: true},
		{Name: "b", Eligible: true},
		{Name: "c", Eligible: true},
	}

	first, err := selector.Select(candidates)
	if err != nil {
		t.Fatalf("first Select(): %v", err)
	}
	if first.Name != "a" {
		t.Fatalf("first Select() = %q, want a", first.Name)
	}

	candidates[0].Eligible = false
	second, err := selector.Select(candidates)
	if err != nil {
		t.Fatalf("second Select(): %v", err)
	}
	if second.Name != "b" {
		t.Fatalf("second Select() = %q, want b", second.Name)
	}
}

func TestWeightedSelectsByEligibleCandidateWeight(t *testing.T) {
	selector := NewWeighted(&fixedRandomSource{values: []int{0, 1, 2, 4}})
	candidates := []Candidate{
		{Name: "disabled", Weight: 100, Eligible: false},
		{Name: "a", Weight: 2, Eligible: true},
		{Name: "b", Weight: 3, Eligible: true},
	}

	for call, want := range []string{"a", "a", "b", "b"} {
		got, err := selector.Select(candidates)
		if err != nil {
			t.Fatalf("Select() call %d: %v", call+1, err)
		}
		if got.Name != want {
			t.Fatalf("Select() call %d = %q, want %q", call+1, got.Name, want)
		}
	}
}

func TestWeightedReturnsStableErrorWhenWeightSumOverflows(t *testing.T) {
	selector := NewWeighted(panicRandomSource{})
	maxInt := int(^uint(0) >> 1)

	defer func() {
		if recovered := recover(); recovered != nil {
			t.Fatalf("Select() panicked: %v", recovered)
		}
	}()
	_, err := selector.Select([]Candidate{
		{Name: "a", Weight: maxInt, Eligible: true},
		{Name: "b", Weight: 1, Eligible: true},
	})
	if !errors.Is(err, ErrNoCandidate) {
		t.Fatalf("Select() error = %v, want ErrNoCandidate", err)
	}
}

type panicRandomSource struct{}

func (panicRandomSource) Intn(int) int {
	panic("random source should not be called")
}

func TestLeastConnectionsChoosesFirstEligibleMinimum(t *testing.T) {
	selector := NewLeastConnections()
	candidates := []Candidate{
		{Name: "disabled", Active: 0, Eligible: false},
		{Name: "busy", Active: 8, Eligible: true},
		{Name: "first-idle", Active: 2, Eligible: true},
		{Name: "second-idle", Active: 2, Eligible: true},
	}

	got, err := selector.Select(candidates)
	if err != nil {
		t.Fatalf("Select(): %v", err)
	}
	if got.Name != "first-idle" {
		t.Fatalf("Select() = %q, want first-idle", got.Name)
	}
}

func TestRoundRobinIsSafeForConcurrentCalls(t *testing.T) {
	selector := NewRoundRobin()
	candidates := []Candidate{
		{Name: "a", Eligible: true},
		{Name: "b", Eligible: true},
	}

	results := make(chan string, 1000)
	var wg sync.WaitGroup
	for range 1000 {
		wg.Add(1)
		go func() {
			defer wg.Done()
			candidate, err := selector.Select(candidates)
			if err != nil {
				t.Errorf("Select(): %v", err)
				return
			}
			results <- candidate.Name
		}()
	}
	wg.Wait()
	close(results)

	counts := map[string]int{}
	for name := range results {
		counts[name]++
	}
	if counts["a"] != 500 || counts["b"] != 500 {
		t.Fatalf("concurrent selections = %v, want a:500 b:500", counts)
	}
}

func TestInjectedRandomSourcesAreSerialized(t *testing.T) {
	for name, selector := range map[string]Selector{
		"random":   NewRandom(&concurrencyDetectingSource{}),
		"weighted": NewWeighted(&concurrencyDetectingSource{}),
	} {
		t.Run(name, func(t *testing.T) {
			candidates := []Candidate{
				{Name: "a", Weight: 1, Eligible: true},
				{Name: "b", Weight: 1, Eligible: true},
			}

			var wg sync.WaitGroup
			for range 100 {
				wg.Add(1)
				go func() {
					defer wg.Done()
					if _, err := selector.Select(candidates); err != nil {
						t.Errorf("Select(): %v", err)
					}
				}()
			}
			wg.Wait()
		})
	}
}

type concurrencyDetectingSource struct {
	active atomic.Bool
}

func (s *concurrencyDetectingSource) Intn(int) int {
	if !s.active.CompareAndSwap(false, true) {
		panic("concurrent RandomSource call")
	}
	time.Sleep(100 * time.Microsecond)
	s.active.Store(false)
	return 0
}