package capacity

import (
	"context"
	"errors"
	"reflect"
	"runtime"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
)

type generateResult struct {
	response *domain.GenerateResponse
	err      error
}

type clientFunc func(
	context.Context,
	domain.GenerateRequest,
) (*domain.GenerateResponse, error)

func (f clientFunc) Generate(
	ctx context.Context,
	req domain.GenerateRequest,
) (*domain.GenerateResponse, error) {
	return f(ctx, req)
}

type blockingClient struct {
	mu       sync.Mutex
	active   int
	peak     int
	calls    map[string]int
	started  chan string
	releases map[string]chan struct{}
}

func newBlockingClient(releases map[string]chan struct{}) *blockingClient {
	return &blockingClient{
		calls:    make(map[string]int),
		started:  make(chan string, 64),
		releases: releases,
	}
}

func (c *blockingClient) Generate(
	ctx context.Context,
	req domain.GenerateRequest,
) (*domain.GenerateResponse, error) {
	id := req.Prompt.SessionID
	c.mu.Lock()
	c.active++
	if c.active > c.peak {
		c.peak = c.active
	}
	c.calls[id]++
	c.mu.Unlock()
	defer func() {
		c.mu.Lock()
		c.active--
		c.mu.Unlock()
	}()

	c.started <- id
	if release := c.releases[id]; release != nil {
		select {
		case <-release:
		case <-ctx.Done():
			return nil, ctx.Err()
		}
	}
	return &domain.GenerateResponse{Content: id}, nil
}

func (c *blockingClient) callCount(id string) int {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.calls[id]
}

func (c *blockingClient) peakConcurrency() int {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.peak
}

func generateAsync(
	client llm.Client,
	ctx context.Context,
	backendID string,
	id string,
) <-chan generateResult {
	result := make(chan generateResult, 1)
	go func() {
		response, err := client.Generate(ctx, domain.GenerateRequest{
			Prompt: domain.RenderedPrompt{SessionID: id},
			Target: domain.ExecutionTarget{BackendID: backendID},
		})
		result <- generateResult{response: response, err: err}
	}()
	return result
}

func waitForWaiterCount(t *testing.T, manager *Manager, backendID string, want int) {
	t.Helper()
	pool := manager.pools[backendID]
	deadline := time.Now().Add(2 * time.Second)
	for {
		pool.mu.Lock()
		got := pool.waiters.Len()
		pool.mu.Unlock()
		if got == want {
			return
		}
		if time.Now().After(deadline) {
			t.Fatalf("waiter count=%d, want %d", got, want)
		}
		runtime.Gosched()
	}
}

func receiveStarted(t *testing.T, started <-chan string) string {
	t.Helper()
	select {
	case id := <-started:
		return id
	case <-time.After(2 * time.Second):
		t.Fatal("timed out waiting for wrapped client invocation")
		return ""
	}
}

func receiveResult(t *testing.T, result <-chan generateResult) generateResult {
	t.Helper()
	select {
	case got := <-result:
		return got
	case <-time.After(2 * time.Second):
		t.Fatal("timed out waiting for generation result")
		return generateResult{}
	}
}

func newTestManager(t *testing.T, policies map[string]domain.BackendCapacityPolicy) *Manager {
	t.Helper()
	manager, err := NewManager(policies)
	if err != nil {
		t.Fatalf("construct manager: %v", err)
	}
	return manager
}

func TestClientLimitsPeakConcurrencyAndServesWaitersFIFO(t *testing.T) {
	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
		"limited": {ConcurrencyLimit: 1},
	})
	firstRelease := make(chan struct{})
	secondRelease := make(chan struct{})
	thirdRelease := make(chan struct{})
	next := newBlockingClient(map[string]chan struct{}{
		"first":  firstRelease,
		"second": secondRelease,
		"third":  thirdRelease,
	})
	client := NewClient(manager, next)

	first := generateAsync(client, context.Background(), "limited", "first")
	if got := receiveStarted(t, next.started); got != "first" {
		t.Fatalf("first invocation=%q, want first", got)
	}
	second := generateAsync(client, context.Background(), "limited", "second")
	waitForWaiterCount(t, manager, "limited", 1)
	third := generateAsync(client, context.Background(), "limited", "third")
	waitForWaiterCount(t, manager, "limited", 2)

	close(firstRelease)
	if got := receiveResult(t, first); got.err != nil {
		t.Fatalf("first generation: %v", got.err)
	}
	if got := receiveStarted(t, next.started); got != "second" {
		t.Fatalf("second invocation=%q, want second", got)
	}
	close(secondRelease)
	if got := receiveResult(t, second); got.err != nil {
		t.Fatalf("second generation: %v", got.err)
	}
	if got := receiveStarted(t, next.started); got != "third" {
		t.Fatalf("third invocation=%q, want third", got)
	}
	close(thirdRelease)
	if got := receiveResult(t, third); got.err != nil {
		t.Fatalf("third generation: %v", got.err)
	}
	if peak := next.peakConcurrency(); peak != 1 {
		t.Fatalf("peak concurrency=%d, want 1", peak)
	}
}

func TestClientPeakConcurrencyDoesNotExceedConfiguredLimit(t *testing.T) {
	const limit = 2
	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
		"limited": {ConcurrencyLimit: limit},
	})
	gate := make(chan struct{})
	releases := make(map[string]chan struct{})
	for i := range 5 {
		releases[string(rune('a'+i))] = gate
	}
	next := newBlockingClient(releases)
	client := NewClient(manager, next)

	results := make([]<-chan generateResult, 0, len(releases))
	for id := range releases {
		results = append(results, generateAsync(client, context.Background(), "limited", id))
	}
	for range limit {
		receiveStarted(t, next.started)
	}
	waitForWaiterCount(t, manager, "limited", len(releases)-limit)

	close(gate)
	for _, result := range results {
		if got := receiveResult(t, result); got.err != nil {
			t.Fatalf("generation: %v", got.err)
		}
	}
	if peak := next.peakConcurrency(); peak != limit {
		t.Fatalf("peak concurrency=%d, want %d", peak, limit)
	}
}

func TestClientRemovesCanceledWaiters(t *testing.T) {
	tests := []struct {
		name      string
		cancelID  string
		wantOrder []string
	}{
		{name: "first waiter", cancelID: "one", wantOrder: []string{"two", "three"}},
		{name: "middle waiter", cancelID: "two", wantOrder: []string{"one", "three"}},
	}

	for _, tc := range tests {
		t.Run(tc.name, func(t *testing.T) {
			manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
				"limited": {ConcurrencyLimit: 1},
			})
			holderRelease := make(chan struct{})
			releases := map[string]chan struct{}{
				"holder": holderRelease,
				"one":    make(chan struct{}),
				"two":    make(chan struct{}),
				"three":  make(chan struct{}),
			}
			next := newBlockingClient(releases)
			client := NewClient(manager, next)

			holder := generateAsync(client, context.Background(), "limited", "holder")
			if got := receiveStarted(t, next.started); got != "holder" {
				t.Fatalf("initial invocation=%q, want holder", got)
			}

			contexts := make(map[string]context.Context)
			cancels := make(map[string]context.CancelFunc)
			results := make(map[string]<-chan generateResult)
			for _, id := range []string{"one", "two", "three"} {
				contexts[id], cancels[id] = context.WithCancel(context.Background())
				results[id] = generateAsync(client, contexts[id], "limited", id)
				waitForWaiterCount(t, manager, "limited", len(results))
			}

			cancels[tc.cancelID]()
			if got := receiveResult(t, results[tc.cancelID]); !errors.Is(got.err, context.Canceled) {
				t.Fatalf("canceled waiter error=%v, want context.Canceled", got.err)
			}
			waitForWaiterCount(t, manager, "limited", 2)

			close(holderRelease)
			if got := receiveResult(t, holder); got.err != nil {
				t.Fatalf("holder generation: %v", got.err)
			}
			for _, id := range tc.wantOrder {
				if got := receiveStarted(t, next.started); got != id {
					t.Fatalf("next invocation=%q, want %q", got, id)
				}
				close(releases[id])
				if got := receiveResult(t, results[id]); got.err != nil {
					t.Fatalf("%s generation: %v", id, got.err)
				}
			}
			if calls := next.callCount(tc.cancelID); calls != 0 {
				t.Fatalf("canceled waiter invoked wrapped client %d times", calls)
			}
			for _, cancel := range cancels {
				cancel()
			}
		})
	}
}

func TestClientGrantCancellationRaceDoesNotLeakPermit(t *testing.T) {
	const iterations = 200
	for i := range iterations {
		manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
			"limited": {ConcurrencyLimit: 1},
		})
		holderRelease := make(chan struct{})
		var waiterCalls atomic.Int64
		next := clientFunc(func(
			_ context.Context,
			req domain.GenerateRequest,
		) (*domain.GenerateResponse, error) {
			if req.Prompt.SessionID == "holder" {
				<-holderRelease
			} else if req.Prompt.SessionID == "waiter" {
				waiterCalls.Add(1)
			}
			return &domain.GenerateResponse{Content: req.Prompt.SessionID}, nil
		})
		client := NewClient(manager, next)

		holder := generateAsync(client, context.Background(), "limited", "holder")
		waitForActiveCount(t, manager, "limited", 1)
		ctx, cancel := context.WithCancel(context.Background())
		waiterResult := generateAsync(client, ctx, "limited", "waiter")
		waitForWaiterCount(t, manager, "limited", 1)

		start := make(chan struct{})
		var race sync.WaitGroup
		race.Add(2)
		go func() {
			defer race.Done()
			<-start
			cancel()
		}()
		go func() {
			defer race.Done()
			<-start
			close(holderRelease)
		}()
		close(start)
		race.Wait()

		if got := receiveResult(t, holder); got.err != nil {
			t.Fatalf("iteration %d holder generation: %v", i, got.err)
		}
		got := receiveResult(t, waiterResult)
		switch calls := waiterCalls.Load(); {
		case calls == 0 && errors.Is(got.err, context.Canceled):
		case calls == 1 && got.err == nil:
		default:
			t.Fatalf("iteration %d waiter calls=%d error=%v", i, calls, got.err)
		}

		probe := generateAsync(client, context.Background(), "limited", "probe")
		if got := receiveResult(t, probe); got.err != nil {
			t.Fatalf("iteration %d probe generation: %v", i, got.err)
		}
		waitForActiveCount(t, manager, "limited", 0)
		waitForWaiterCount(t, manager, "limited", 0)
	}
}

func waitForActiveCount(t *testing.T, manager *Manager, backendID string, want int) {
	t.Helper()
	pool := manager.pools[backendID]
	deadline := time.Now().Add(2 * time.Second)
	for {
		pool.mu.Lock()
		got := pool.active
		pool.mu.Unlock()
		if got == want {
			return
		}
		if time.Now().After(deadline) {
			t.Fatalf("active count=%d, want %d", got, want)
		}
		runtime.Gosched()
	}
}

func TestClientUsesIndependentPoolsAndUnlimitedFastPaths(t *testing.T) {
	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
		"alpha": {ConcurrencyLimit: 1},
		"beta":  {ConcurrencyLimit: 1},
	})
	alphaRelease := make(chan struct{})
	betaRelease := make(chan struct{})
	next := newBlockingClient(map[string]chan struct{}{
		"alpha": alphaRelease,
		"beta":  betaRelease,
	})
	client := NewClient(manager, next)

	alpha := generateAsync(client, context.Background(), "alpha", "alpha")
	beta := generateAsync(client, context.Background(), "beta", "beta")
	started := map[string]bool{
		receiveStarted(t, next.started): true,
		receiveStarted(t, next.started): true,
	}
	if !started["alpha"] || !started["beta"] {
		t.Fatalf("independent pools did not both start: %#v", started)
	}
	close(alphaRelease)
	close(betaRelease)
	if got := receiveResult(t, alpha); got.err != nil {
		t.Fatalf("alpha generation: %v", got.err)
	}
	if got := receiveResult(t, beta); got.err != nil {
		t.Fatalf("beta generation: %v", got.err)
	}

	for _, backendID := range []string{"", "unknown"} {
		response, err := client.Generate(context.Background(), domain.GenerateRequest{
			Prompt: domain.RenderedPrompt{SessionID: backendID},
			Target: domain.ExecutionTarget{BackendID: backendID},
		})
		if err != nil || response == nil {
			t.Fatalf("unlimited backend %q response=(%#v, %v)", backendID, response, err)
		}
	}
	if got := NewClient(nil, next); got != next {
		t.Fatal("nil manager did not return the wrapped client unchanged")
	}
}

func TestClientPreservesRequestsResponsesAndErrors(t *testing.T) {
	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
		"limited": {ConcurrencyLimit: 1},
	})
	request := domain.GenerateRequest{
		Prompt: domain.RenderedPrompt{
			SessionID: "session",
			Messages: []domain.RenderedMessage{
				{Role: "user", Content: "content"},
			},
		},
		Target: domain.ExecutionTarget{
			BackendID:   "limited",
			Model:       "model",
			ExtraParams: map[string]any{"key": "value"},
		},
	}
	response := &domain.GenerateResponse{
		Content: "output",
		Usage:   domain.TokenUsage{TotalTokens: 7},
	}
	collaboratorErr := errors.New("collaborator failure")
	tests := []struct {
		name     string
		response *domain.GenerateResponse
		err      error
	}{
		{name: "successful response", response: response},
		{name: "nil response"},
		{name: "collaborator error", response: response, err: collaboratorErr},
	}

	for _, tc := range tests {
		t.Run(tc.name, func(t *testing.T) {
			var captured domain.GenerateRequest
			next := clientFunc(func(
				_ context.Context,
				req domain.GenerateRequest,
			) (*domain.GenerateResponse, error) {
				captured = req
				return tc.response, tc.err
			})
			gotResponse, gotErr := NewClient(manager, next).Generate(context.Background(), request)
			if !reflect.DeepEqual(captured, request) {
				t.Fatalf("request changed: %#v", captured)
			}
			if gotResponse != tc.response || gotErr != tc.err {
				t.Fatalf("response=(%p, %v), want (%p, %v)",
					gotResponse, gotErr, tc.response, tc.err)
			}
		})
	}
}

func TestClientReleasesPermitDuringPanicUnwinding(t *testing.T) {
	manager := newTestManager(t, map[string]domain.BackendCapacityPolicy{
		"limited": {ConcurrencyLimit: 1},
	})
	var calls atomic.Int64
	next := clientFunc(func(
		_ context.Context,
		_ domain.GenerateRequest,
	) (*domain.GenerateResponse, error) {
		if calls.Add(1) == 1 {
			panic("test panic")
		}
		return &domain.GenerateResponse{Content: "recovered"}, nil
	})
	client := NewClient(manager, next)
	request := domain.GenerateRequest{
		Target: domain.ExecutionTarget{BackendID: "limited"},
	}

	func() {
		defer func() {
			if recover() == nil {
				t.Fatal("expected wrapped client panic")
			}
		}()
		_, _ = client.Generate(context.Background(), request)
	}()

	response, err := client.Generate(context.Background(), request)
	if err != nil || response == nil || response.Content != "recovered" {
		t.Fatalf("generation after panic=(%#v, %v)", response, err)
	}
}