package vidmod

import (
	"context"
	"errors"
	"fmt"
	"reflect"
	"sort"
	"strings"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
)

type fakeRecommendScoreStore struct {
	maxEligibleID      func(context.Context) (ObjectID, error)
	countUninitialized func(context.Context, ObjectID, int64) (int64, error)
	openCursor         func(context.Context, ObjectID, int32) (recommendScoreCursor, error)
	findBatch          func(context.Context, ObjectID, ObjectID, int) ([]recommendScoreDocument, error)
	bulkWrite          func(context.Context, []mongo.WriteModel) error
}

func (f fakeRecommendScoreStore) MaxEligibleID(ctx context.Context) (ObjectID, error) {
	return f.maxEligibleID(ctx)
}

func (f fakeRecommendScoreStore) CountUninitialized(
	ctx context.Context,
	maxID ObjectID,
	limit int64,
) (int64, error) {
	if f.countUninitialized == nil {
		return 0, nil
	}
	return f.countUninitialized(ctx, maxID, limit)
}

func (f fakeRecommendScoreStore) OpenCursor(
	ctx context.Context,
	maxID ObjectID,
	batchSize int32,
) (recommendScoreCursor, error) {
	if f.openCursor != nil {
		return f.openCursor(ctx, maxID, batchSize)
	}
	if f.findBatch == nil {
		return nil, errors.New("fake recommend score cursor is not configured")
	}
	documents := make([]recommendScoreDocument, 0)
	afterID := ObjectID{}
	limit := int(batchSize)
	for {
		batch, err := f.findBatch(ctx, afterID, maxID, limit)
		if err != nil {
			return nil, err
		}
		if len(batch) == 0 {
			break
		}
		documents = append(documents, batch...)
		afterID = batch[len(batch)-1].ID
		if len(batch) < limit || afterID == maxID {
			break
		}
	}
	return &fakeRecommendScoreCursor{documents: documents, current: -1}, nil
}

func (f fakeRecommendScoreStore) BulkWrite(ctx context.Context, writes []mongo.WriteModel) error {
	return f.bulkWrite(ctx, writes)
}

type fakeRecommendScoreCursor struct {
	documents            []recommendScoreDocument
	index                int
	current              int
	cursorErr            error
	decodeErrAt          int
	decodeErr            error
	closeErr             error
	closeCalls           atomic.Int32
	closeContextCanceled atomic.Bool
}

func (f *fakeRecommendScoreCursor) Next(ctx context.Context) bool {
	if err := ctx.Err(); err != nil {
		f.cursorErr = err
		return false
	}
	if f.index >= len(f.documents) {
		return false
	}
	f.current = f.index
	f.index++
	return true
}

func (f *fakeRecommendScoreCursor) Decode(value interface{}) error {
	if f.decodeErr != nil && f.current == f.decodeErrAt {
		return f.decodeErr
	}
	document, ok := value.(*recommendScoreDocument)
	if !ok {
		return fmt.Errorf("unexpected decode target %T", value)
	}
	if f.current < 0 || f.current >= len(f.documents) {
		return errors.New("decode called without current document")
	}
	*document = f.documents[f.current]
	return nil
}

func (f *fakeRecommendScoreCursor) Err() error {
	return f.cursorErr
}

func (f *fakeRecommendScoreCursor) Close(ctx context.Context) error {
	f.closeCalls.Add(1)
	f.closeContextCanceled.Store(ctx.Err() != nil)
	return f.closeErr
}

type refreshRecommendScoresResult struct {
	candidates []RecommendCandidate
	err        error
}

func recommendBatchTestID(value byte) ObjectID {
	var id ObjectID
	id[len(id)-1] = value
	return id
}

func recommendBatchTestScore(value int64) *int64 {
	return &value
}

func recommendBatchTestDocuments(count int) []recommendScoreDocument {
	documents := make([]recommendScoreDocument, count)
	for i := range documents {
		documents[i] = recommendScoreDocument{
			ID:                   recommendBatchTestID(byte(i + 1)),
			RecommendLikeCount:   int64(i + 1),
			RecommendInitialized: true,
		}
	}
	return documents
}

func recommendBatchTestFind(
	documents []recommendScoreDocument,
	afterID, maxID ObjectID,
	limit int,
) []recommendScoreDocument {
	batch := make([]recommendScoreDocument, 0, limit)
	for _, document := range documents {
		if compareObjectID(document.ID, afterID) <= 0 ||
			compareObjectID(document.ID, maxID) > 0 {
			continue
		}
		batch = append(batch, document)
		if len(batch) == limit {
			break
		}
	}
	return batch
}

func TestRefreshRecommendScoresBatchesAtFixedHighWater(t *testing.T) {
	const (
		batchSize       = 2
		cursorBatchSize = 4
	)
	initialDocuments := recommendBatchTestDocuments(5)
	highWater := initialDocuments[len(initialDocuments)-1].ID
	documents := append([]recommendScoreDocument(nil), initialDocuments...)

	var stateMu sync.Mutex
	maxEligibleIDCalls := 0
	openCursorCalls := 0
	var openedMaxID ObjectID
	var openedBatchSize int32
	var cursor *fakeRecommendScoreCursor
	bulkSizes := make([]int, 0, 3)

	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			stateMu.Lock()
			defer stateMu.Unlock()
			maxEligibleIDCalls++
			return highWater, nil
		},
		openCursor: func(
			_ context.Context,
			maxID ObjectID,
			serverBatchSize int32,
		) (recommendScoreCursor, error) {
			stateMu.Lock()
			defer stateMu.Unlock()
			openCursorCalls++
			openedMaxID = maxID
			openedBatchSize = serverBatchSize
			// 模拟扫描开始后插入的数据；固定高水位必须把它排除在本轮之外。
			documents = append(documents, recommendScoreDocument{
				ID:                   recommendBatchTestID(6),
				RecommendLikeCount:   6,
				RecommendInitialized: true,
			})
			filtered := make([]recommendScoreDocument, 0, len(documents))
			for _, document := range documents {
				if compareObjectID(document.ID, maxID) <= 0 {
					filtered = append(filtered, document)
				}
			}
			cursor = &fakeRecommendScoreCursor{documents: filtered, current: -1}
			return cursor, nil
		},
		bulkWrite: func(_ context.Context, writes []mongo.WriteModel) error {
			stateMu.Lock()
			defer stateMu.Unlock()
			bulkSizes = append(bulkSizes, len(writes))
			return nil
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		batchSize,
		cursorBatchSize,
		2,
		store,
	)
	if err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", err)
	}

	stateMu.Lock()
	gotMaxEligibleIDCalls := maxEligibleIDCalls
	gotOpenCursorCalls := openCursorCalls
	gotOpenedMaxID := openedMaxID
	gotOpenedBatchSize := openedBatchSize
	gotBulkSizes := append([]int(nil), bulkSizes...)
	stateMu.Unlock()

	if gotMaxEligibleIDCalls != 1 {
		t.Fatalf("MaxEligibleID calls = %d, want 1", gotMaxEligibleIDCalls)
	}
	if gotOpenCursorCalls != 1 {
		t.Fatalf("OpenCursor calls = %d, want 1", gotOpenCursorCalls)
	}
	if gotOpenedMaxID != highWater {
		t.Fatalf("OpenCursor maxID = %s, want fixed high water %s", gotOpenedMaxID.Hex(), highWater.Hex())
	}
	if gotOpenedBatchSize != cursorBatchSize {
		t.Fatalf("OpenCursor batch size = %d, want %d", gotOpenedBatchSize, cursorBatchSize)
	}
	if cursor == nil {
		t.Fatal("OpenCursor returned a nil cursor")
	}
	if cursor.closeCalls.Load() != 1 {
		t.Fatalf("Cursor close calls = %d, want 1", cursor.closeCalls.Load())
	}
	sort.Ints(gotBulkSizes)
	if got, want := gotBulkSizes, []int{1, 2, 2}; len(got) != len(want) ||
		got[0] != want[0] || got[1] != want[1] || got[2] != want[2] {
		t.Fatalf("BulkWrite batch sizes = %v, want %v", got, want)
	}

	if len(candidates) != len(initialDocuments) {
		t.Fatalf("candidate count = %d, want %d", len(candidates), len(initialDocuments))
	}
	seen := make(map[ObjectID]bool, len(candidates))
	for _, candidate := range candidates {
		seen[candidate.ID] = true
	}
	for _, document := range initialDocuments {
		if !seen[document.ID] {
			t.Errorf("candidate %s is missing", document.ID.Hex())
		}
	}
	if seen[recommendBatchTestID(6)] {
		t.Error("candidate inserted above the fixed high water was included")
	}
}

func TestPrepareRecommendScoreBatchWritesOnlyChangedOrUninitialized(t *testing.T) {
	now := time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC)
	documents := []recommendScoreDocument{
		{
			ID:                 recommendBatchTestID(1),
			LikeCount:          1,
			RecommendLikeCount: 10, RecommendCollectCount: 2,
			RecommendCommentCount: 3, RecommendShareCount: 4,
			RecommendScore:       recommendBatchTestScore(CalculateRecommendScore(10, 2, 3, 4)),
			RecommendInitialized: true,
		},
		{
			ID:                 recommendBatchTestID(2),
			LikeCount:          8,
			RecommendLikeCount: 5, RecommendCollectCount: 1,
			RecommendScore:       recommendBatchTestScore(0),
			RecommendInitialized: true,
		},
		{
			ID:                   recommendBatchTestID(3),
			LikeCount:            7,
			RecommendScore:       recommendBatchTestScore(7),
			RecommendInitialized: false,
		},
		{
			ID:                   recommendBatchTestID(4),
			RecommendScore:       recommendBatchTestScore(0),
			RecommendInitialized: true,
		},
		{
			ID:                   recommendBatchTestID(5),
			RecommendScore:       nil,
			RecommendInitialized: true,
		},
	}

	candidates, writes := prepareRecommendScoreBatch(documents, now)
	if len(candidates) != len(documents) {
		t.Fatalf("candidate count = %d, want %d", len(candidates), len(documents))
	}
	if len(writes) != 3 {
		t.Fatalf("write count = %d, want 3 changed/uninitialized/missing-score documents", len(writes))
	}
	wantWriteIDs := []ObjectID{documents[1].ID, documents[2].ID, documents[4].ID}
	for i, write := range writes {
		update, ok := write.(*mongo.UpdateOneModel)
		if !ok {
			t.Fatalf("write %d type = %T, want *mongo.UpdateOneModel", i, write)
		}
		filter, ok := update.Filter.(bson.M)
		if !ok || filter["_id"] != wantWriteIDs[i] {
			t.Fatalf("write %d filter = %#v, want _id %s", i, update.Filter, wantWriteIDs[i].Hex())
		}
		pipeline, ok := update.Update.(mongo.Pipeline)
		if !ok {
			t.Fatalf("write %d update type = %T, want mongo.Pipeline", i, update.Update)
		}
		pipelineText := fmt.Sprint(pipeline)
		for _, field := range []string{
			"$recommendLikeCount",
			"$recommendCollectCount",
			"$recommendCommentCount",
			"$recommendShareCount",
		} {
			if !strings.Contains(pipelineText, field) {
				t.Fatalf("write %d pipeline does not reference %s: %s", i, field, pipelineText)
			}
		}
	}
	wantScores := []int64{
		CalculateRecommendScore(10, 2, 3, 4),
		CalculateRecommendScore(8, 1, 0, 0),
		7,
		0,
		0,
	}
	for i, candidate := range candidates {
		if candidate.RecommendScore != wantScores[i] {
			t.Errorf("candidate %d score = %d, want %d", i, candidate.RecommendScore, wantScores[i])
		}
	}
}

func TestRefreshRecommendScoresSkipsBulkWriteForUnchangedBatches(t *testing.T) {
	documents := recommendBatchTestDocuments(5)
	for i := range documents {
		documents[i].RecommendScore = recommendBatchTestScore(CalculateRecommendScore(
			documents[i].RecommendLikeCount,
			documents[i].RecommendCollectCount,
			documents[i].RecommendCommentCount,
			documents[i].RecommendShareCount,
		))
	}
	highWater := documents[len(documents)-1].ID
	var bulkCalls atomic.Int32
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		findBatch: func(
			_ context.Context,
			afterID, maxID ObjectID,
			limit int,
		) ([]recommendScoreDocument, error) {
			return recommendBatchTestFind(documents, afterID, maxID, limit), nil
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			bulkCalls.Add(1)
			return errors.New("unchanged batch must not be written")
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		2,
		2,
		2,
		store,
	)
	if err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", err)
	}
	if len(candidates) != len(documents) {
		t.Fatalf("candidate count = %d, want %d", len(candidates), len(documents))
	}
	if got := bulkCalls.Load(); got != 0 {
		t.Fatalf("BulkWrite calls = %d, want 0 for unchanged documents", got)
	}
}

func TestRefreshRecommendScoresInitializationGuardStopsBeforeWrites(t *testing.T) {
	documents := recommendBatchTestDocuments(2)
	highWater := documents[len(documents)-1].ID
	var cursorCalls, bulkCalls atomic.Int32
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		countUninitialized: func(
			_ context.Context,
			gotMaxID ObjectID,
			limit int64,
		) (int64, error) {
			if gotMaxID != highWater {
				t.Fatalf("CountUninitialized maxID = %s, want %s", gotMaxID.Hex(), highWater.Hex())
			}
			if limit != 11 {
				t.Fatalf("CountUninitialized limit = %d, want 11", limit)
			}
			return 11, nil
		},
		openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
			cursorCalls.Add(1)
			return nil, errors.New("cursor must not open after guard rejection")
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			bulkCalls.Add(1)
			return errors.New("bulk write must not run after guard rejection")
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		2,
		2,
		2,
		store,
		10,
	)
	var limitErr RecommendInitializationLimitError
	if !errors.As(err, &limitErr) {
		t.Fatalf("error = %v, want RecommendInitializationLimitError", err)
	}
	if limitErr.Limit != 10 || limitErr.ObservedAtLeast != 11 {
		t.Fatalf("limit error = %+v", limitErr)
	}
	if candidates != nil {
		t.Fatalf("candidates = %#v, want nil", candidates)
	}
	if cursorCalls.Load() != 0 || bulkCalls.Load() != 0 {
		t.Fatalf("cursorCalls=%d bulkCalls=%d, want zero", cursorCalls.Load(), bulkCalls.Load())
	}
}

func TestRefreshRecommendScoresInitializationGuardAllowsExactLimit(t *testing.T) {
	documents := recommendBatchTestDocuments(2)
	for i := range documents {
		documents[i].RecommendScore = recommendBatchTestScore(documents[i].RecommendLikeCount)
	}
	highWater := documents[len(documents)-1].ID
	var countCalls atomic.Int32
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		countUninitialized: func(context.Context, ObjectID, int64) (int64, error) {
			countCalls.Add(1)
			return 10, nil
		},
		findBatch: func(
			_ context.Context,
			afterID, maxID ObjectID,
			limit int,
		) ([]recommendScoreDocument, error) {
			return recommendBatchTestFind(documents, afterID, maxID, limit), nil
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			return errors.New("unchanged documents must not be written")
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		2,
		2,
		2,
		store,
		10,
	)
	if err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", err)
	}
	if countCalls.Load() != 1 || len(candidates) != len(documents) {
		t.Fatalf("countCalls=%d candidateCount=%d", countCalls.Load(), len(candidates))
	}
}

func TestRefreshRecommendScoresWritesOnlyChangedMixedBatch(t *testing.T) {
	documents := recommendBatchTestDocuments(4)
	for i := range documents {
		documents[i].RecommendScore = recommendBatchTestScore(documents[i].RecommendLikeCount)
	}
	documents[2].RecommendScore = recommendBatchTestScore(0)
	highWater := documents[len(documents)-1].ID
	var stateMu sync.Mutex
	var bulkSizes []int
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		findBatch: func(
			_ context.Context,
			afterID, maxID ObjectID,
			limit int,
		) ([]recommendScoreDocument, error) {
			return recommendBatchTestFind(documents, afterID, maxID, limit), nil
		},
		bulkWrite: func(_ context.Context, writes []mongo.WriteModel) error {
			stateMu.Lock()
			defer stateMu.Unlock()
			bulkSizes = append(bulkSizes, len(writes))
			return nil
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		2,
		2,
		2,
		store,
	)
	if err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", err)
	}
	if len(candidates) != len(documents) {
		t.Fatalf("candidate count = %d, want %d", len(candidates), len(documents))
	}
	stateMu.Lock()
	gotBulkSizes := append([]int(nil), bulkSizes...)
	stateMu.Unlock()
	if len(gotBulkSizes) != 1 || gotBulkSizes[0] != 1 {
		t.Fatalf("BulkWrite batch sizes = %v, want [1]", gotBulkSizes)
	}
}

func TestRefreshRecommendScoresBoundsAndUsesConcurrency(t *testing.T) {
	const workerCount = 3
	documents := recommendBatchTestDocuments(8)
	highWater := documents[len(documents)-1].ID
	release := make(chan struct{})
	started := make(chan struct{}, len(documents))

	var stateMu sync.Mutex
	active := 0
	maxActive := 0
	bulkCalls := 0
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		findBatch: func(
			_ context.Context,
			afterID, maxID ObjectID,
			limit int,
		) ([]recommendScoreDocument, error) {
			return recommendBatchTestFind(documents, afterID, maxID, limit), nil
		},
		bulkWrite: func(ctx context.Context, _ []mongo.WriteModel) error {
			stateMu.Lock()
			active++
			bulkCalls++
			if active > maxActive {
				maxActive = active
			}
			stateMu.Unlock()
			defer func() {
				stateMu.Lock()
				active--
				stateMu.Unlock()
			}()

			started <- struct{}{}
			select {
			case <-release:
				return nil
			case <-ctx.Done():
				return ctx.Err()
			}
		},
	}

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	result := make(chan refreshRecommendScoresResult, 1)
	go func() {
		candidates, err := refreshRecommendScores(
			ctx,
			time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
			1,
			1,
			workerCount,
			store,
		)
		result <- refreshRecommendScoresResult{candidates: candidates, err: err}
	}()

	for i := 0; i < workerCount; i++ {
		select {
		case <-started:
		case <-ctx.Done():
			t.Fatalf("only %d/%d workers reached the barrier: %v", i, workerCount, ctx.Err())
		}
	}
	stateMu.Lock()
	gotMaxActiveAtBarrier := maxActive
	stateMu.Unlock()
	if gotMaxActiveAtBarrier != workerCount {
		t.Fatalf("active BulkWrite calls at barrier = %d, want %d", gotMaxActiveAtBarrier, workerCount)
	}
	close(release)

	var got refreshRecommendScoresResult
	select {
	case got = <-result:
	case <-ctx.Done():
		t.Fatalf("refreshRecommendScores() did not finish: %v", ctx.Err())
	}
	if got.err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", got.err)
	}
	if len(got.candidates) != len(documents) {
		t.Fatalf("candidate count = %d, want %d", len(got.candidates), len(documents))
	}

	stateMu.Lock()
	gotActive := active
	gotMaxActive := maxActive
	gotBulkCalls := bulkCalls
	stateMu.Unlock()
	if gotActive != 0 {
		t.Errorf("active BulkWrite calls after return = %d, want 0", gotActive)
	}
	if gotMaxActive != workerCount {
		t.Errorf("maximum concurrent BulkWrite calls = %d, want %d", gotMaxActive, workerCount)
	}
	if gotBulkCalls != len(documents) {
		t.Errorf("BulkWrite calls = %d, want %d", gotBulkCalls, len(documents))
	}
}

func TestRefreshRecommendScoresFirstErrorCancelsWorkers(t *testing.T) {
	const workerCount = 2
	writeErr := errors.New("bulk write failed")
	documents := recommendBatchTestDocuments(8)
	highWater := documents[len(documents)-1].ID
	started := make(chan int32, workerCount)
	releaseFailure := make(chan struct{})
	cancelObserved := make(chan struct{})
	var cancelObservedOnce sync.Once
	var releaseOnce sync.Once
	release := func() {
		releaseOnce.Do(func() {
			close(releaseFailure)
		})
	}
	defer release()

	var bulkCalls atomic.Int32
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		findBatch: func(
			_ context.Context,
			afterID, maxID ObjectID,
			limit int,
		) ([]recommendScoreDocument, error) {
			return recommendBatchTestFind(documents, afterID, maxID, limit), nil
		},
		bulkWrite: func(ctx context.Context, _ []mongo.WriteModel) error {
			call := bulkCalls.Add(1)
			started <- call
			if call == 1 {
				select {
				case <-releaseFailure:
					return writeErr
				case <-ctx.Done():
					return ctx.Err()
				}
			}
			<-ctx.Done()
			cancelObservedOnce.Do(func() {
				close(cancelObserved)
			})
			return ctx.Err()
		},
	}

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	result := make(chan refreshRecommendScoresResult, 1)
	go func() {
		candidates, err := refreshRecommendScores(
			ctx,
			time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
			1,
			1,
			workerCount,
			store,
		)
		result <- refreshRecommendScoresResult{candidates: candidates, err: err}
	}()

	for i := 0; i < workerCount; i++ {
		select {
		case <-started:
		case <-ctx.Done():
			t.Fatalf("only %d/%d writes reached the barrier: %v", i, workerCount, ctx.Err())
		}
	}
	release()

	var got refreshRecommendScoresResult
	select {
	case got = <-result:
	case <-ctx.Done():
		t.Fatalf("refreshRecommendScores() did not finish after first error: %v", ctx.Err())
	}
	if !errors.Is(got.err, writeErr) {
		t.Fatalf("refreshRecommendScores() error = %v, want %v", got.err, writeErr)
	}
	if got.candidates != nil {
		t.Fatalf("candidates = %v, want nil on write error", got.candidates)
	}
	select {
	case <-cancelObserved:
	default:
		t.Fatal("peer worker did not observe cancellation from the first error")
	}
	if gotCalls := bulkCalls.Load(); gotCalls != workerCount {
		t.Fatalf("BulkWrite calls = %d, want %d blocked workers only", gotCalls, workerCount)
	}
}

func TestRefreshRecommendScoresEmptyHighWaterSkipsWork(t *testing.T) {
	var findCalls atomic.Int32
	var bulkCalls atomic.Int32
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return ObjectID{}, nil
		},
		findBatch: func(
			context.Context,
			ObjectID, ObjectID,
			int,
		) ([]recommendScoreDocument, error) {
			findCalls.Add(1)
			return nil, nil
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			bulkCalls.Add(1)
			return nil
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		2,
		2,
		2,
		store,
	)
	if err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", err)
	}
	if candidates == nil || len(candidates) != 0 {
		t.Fatalf("candidates = %#v, want non-nil empty slice", candidates)
	}
	if got := findCalls.Load(); got != 0 {
		t.Errorf("FindBatch calls = %d, want 0", got)
	}
	if got := bulkCalls.Load(); got != 0 {
		t.Errorf("BulkWrite calls = %d, want 0", got)
	}
}

func TestRefreshRecommendScoresPropagatesContextCancellation(t *testing.T) {
	documents := recommendBatchTestDocuments(2)
	highWater := documents[len(documents)-1].ID
	writeStarted := make(chan struct{}, 1)
	cancelObserved := make(chan struct{})
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		findBatch: func(
			_ context.Context,
			afterID, maxID ObjectID,
			limit int,
		) ([]recommendScoreDocument, error) {
			return recommendBatchTestFind(documents, afterID, maxID, limit), nil
		},
		bulkWrite: func(ctx context.Context, _ []mongo.WriteModel) error {
			writeStarted <- struct{}{}
			<-ctx.Done()
			close(cancelObserved)
			return ctx.Err()
		},
	}

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	guard, stopGuard := context.WithTimeout(context.Background(), 5*time.Second)
	defer stopGuard()
	result := make(chan refreshRecommendScoresResult, 1)
	go func() {
		candidates, err := refreshRecommendScores(
			ctx,
			time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
			1,
			1,
			1,
			store,
		)
		result <- refreshRecommendScoresResult{candidates: candidates, err: err}
	}()

	select {
	case <-writeStarted:
	case <-guard.Done():
		t.Fatalf("BulkWrite did not start: %v", guard.Err())
	}
	cancel()

	var got refreshRecommendScoresResult
	select {
	case got = <-result:
	case <-guard.Done():
		t.Fatalf("refreshRecommendScores() did not stop after cancellation: %v", guard.Err())
	}
	if !errors.Is(got.err, context.Canceled) {
		t.Fatalf("refreshRecommendScores() error = %v, want %v", got.err, context.Canceled)
	}
	if got.candidates != nil {
		t.Fatalf("candidates = %v, want nil after cancellation", got.candidates)
	}
	select {
	case <-cancelObserved:
	default:
		t.Fatal("BulkWrite did not observe context cancellation")
	}
}

func TestRecommendScoreCursorQueryUsesBoundedIndexedScan(t *testing.T) {
	maxID := recommendBatchTestID(9)
	generatedAt := time.Date(2026, 7, 31, 8, 0, 0, 0, time.UTC)
	scope := newRecommendScoreScope(generatedAt, []string{" blocked ", "", "blocked", "other"})
	filter, opts := recommendScoreCursorQuery(maxID, 4000, scope)

	if filter["status"] != CheckPass || filter["newsType"] != SHORT || filter["deleteAt"] != nil {
		t.Fatalf("filter = %#v, want eligible short-video filter", filter)
	}
	if !reflect.DeepEqual(filter["recoWeight"], bson.M{"$ne": -1}) {
		t.Fatalf("recoWeight filter = %#v, want explicit non-recommend exclusion", filter["recoWeight"])
	}
	reviewAt, ok := filter["reviewAt"].(bson.M)
	if !ok || !reflect.DeepEqual(reviewAt, bson.M{"$lte": generatedAt}) {
		t.Fatalf("reviewAt filter = %#v, want <= %s", filter["reviewAt"], generatedAt)
	}
	moduleIDs, ok := filter["mId"].(bson.M)
	if !ok || !reflect.DeepEqual(moduleIDs["$nin"], []string{"blocked", "other"}) {
		t.Fatalf("mId filter = %#v, want normalized excluded modules", filter["mId"])
	}
	idRange, ok := filter["_id"].(bson.M)
	if !ok || idRange["$lte"] != maxID || len(idRange) != 1 {
		t.Fatalf("_id filter = %#v, want $lte %s", filter["_id"], maxID.Hex())
	}
	if opts.BatchSize == nil || *opts.BatchSize != 4000 {
		t.Fatalf("cursor batch size = %v, want 4000", opts.BatchSize)
	}
	if opts.Limit != nil {
		t.Fatalf("cursor limit = %v, want nil for one streaming cursor", *opts.Limit)
	}
	if opts.Hint != shortRecommendRefreshIndexName {
		t.Fatalf("cursor hint = %#v, want %s", opts.Hint, shortRecommendRefreshIndexName)
	}
	wantSort := bson.D{{Key: "_id", Value: 1}}
	if !reflect.DeepEqual(opts.Sort, wantSort) {
		t.Fatalf("cursor sort = %#v, want %#v", opts.Sort, wantSort)
	}
	projection, ok := opts.Projection.(bson.M)
	if !ok {
		t.Fatalf("cursor projection type = %T, want bson.M", opts.Projection)
	}
	for _, field := range []string{
		"_id", "mId", "reviewAt", "likeCount", "collectCount", "commentCount", "shareCount",
		"recommendLikeCount", "recommendCollectCount", "recommendCommentCount",
		"recommendShareCount", "recommendScore", "recommendInitialized",
	} {
		if projection[field] != 1 {
			t.Fatalf("projection[%q] = %#v, want 1", field, projection[field])
		}
	}
}

func TestRecommendScoreMaxEligibleIDQueryUsesSameEligibilityScope(t *testing.T) {
	generatedAt := time.Date(2026, 7, 31, 8, 0, 0, 0, time.UTC)
	scope := newRecommendScoreScope(generatedAt, []string{"blocked"})
	filter, opts := recommendScoreMaxEligibleIDQuery(scope)
	if filter["status"] != CheckPass || filter["newsType"] != SHORT || filter["deleteAt"] != nil {
		t.Fatalf("filter = %#v, want eligible short-video filter", filter)
	}
	if !reflect.DeepEqual(filter["recoWeight"], bson.M{"$ne": -1}) {
		t.Fatalf("recoWeight filter = %#v, want explicit non-recommend exclusion", filter["recoWeight"])
	}
	if !reflect.DeepEqual(filter["reviewAt"], bson.M{"$lte": generatedAt}) {
		t.Fatalf("reviewAt filter = %#v, want <= %s", filter["reviewAt"], generatedAt)
	}
	if !reflect.DeepEqual(filter["mId"], bson.M{"$nin": []string{"blocked"}}) {
		t.Fatalf("mId filter = %#v, want blocked module", filter["mId"])
	}
	if opts.Hint != shortRecommendRefreshIndexName {
		t.Fatalf("hint = %#v, want %s", opts.Hint, shortRecommendRefreshIndexName)
	}
	if !reflect.DeepEqual(opts.Sort, bson.D{{Key: "_id", Value: -1}}) {
		t.Fatalf("sort = %#v, want descending _id", opts.Sort)
	}
}

func TestRecommendScoreMaxEligibleIDResultTreatsEmptyCollectionAsEmptyQueue(t *testing.T) {
	id, err := recommendScoreMaxEligibleIDResult(
		recommendBatchTestID(9),
		fmt.Errorf("wrapped: %w", mongo.ErrNoDocuments),
	)
	if err != nil {
		t.Fatalf("recommendScoreMaxEligibleIDResult() error = %v", err)
	}
	if !id.IsZero() {
		t.Fatalf("id = %s, want zero ObjectID", id.Hex())
	}

	wantErr := errors.New("find failed")
	id, err = recommendScoreMaxEligibleIDResult(recommendBatchTestID(9), wantErr)
	if !errors.Is(err, wantErr) || !id.IsZero() {
		t.Fatalf("id = %s, error = %v, want zero id and %v", id.Hex(), err, wantErr)
	}
}

func TestRecommendInitializationCountQueryUsesSameBoundedIndex(t *testing.T) {
	maxID := recommendBatchTestID(9)
	generatedAt := time.Date(2026, 7, 31, 8, 0, 0, 0, time.UTC)
	scope := newRecommendScoreScope(generatedAt, []string{"blocked"})
	filter, opts := recommendInitializationCountQuery(maxID, 50_001, scope)
	if filter["status"] != CheckPass || filter["newsType"] != SHORT || filter["deleteAt"] != nil {
		t.Fatalf("filter = %#v, want eligible short-video filter", filter)
	}
	if !reflect.DeepEqual(filter["recoWeight"], bson.M{"$ne": -1}) {
		t.Fatalf("recoWeight filter = %#v, want explicit non-recommend exclusion", filter["recoWeight"])
	}
	reviewAt, ok := filter["reviewAt"].(bson.M)
	if !ok || !reflect.DeepEqual(reviewAt, bson.M{"$lte": generatedAt}) {
		t.Fatalf("reviewAt filter = %#v, want <= %s", filter["reviewAt"], generatedAt)
	}
	moduleIDs, ok := filter["mId"].(bson.M)
	if !ok || !reflect.DeepEqual(moduleIDs["$nin"], []string{"blocked"}) {
		t.Fatalf("mId filter = %#v, want blocked module", filter["mId"])
	}
	idRange, ok := filter["_id"].(bson.M)
	if !ok || idRange["$lte"] != maxID {
		t.Fatalf("_id filter = %#v", filter["_id"])
	}
	initialized, ok := filter["recommendInitialized"].(bson.M)
	if !ok || initialized["$ne"] != true {
		t.Fatalf("recommendInitialized filter = %#v", filter["recommendInitialized"])
	}
	if opts.Limit == nil || *opts.Limit != 50_001 {
		t.Fatalf("limit = %v, want 50001", opts.Limit)
	}
	if opts.Hint != shortRecommendRefreshIndexName {
		t.Fatalf("hint = %#v, want %s", opts.Hint, shortRecommendRefreshIndexName)
	}
}

func TestRecommendVideosByIDsFilterRevalidatesEligibility(t *testing.T) {
	ids := []ObjectID{recommendBatchTestID(1), recommendBatchTestID(2)}
	filter := recommendVideosByIDsFilter(ids)
	idMatch, ok := filter["_id"].(bson.M)
	if !ok || !reflect.DeepEqual(idMatch["$in"], ids) {
		t.Fatalf("_id filter = %#v", filter["_id"])
	}
	if filter["status"] != CheckPass ||
		filter["newsType"] != SHORT ||
		filter["deleteAt"] != nil ||
		!reflect.DeepEqual(filter["recoWeight"], bson.M{"$ne": -1}) {
		t.Fatalf("filter = %#v, want approved non-deleted short video", filter)
	}
}

func TestGetRecommendVideosByIDsContextEmptyInputDoesNotAccessMongo(t *testing.T) {
	originalDB := mdb
	mdb = nil
	t.Cleanup(func() { mdb = originalDB })
	videos, err := GetRecommendVideosByIDsContext(context.Background(), nil)
	if err != nil {
		t.Fatalf("GetRecommendVideosByIDsContext() error = %v", err)
	}
	if len(videos) != 0 {
		t.Fatalf("videos = %#v, want empty", videos)
	}
}

func TestStreamRecommendScoreBatchesOwnsSlicesAndClosesCursor(t *testing.T) {
	documents := recommendBatchTestDocuments(5)
	maxID := documents[len(documents)-1].ID
	cursor := &fakeRecommendScoreCursor{documents: documents, current: -1}
	openCalls := 0
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return maxID, nil
		},
		openCursor: func(_ context.Context, gotMaxID ObjectID, gotBatchSize int32) (recommendScoreCursor, error) {
			openCalls++
			if gotMaxID != maxID {
				t.Fatalf("OpenCursor maxID = %s, want %s", gotMaxID.Hex(), maxID.Hex())
			}
			if gotBatchSize != 4 {
				t.Fatalf("OpenCursor batch size = %d, want 4", gotBatchSize)
			}
			return cursor, nil
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			return nil
		},
	}

	var batches [][]recommendScoreDocument
	err := streamRecommendScoreBatches(
		context.Background(),
		maxID,
		2,
		4,
		store,
		func(batch []recommendScoreDocument) error {
			batches = append(batches, batch)
			return nil
		},
	)
	if err != nil {
		t.Fatalf("streamRecommendScoreBatches() error = %v", err)
	}
	if openCalls != 1 {
		t.Fatalf("OpenCursor calls = %d, want 1", openCalls)
	}
	if got, want := cursor.closeCalls.Load(), int32(1); got != want {
		t.Fatalf("Cursor close calls = %d, want %d", got, want)
	}
	if cursor.closeContextCanceled.Load() {
		t.Fatal("Cursor Close received an already-canceled cleanup context")
	}
	wantBatchSizes := []int{2, 2, 1}
	if len(batches) != len(wantBatchSizes) {
		t.Fatalf("batch count = %d, want %d", len(batches), len(wantBatchSizes))
	}
	gotIDs := make([]ObjectID, 0, len(documents))
	for i, batch := range batches {
		if len(batch) != wantBatchSizes[i] {
			t.Errorf("batch %d size = %d, want %d", i, len(batch), wantBatchSizes[i])
		}
		for _, document := range batch {
			gotIDs = append(gotIDs, document.ID)
		}
	}
	for i := range documents {
		if gotIDs[i] != documents[i].ID {
			t.Fatalf("streamed ID %d = %s, want %s", i, gotIDs[i].Hex(), documents[i].ID.Hex())
		}
	}
}

func TestStreamRecommendScoreBatchesErrorLifecycle(t *testing.T) {
	openErr := errors.New("open cursor failed")
	decodeErr := errors.New("decode failed")
	cursorErr := errors.New("cursor failed")
	closeErr := errors.New("close failed")

	tests := []struct {
		name          string
		open          func() (recommendScoreCursor, error)
		wantErr       error
		wantClose     int32
		wantYieldCall int
	}{
		{
			name: "open error",
			open: func() (recommendScoreCursor, error) {
				return nil, openErr
			},
			wantErr: openErr,
		},
		{
			name: "nil cursor",
			open: func() (recommendScoreCursor, error) {
				return nil, nil
			},
			wantErr: errors.New("recommend score cursor is nil"),
		},
		{
			name: "decode error wins over close error",
			open: func() (recommendScoreCursor, error) {
				return &fakeRecommendScoreCursor{
					documents:   recommendBatchTestDocuments(2),
					current:     -1,
					decodeErrAt: 0,
					decodeErr:   decodeErr,
					closeErr:    closeErr,
				}, nil
			},
			wantErr:   decodeErr,
			wantClose: 1,
		},
		{
			name: "cursor error prevents partial tail",
			open: func() (recommendScoreCursor, error) {
				return &fakeRecommendScoreCursor{
					documents: recommendBatchTestDocuments(1),
					current:   -1,
					cursorErr: cursorErr,
				}, nil
			},
			wantErr:   cursorErr,
			wantClose: 1,
		},
		{
			name: "close error",
			open: func() (recommendScoreCursor, error) {
				return &fakeRecommendScoreCursor{
					current:  -1,
					closeErr: closeErr,
				}, nil
			},
			wantErr:   closeErr,
			wantClose: 1,
		},
	}

	for _, test := range tests {
		t.Run(test.name, func(t *testing.T) {
			var opened recommendScoreCursor
			store := fakeRecommendScoreStore{
				maxEligibleID: func(context.Context) (ObjectID, error) {
					return recommendBatchTestID(2), nil
				},
				openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
					cursor, err := test.open()
					opened = cursor
					return cursor, err
				},
				bulkWrite: func(context.Context, []mongo.WriteModel) error {
					return nil
				},
			}
			yieldCalls := 0
			err := streamRecommendScoreBatches(
				context.Background(),
				recommendBatchTestID(2),
				2,
				4,
				store,
				func([]recommendScoreDocument) error {
					yieldCalls++
					return nil
				},
			)
			if test.name == "nil cursor" {
				if err == nil || !strings.Contains(err.Error(), test.wantErr.Error()) {
					t.Fatalf("error = %v, want containing %q", err, test.wantErr)
				}
			} else if !errors.Is(err, test.wantErr) {
				t.Fatalf("error = %v, want %v", err, test.wantErr)
			}
			if yieldCalls != test.wantYieldCall {
				t.Fatalf("yield calls = %d, want %d", yieldCalls, test.wantYieldCall)
			}
			if cursor, ok := opened.(*fakeRecommendScoreCursor); ok {
				if got := cursor.closeCalls.Load(); got != test.wantClose {
					t.Fatalf("Cursor close calls = %d, want %d", got, test.wantClose)
				}
			}
		})
	}
}

func TestStreamRecommendScoreBatchesCancellationUsesCleanupContext(t *testing.T) {
	documents := recommendBatchTestDocuments(2)
	cursor := &fakeRecommendScoreCursor{documents: documents, current: -1}
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return documents[len(documents)-1].ID, nil
		},
		openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
			return cursor, nil
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			return nil
		},
	}
	ctx, cancel := context.WithCancel(context.Background())
	err := streamRecommendScoreBatches(
		ctx,
		documents[len(documents)-1].ID,
		1,
		2,
		store,
		func([]recommendScoreDocument) error {
			cancel()
			return context.Canceled
		},
	)
	if !errors.Is(err, context.Canceled) {
		t.Fatalf("error = %v, want %v", err, context.Canceled)
	}
	if got := cursor.closeCalls.Load(); got != 1 {
		t.Fatalf("Cursor close calls = %d, want 1", got)
	}
	if cursor.closeContextCanceled.Load() {
		t.Fatal("Cursor Close received the canceled work context")
	}
}

func TestStreamRecommendScoreBatchesRejectsInvalidIDSequence(t *testing.T) {
	id1 := recommendBatchTestID(1)
	id2 := recommendBatchTestID(2)
	id3 := recommendBatchTestID(3)
	tests := []struct {
		name      string
		documents []recommendScoreDocument
		maxID     ObjectID
	}{
		{
			name:      "zero ID",
			documents: []recommendScoreDocument{{}},
			maxID:     id2,
		},
		{
			name:      "duplicate across batch boundary",
			documents: []recommendScoreDocument{{ID: id1}, {ID: id1}},
			maxID:     id2,
		},
		{
			name:      "descending across batch boundary",
			documents: []recommendScoreDocument{{ID: id2}, {ID: id1}},
			maxID:     id2,
		},
		{
			name:      "above fixed high water",
			documents: []recommendScoreDocument{{ID: id1}, {ID: id3}},
			maxID:     id2,
		},
	}

	for _, test := range tests {
		t.Run(test.name, func(t *testing.T) {
			cursor := &fakeRecommendScoreCursor{documents: test.documents, current: -1}
			store := fakeRecommendScoreStore{
				maxEligibleID: func(context.Context) (ObjectID, error) {
					return test.maxID, nil
				},
				openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
					return cursor, nil
				},
				bulkWrite: func(context.Context, []mongo.WriteModel) error {
					return nil
				},
			}
			err := streamRecommendScoreBatches(
				context.Background(),
				test.maxID,
				1,
				2,
				store,
				func([]recommendScoreDocument) error { return nil },
			)
			if err == nil || !strings.Contains(err.Error(), "invalid _id sequence") {
				t.Fatalf("error = %v, want invalid _id sequence", err)
			}
			if got := cursor.closeCalls.Load(); got != 1 {
				t.Fatalf("Cursor close calls = %d, want 1", got)
			}
		})
	}
}

func TestRefreshRecommendScoresNonZeroHighWaterEmptyCursor(t *testing.T) {
	highWater := recommendBatchTestID(1)
	cursor := &fakeRecommendScoreCursor{current: -1}
	var bulkCalls atomic.Int32
	store := fakeRecommendScoreStore{
		maxEligibleID: func(context.Context) (ObjectID, error) {
			return highWater, nil
		},
		openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
			return cursor, nil
		},
		bulkWrite: func(context.Context, []mongo.WriteModel) error {
			bulkCalls.Add(1)
			return nil
		},
	}

	candidates, err := refreshRecommendScores(
		context.Background(),
		time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
		2,
		4,
		2,
		store,
	)
	if err != nil {
		t.Fatalf("refreshRecommendScores() error = %v", err)
	}
	if candidates == nil || len(candidates) != 0 {
		t.Fatalf("candidates = %#v, want non-nil empty slice", candidates)
	}
	if got := cursor.closeCalls.Load(); got != 1 {
		t.Fatalf("Cursor close calls = %d, want 1", got)
	}
	if got := bulkCalls.Load(); got != 0 {
		t.Fatalf("BulkWrite calls = %d, want 0", got)
	}
}

func TestRefreshRecommendScoresRejectsInvalidLimitsBeforeDatabaseAccess(t *testing.T) {
	tests := []struct {
		name            string
		batchSize       int
		cursorBatchSize int32
		workerCount     int
		wantMessage     string
	}{
		{
			name:            "process batch",
			batchSize:       0,
			cursorBatchSize: 4,
			workerCount:     2,
			wantMessage:     "batch size must be positive",
		},
		{
			name:            "cursor batch",
			batchSize:       2,
			cursorBatchSize: 0,
			workerCount:     2,
			wantMessage:     "cursor batch size must be positive",
		},
		{
			name:            "worker count",
			batchSize:       2,
			cursorBatchSize: 4,
			workerCount:     0,
			wantMessage:     "worker count must be positive",
		},
	}
	for _, test := range tests {
		t.Run(test.name, func(t *testing.T) {
			var databaseCalls atomic.Int32
			store := fakeRecommendScoreStore{
				maxEligibleID: func(context.Context) (ObjectID, error) {
					databaseCalls.Add(1)
					return recommendBatchTestID(1), nil
				},
				openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
					databaseCalls.Add(1)
					return nil, errors.New("must not open cursor")
				},
				bulkWrite: func(context.Context, []mongo.WriteModel) error {
					databaseCalls.Add(1)
					return nil
				},
			}
			candidates, err := refreshRecommendScores(
				context.Background(),
				time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
				test.batchSize,
				test.cursorBatchSize,
				test.workerCount,
				store,
			)
			if err == nil || !strings.Contains(err.Error(), test.wantMessage) {
				t.Fatalf("error = %v, want containing %q", err, test.wantMessage)
			}
			if candidates != nil {
				t.Fatalf("candidates = %#v, want nil", candidates)
			}
			if got := databaseCalls.Load(); got != 0 {
				t.Fatalf("database calls = %d, want 0", got)
			}
		})
	}
}

func TestRefreshRecommendScoresPropagatesSourceErrors(t *testing.T) {
	maxErr := errors.New("max ID failed")
	openErr := errors.New("open cursor failed")
	tests := []struct {
		name       string
		maxID      func(context.Context) (ObjectID, error)
		openCursor func(context.Context, ObjectID, int32) (recommendScoreCursor, error)
		wantErr    error
		wantOpens  int32
	}{
		{
			name: "max ID",
			maxID: func(context.Context) (ObjectID, error) {
				return ObjectID{}, maxErr
			},
			openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
				return nil, errors.New("must not open cursor")
			},
			wantErr: maxErr,
		},
		{
			name: "open cursor",
			maxID: func(context.Context) (ObjectID, error) {
				return recommendBatchTestID(1), nil
			},
			openCursor: func(context.Context, ObjectID, int32) (recommendScoreCursor, error) {
				return nil, openErr
			},
			wantErr:   openErr,
			wantOpens: 1,
		},
	}

	for _, test := range tests {
		t.Run(test.name, func(t *testing.T) {
			var openCalls atomic.Int32
			store := fakeRecommendScoreStore{
				maxEligibleID: test.maxID,
				openCursor: func(ctx context.Context, maxID ObjectID, batchSize int32) (recommendScoreCursor, error) {
					openCalls.Add(1)
					return test.openCursor(ctx, maxID, batchSize)
				},
				bulkWrite: func(context.Context, []mongo.WriteModel) error {
					return errors.New("must not write")
				},
			}
			candidates, err := refreshRecommendScores(
				context.Background(),
				time.Date(2026, 7, 31, 0, 0, 0, 0, time.UTC),
				2,
				4,
				2,
				store,
			)
			if !errors.Is(err, test.wantErr) {
				t.Fatalf("error = %v, want %v", err, test.wantErr)
			}
			if candidates != nil {
				t.Fatalf("candidates = %#v, want nil", candidates)
			}
			if got := openCalls.Load(); got != test.wantOpens {
				t.Fatalf("OpenCursor calls = %d, want %d", got, test.wantOpens)
			}
		})
	}
}