package service

import (
	"context"
	"errors"
	"fmt"
	"math"
	"sort"
	"strings"
	"time"

	"github.com/sirupsen/logrus"
	commonRepo "gitlab.com/mbugroup/lti-api.git/internal/common/repository"
	"gitlab.com/mbugroup/lti-api.git/internal/entities"
	productWarehouseRepo "gitlab.com/mbugroup/lti-api.git/internal/modules/inventory/product-warehouses/repositories"
	"gitlab.com/mbugroup/lti-api.git/internal/utils/fifo"
	"gorm.io/gorm"
	"gorm.io/gorm/clause"
)

type FifoService interface {
	RegisterStockable(cfg fifo.StockableConfig) error
	RegisterUsable(cfg fifo.UsableConfig) error

	Replenish(ctx context.Context, req StockReplenishRequest) (*StockReplenishResult, error)
	Consume(ctx context.Context, req StockConsumeRequest) (*StockConsumeResult, error)
	ReleaseUsage(ctx context.Context, req StockReleaseRequest) error
}

type fifoService struct {
	db                     *gorm.DB
	logger                 *logrus.Logger
	allocations            commonRepo.StockAllocationRepository
	productWarehouseRepo   productWarehouseRepo.ProductWarehouseRepository
	defaultOrderBy         []string
	pendingBatchPerUsable  int
	maxLotsPerStockable    int
	defaultAllocationNotes string
}

func NewFifoService(
	db *gorm.DB,
	allocations commonRepo.StockAllocationRepository,
	productWarehouseRepo productWarehouseRepo.ProductWarehouseRepository,
	logger *logrus.Logger,
) FifoService {
	if logger == nil {
		logger = logrus.StandardLogger()
	}
	return &fifoService{
		db:                    db,
		logger:                logger,
		allocations:           allocations,
		productWarehouseRepo:  productWarehouseRepo,
		defaultOrderBy:        []string{"created_at ASC", "id ASC"},
		pendingBatchPerUsable: 25,
		maxLotsPerStockable:   50,
	}
}

func (s *fifoService) withTransaction(
	ctx context.Context,
	tx *gorm.DB,
	fn func(*gorm.DB) error,
) error {
	if tx != nil {
		return fn(tx.WithContext(ctx))
	}
	return s.db.WithContext(ctx).Transaction(func(inner *gorm.DB) error {
		return fn(inner)
	})
}

func (s *fifoService) txOrDB(tx, db *gorm.DB) *gorm.DB {
	if tx != nil {
		return tx
	}
	return db
}

func (s *fifoService) RegisterStockable(cfg fifo.StockableConfig) error {
	return fifo.RegisterStockable(cfg)
}

func (s *fifoService) RegisterUsable(cfg fifo.UsableConfig) error {
	return fifo.RegisterUsable(cfg)
}

type StockReplenishRequest struct {
	StockableKey       fifo.StockableKey
	StockableID        uint
	ProductWarehouseID uint
	Quantity           float64
	Note               *string
	Tx                 *gorm.DB
}

type PendingResolution struct {
	UsableKey fifo.UsableKey
	UsableID  uint
	Quantity  float64
}

type StockReplenishResult struct {
	AddedQuantity    float64
	PendingResolved  []PendingResolution
	RemainingPending float64
}

type StockConsumeRequest struct {
	UsableKey          fifo.UsableKey
	UsableID           uint
	ProductWarehouseID uint
	Quantity           float64
	AllowPending       bool
	Note               *string
	Tx                 *gorm.DB
}

type AllocationDetail struct {
	StockableKey fifo.StockableKey
	StockableID  uint
	Quantity     float64
}

type StockConsumeResult struct {
	RequestedQuantity float64
	UsageQuantity     float64
	PendingQuantity   float64
	AddedAllocations  []AllocationDetail
	ReleasedQuantity  float64
}

type StockReleaseRequest struct {
	UsableKey fifo.UsableKey
	UsableID  uint
	Reason    *string
	Tx        *gorm.DB
}

func (s *fifoService) Replenish(ctx context.Context, req StockReplenishRequest) (*StockReplenishResult, error) {
	if req.StockableID == 0 || strings.TrimSpace(req.StockableKey.String()) == "" {
		return nil, errors.New("stockable key and id are required")
	}
	if req.ProductWarehouseID == 0 {
		return nil, errors.New("product warehouse id is required")
	}
	if req.Quantity <= 0 {
		return nil, errors.New("quantity must be greater than zero")
	}

	cfg, ok := fifo.Stockable(req.StockableKey)
	if !ok {
		return nil, fmt.Errorf("stockable %q is not registered", req.StockableKey)
	}

	result := &StockReplenishResult{
		AddedQuantity: req.Quantity,
	}

	err := s.withTransaction(ctx, req.Tx, func(tx *gorm.DB) error {
		if err := s.incrementStockableQty(ctx, tx, cfg, req.StockableID, req.Quantity); err != nil {
			return err
		}

		if err := s.productWarehouseRepo.AdjustQuantities(ctx, map[uint]float64{
			req.ProductWarehouseID: req.Quantity,
		}, func(db *gorm.DB) *gorm.DB {
			return s.txOrDB(tx, db)
		}); err != nil {
			return err
		}

		resolved, err := s.resolvePendingForWarehouse(ctx, tx, req.ProductWarehouseID)
		if err != nil {
			return err
		}
		result.PendingResolved = resolved
		return nil
	})
	if err != nil {
		return nil, err
	}

	return result, nil
}

func (s *fifoService) Consume(ctx context.Context, req StockConsumeRequest) (*StockConsumeResult, error) {
	if req.UsableID == 0 || strings.TrimSpace(req.UsableKey.String()) == "" {
		return nil, errors.New("usable key and id are required")
	}
	if req.Quantity < 0 {
		return nil, errors.New("quantity must be zero or greater")
	}
	cfg, ok := fifo.Usable(req.UsableKey)
	if !ok {
		return nil, fmt.Errorf("usable %q is not registered", req.UsableKey)
	}

	result := &StockConsumeResult{
		RequestedQuantity: req.Quantity,
	}

	err := s.withTransaction(ctx, req.Tx, func(tx *gorm.DB) error {
		ctxRow, err := s.loadUsableContext(ctx, tx, cfg, req.UsableID)
		if err != nil {
			return err
		}

		productWarehouseID := ctxRow.ProductWarehouseID
		if productWarehouseID == 0 {
			return fmt.Errorf("usable %q (id: %d) has no product warehouse reference", req.UsableKey, req.UsableID)
		}
		if req.ProductWarehouseID != 0 && req.ProductWarehouseID != productWarehouseID {
			return fmt.Errorf("usable %q (id: %d) references product warehouse %d but %d was provided", req.UsableKey, req.UsableID, productWarehouseID, req.ProductWarehouseID)
		}

		currentUsage := ctxRow.UsageQty
		currentPending := ctxRow.PendingQty
		currentTotal := currentUsage + currentPending
		delta := req.Quantity - currentTotal
		var (
			usageDelta     float64
			pendingDelta   float64
			addedAlloc     []AllocationDetail
			releasedAmount float64
		)

		switch {
		case delta > 0:

			var excludedStockables []fifo.StockableKey
			if cfg.ExcludedStockables != nil {
				excludedStockables = cfg.ExcludedStockables
			}

			allocationRes, err := s.allocateFromStock(ctx, tx, productWarehouseID, req.UsableKey, req.UsableID, delta, excludedStockables)
			if err != nil {
				return err
			}
			if allocationRes.pending > 0 && !req.AllowPending {
				return fmt.Errorf("insufficient stock: requested %.3f, allocated %.3f", req.Quantity, currentUsage+allocationRes.allocated)
			}

			usageDelta += allocationRes.allocated
			pendingDelta += allocationRes.pending
			addedAlloc = allocationRes.allocations

			if allocationRes.allocated > 0 {
				if err := s.productWarehouseRepo.AdjustQuantities(ctx, map[uint]float64{
					productWarehouseID: -allocationRes.allocated,
				}, func(db *gorm.DB) *gorm.DB {
					return s.txOrDB(tx, db)
				}); err != nil {
					return err
				}
			}
		case delta < 0:
			reductionTarget := -delta

			if currentPending > 0 {
				pendingReduction := math.Min(currentPending, reductionTarget)
				if pendingReduction > 0 {
					pendingDelta -= pendingReduction
					reductionTarget -= pendingReduction
				}
			}

			if reductionTarget > 0 {
				released, err := s.releaseUsagePortion(ctx, tx, req.UsableKey, req.UsableID, reductionTarget)
				if err != nil {
					return err
				}
				if released+1e-6 < reductionTarget {
					return fmt.Errorf("unable to release %.3f from usable %d, only %.3f available", reductionTarget, req.UsableID, released)
				}
				usageDelta -= released
				releasedAmount = released
			}
		default:
			// no change
		}

		if err := s.applyUsableDeltas(ctx, tx, cfg, req.UsableID, usageDelta, pendingDelta); err != nil {
			return err
		}

		result.AddedAllocations = addedAlloc
		result.ReleasedQuantity = releasedAmount
		result.UsageQuantity = currentUsage + usageDelta
		result.PendingQuantity = currentPending + pendingDelta
		return nil
	})
	if err != nil {
		return nil, err
	}

	return result, nil
}

func (s *fifoService) ReleaseUsage(ctx context.Context, req StockReleaseRequest) error {
	if req.UsableID == 0 || strings.TrimSpace(req.UsableKey.String()) == "" {
		return errors.New("usable key and id are required")
	}
	return s.withTransaction(ctx, req.Tx, func(tx *gorm.DB) error {
		cfg, ok := fifo.Usable(req.UsableKey)
		if !ok {
			return fmt.Errorf("usable %q is not registered", req.UsableKey)
		}

		ctxRow, err := s.loadUsableContext(ctx, tx, cfg, req.UsableID)
		if err != nil {
			return err
		}
		var usageDelta, pendingDelta float64
		if ctxRow.UsageQty > 0 {
			if _, err := s.releaseUsagePortion(ctx, tx, req.UsableKey, req.UsableID, ctxRow.UsageQty); err != nil {
				return err
			}
			usageDelta -= ctxRow.UsageQty
		}
		if ctxRow.PendingQty > 0 {
			pendingDelta -= ctxRow.PendingQty
		}

		if err := s.applyUsableDeltas(ctx, tx, cfg, req.UsableID, usageDelta, pendingDelta); err != nil {
			return err
		}

		return s.allocations.ReleaseByUsable(ctx, req.UsableKey.String(), req.UsableID, req.Reason, func(db *gorm.DB) *gorm.DB {
			return s.txOrDB(tx, db)
		})
	})
}

// --- helpers ---

type usableContextRow struct {
	ProductWarehouseID uint
	UsageQty           float64
	PendingQty         float64
}

func (s *fifoService) loadUsableContext(ctx context.Context, tx *gorm.DB, cfg fifo.UsableConfig, id uint) (*usableContextRow, error) {
	var row usableContextRow

	query := tx.Table(cfg.Table).
		Select(fmt.Sprintf("%s AS product_warehouse_id, COALESCE(%s,0) AS usage_qty, COALESCE(%s,0) AS pending_qty", cfg.Columns.ProductWarehouseID, cfg.Columns.UsageQuantity, cfg.Columns.PendingQuantity)).
		Where(fmt.Sprintf("%s = ?", cfg.Columns.ID), id).
		Clauses(clause.Locking{Strength: "UPDATE"})

	if cfg.Scope != nil {
		query = cfg.Scope(query)
	}

	if err := query.Take(&row).Error; err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, fmt.Errorf("usable record %d not found", id)
		}
		return nil, err
	}

	return &row, nil
}

func (s *fifoService) incrementStockableQty(ctx context.Context, tx *gorm.DB, cfg fifo.StockableConfig, id uint, qty float64) error {
	column := cfg.Columns.TotalQuantity

	query := tx.Table(cfg.Table).
		Where(fmt.Sprintf("%s = ?", cfg.Columns.ID), id)
	if cfg.Scope != nil {
		query = cfg.Scope(query)
	}

	updates := map[string]any{
		column: gorm.Expr(fmt.Sprintf("COALESCE(%s,0) + ?", column), qty),
	}
	if cfg.Columns.TotalUsedQuantity != "" {
		updates[cfg.Columns.TotalUsedQuantity] = gorm.Expr(fmt.Sprintf("COALESCE(%s,0)", cfg.Columns.TotalUsedQuantity))
	}

	return query.Updates(updates).Error
}

func (s *fifoService) incrementStockableUsage(ctx context.Context, tx *gorm.DB, cfg fifo.StockableConfig, id uint, qty float64) error {
	if qty == 0 {
		return nil
	}
	column := cfg.Columns.TotalUsedQuantity
	query := tx.Table(cfg.Table).
		Where(fmt.Sprintf("%s = ?", cfg.Columns.ID), id)
	if cfg.Scope != nil {
		query = cfg.Scope(query)
	}

	return query.Update(column, gorm.Expr(fmt.Sprintf("COALESCE(%s,0) + ?", column), qty)).Error
}

type allocationOutcome struct {
	allocated   float64
	pending     float64
	allocations []AllocationDetail
}

type stockLot struct {
	StockableKey fifo.StockableKey
	RecordID     uint
	AvailableQty float64
	CreatedAt    time.Time
}

func (s *fifoService) allocateFromStock(
	ctx context.Context,
	tx *gorm.DB,
	productWarehouseID uint,
	usableKey fifo.UsableKey,
	usableID uint,
	requestQty float64,
	excludedStockables []fifo.StockableKey,
) (*allocationOutcome, error) {
	lots, err := s.fetchStockLots(ctx, tx, productWarehouseID, excludedStockables)
	if err != nil {
		return nil, err
	}
	if len(lots) == 0 {
		return &allocationOutcome{pending: requestQty}, nil
	}

	var (
		remaining           = requestQty
		applied             float64
		allocations         []*entities.StockAllocation
		allocationSummaries []AllocationDetail
		usageAdjustments    = make(map[fifo.StockableKey]map[uint]float64)
	)

	for _, lot := range lots {
		if remaining <= 0 {
			break
		}
		if lot.AvailableQty <= 0 {
			continue
		}

		portion := lot.AvailableQty
		if portion > remaining {
			portion = remaining
		}

		applied += portion
		remaining -= portion

		allocationSummaries = append(allocationSummaries, AllocationDetail{
			StockableKey: lot.StockableKey,
			StockableID:  lot.RecordID,
			Quantity:     portion,
		})

		allocations = append(allocations, &entities.StockAllocation{
			ProductWarehouseId: productWarehouseID,
			StockableType:      lot.StockableKey.String(),
			StockableId:        lot.RecordID,
			UsableType:         usableKey.String(),
			UsableId:           usableID,
			Qty:                portion,
			Status:             entities.StockAllocationStatusActive,
		})

		if _, ok := usageAdjustments[lot.StockableKey]; !ok {
			usageAdjustments[lot.StockableKey] = make(map[uint]float64)
		}
		usageAdjustments[lot.StockableKey][lot.RecordID] += portion
	}

	if len(allocations) > 0 {
		if err := s.allocations.CreateMany(ctx, allocations, func(db *gorm.DB) *gorm.DB {
			return s.txOrDB(tx, db)
		}); err != nil {
			return nil, err
		}

		for key, deltas := range usageAdjustments {
			cfg, ok := fifo.Stockable(key)
			if !ok {
				continue
			}
			for id, qty := range deltas {
				if err := s.incrementStockableUsage(ctx, tx, cfg, id, qty); err != nil {
					return nil, err
				}
			}
		}
	}

	return &allocationOutcome{
		allocated:   applied,
		pending:     remaining,
		allocations: allocationSummaries,
	}, nil
}

func (s *fifoService) fetchStockLots(ctx context.Context, tx *gorm.DB, productWarehouseID uint, excludedStockables []fifo.StockableKey) ([]stockLot, error) {
	configs := fifo.Stockables()
	if len(configs) == 0 {
		return nil, nil
	}

	// Create exclusion set for faster lookup
	excludedSet := make(map[fifo.StockableKey]bool)
	for _, key := range excludedStockables {
		excludedSet[key] = true
	}

	var lots []stockLot
	for key, cfg := range configs {
		// Skip excluded stockables
		if excludedSet[key] {
			continue
		}

		usesNumericTime := cfg.Columns.CreatedAt == cfg.Columns.ID

		var selectStmt string
		if usesNumericTime {

			selectStmt = fmt.Sprintf(
				"%s AS id, %s AS available_qty, '1970-01-01 00:00:00 UTC'::timestamp AS created_at",
				cfg.Columns.ID,
				fmt.Sprintf("%s - COALESCE(%s,0)", cfg.Columns.TotalQuantity, cfg.Columns.TotalUsedQuantity),
			)
		} else {
			selectStmt = fmt.Sprintf(
				"%s AS id, %s AS available_qty, %s AS created_at",
				cfg.Columns.ID,
				fmt.Sprintf("%s - COALESCE(%s,0)", cfg.Columns.TotalQuantity, cfg.Columns.TotalUsedQuantity),
				cfg.Columns.CreatedAt,
			)
		}

		var rows []struct {
			ID           uint
			AvailableQty float64
			CreatedAt    time.Time
		}

		query := tx.Table(cfg.Table).
			Select(selectStmt).
			Where(fmt.Sprintf("%s = ?", cfg.Columns.ProductWarehouseID), productWarehouseID).
			Where(fmt.Sprintf("%s > %s", cfg.Columns.TotalQuantity, cfg.Columns.TotalUsedQuantity))

		if cfg.Scope != nil {
			query = cfg.Scope(query)
		}

		for _, order := range s.orderClauses(cfg.OrderBy) {
			query = query.Order(order)
		}
		query = query.Limit(s.maxLotsPerStockable)

		if err := query.Find(&rows).Error; err != nil {
			return nil, err
		}

		for _, row := range rows {
			if row.AvailableQty <= 0 {
				continue
			}
			lots = append(lots, stockLot{
				StockableKey: key,
				RecordID:     row.ID,
				AvailableQty: row.AvailableQty,
				CreatedAt:    row.CreatedAt,
			})
		}
	}

	if len(lots) == 0 {
		return nil, nil
	}

	sort.SliceStable(lots, func(i, j int) bool {
		if lots[i].CreatedAt.Equal(lots[j].CreatedAt) {
			return lots[i].RecordID < lots[j].RecordID
		}
		return lots[i].CreatedAt.Before(lots[j].CreatedAt)
	})

	return lots, nil
}

func (s *fifoService) applyUsableDeltas(ctx context.Context, tx *gorm.DB, cfg fifo.UsableConfig, id uint, usageDelta, pendingDelta float64) error {
	if usageDelta == 0 && pendingDelta == 0 {
		return nil
	}

	updates := map[string]any{}
	if usageDelta != 0 {
		updates[cfg.Columns.UsageQuantity] = gorm.Expr(fmt.Sprintf("COALESCE(%s,0) + ?", cfg.Columns.UsageQuantity), usageDelta)
	}
	if pendingDelta != 0 {
		updates[cfg.Columns.PendingQuantity] = gorm.Expr(fmt.Sprintf("COALESCE(%s,0) + ?", cfg.Columns.PendingQuantity), pendingDelta)
	}

	query := tx.Table(cfg.Table).Where(fmt.Sprintf("%s = ?", cfg.Columns.ID), id)
	if cfg.Scope != nil {
		query = cfg.Scope(query)
	}

	return query.Updates(updates).Error
}

type pendingCandidate struct {
	UsableKey fifo.UsableKey
	Config    fifo.UsableConfig
	UsableID  uint
	Pending   float64
	CreatedAt time.Time
}

func (s *fifoService) resolvePendingForWarehouse(ctx context.Context, tx *gorm.DB, productWarehouseID uint) ([]PendingResolution, error) {
	candidates, err := s.fetchPendingCandidates(ctx, tx, productWarehouseID)
	if err != nil {
		return nil, err
	}
	if len(candidates) == 0 {
		return nil, nil
	}

	var resolutions []PendingResolution

	for _, candidate := range candidates {
		if candidate.Pending <= 0 {
			continue
		}

		// Get excluded stockables from candidate usable config
		var excludedStockables []fifo.StockableKey
		if candidate.Config.ExcludedStockables != nil {
			excludedStockables = candidate.Config.ExcludedStockables
		}

		outcome, err := s.allocateFromStock(ctx, tx, productWarehouseID, candidate.UsableKey, candidate.UsableID, candidate.Pending, excludedStockables)
		if err != nil {
			return nil, err
		}
		if outcome.allocated <= 0 {
			break
		}

		if err := s.applyUsableDeltas(ctx, tx, candidate.Config, candidate.UsableID, outcome.allocated, -outcome.allocated); err != nil {
			return nil, err
		}

		if err := s.productWarehouseRepo.AdjustQuantities(ctx, map[uint]float64{
			productWarehouseID: -outcome.allocated,
		}, func(db *gorm.DB) *gorm.DB {
			return s.txOrDB(tx, db)
		}); err != nil {
			return nil, err
		}

		resolutions = append(resolutions, PendingResolution{
			UsableKey: candidate.UsableKey,
			UsableID:  candidate.UsableID,
			Quantity:  outcome.allocated,
		})

		if outcome.pending > 0 {
			// No more stock available for this warehouse at the moment.
			break
		}
	}

	return resolutions, nil
}

func (s *fifoService) releaseUsagePortion(
	ctx context.Context,
	tx *gorm.DB,
	usableKey fifo.UsableKey,
	usableID uint,
	target float64,
) (float64, error) {
	if target <= 0 {
		return 0, nil
	}

	allocations, err := s.allocations.FindActiveByUsable(ctx, usableKey.String(), usableID, func(db *gorm.DB) *gorm.DB {
		target := s.txOrDB(tx, db)
		return target.Clauses(clause.Locking{Strength: "UPDATE"})
	})
	if err != nil {
		return 0, err
	}
	if len(allocations) == 0 {
		return 0, nil
	}

	var (
		remaining            = target
		totalReleased        float64
		warehouseAdjustments = make(map[uint]float64)
		stockableAdjustments = make(map[fifo.StockableKey]map[uint]float64)
	)

	now := time.Now()

	for i := len(allocations) - 1; i >= 0 && remaining > 0; i-- {
		allocation := allocations[i]
		releaseAmt := allocation.Qty
		if releaseAmt > remaining {
			releaseAmt = remaining
		}

		remaining -= releaseAmt
		totalReleased += releaseAmt
		warehouseAdjustments[allocation.ProductWarehouseId] += releaseAmt

		key := fifo.StockableKey(allocation.StockableType)
		if _, ok := stockableAdjustments[key]; !ok {
			stockableAdjustments[key] = make(map[uint]float64)
		}
		stockableAdjustments[key][allocation.StockableId] += releaseAmt

		if releaseAmt == allocation.Qty {
			if err := s.allocations.PatchOne(ctx, allocation.Id, map[string]any{
				"status":      entities.StockAllocationStatusReleased,
				"released_at": now,
			}, func(db *gorm.DB) *gorm.DB {
				return s.txOrDB(tx, db)
			}); err != nil {
				return 0, err
			}
		} else {
			if err := s.allocations.PatchOne(ctx, allocation.Id, map[string]any{
				"qty": allocation.Qty - releaseAmt,
			}, func(db *gorm.DB) *gorm.DB {
				return s.txOrDB(tx, db)
			}); err != nil {
				return 0, err
			}
		}
	}

	if totalReleased == 0 {
		return 0, nil
	}

	for key, deltas := range stockableAdjustments {
		cfg, ok := fifo.Stockable(key)
		if !ok {
			continue
		}
		for id, qty := range deltas {
			if err := s.incrementStockableUsage(ctx, tx, cfg, id, -qty); err != nil {
				return 0, err
			}
		}
	}

	if len(warehouseAdjustments) > 0 {
		if err := s.productWarehouseRepo.AdjustQuantities(ctx, warehouseAdjustments, func(db *gorm.DB) *gorm.DB {
			return s.txOrDB(tx, db)
		}); err != nil {
			return 0, err
		}

		for warehouseID := range warehouseAdjustments {
			if _, err := s.resolvePendingForWarehouse(ctx, tx, warehouseID); err != nil {
				return 0, err
			}
		}
	}

	return totalReleased, nil
}

func (s *fifoService) fetchPendingCandidates(ctx context.Context, tx *gorm.DB, productWarehouseID uint) ([]pendingCandidate, error) {
	configs := fifo.Usables()
	if len(configs) == 0 {
		return nil, nil
	}

	var candidates []pendingCandidate

	for key, cfg := range configs {
		selectStmt := fmt.Sprintf(
			"%s AS id, %s AS pending_qty, %s AS created_at",
			cfg.Columns.ID,
			cfg.Columns.PendingQuantity,
			cfg.Columns.CreatedAt,
		)

		var rows []struct {
			ID        uint
			Pending   float64
			CreatedAt time.Time
		}

		query := tx.Table(cfg.Table).
			Select(selectStmt).
			Where(fmt.Sprintf("%s = ?", cfg.Columns.ProductWarehouseID), productWarehouseID).
			Where(fmt.Sprintf("%s > 0", cfg.Columns.PendingQuantity)).
			Limit(s.pendingBatchPerUsable)

		if cfg.Scope != nil {
			query = cfg.Scope(query)
		}

		for _, order := range s.orderClauses(cfg.OrderBy) {
			query = query.Order(order)
		}

		if err := query.Find(&rows).Error; err != nil {
			return nil, err
		}

		for _, row := range rows {
			if row.Pending <= 0 {
				continue
			}
			candidates = append(candidates, pendingCandidate{
				UsableKey: key,
				Config:    cfg,
				UsableID:  row.ID,
				Pending:   row.Pending,
				CreatedAt: row.CreatedAt,
			})
		}
	}

	if len(candidates) == 0 {
		return nil, nil
	}

	sort.SliceStable(candidates, func(i, j int) bool {
		if candidates[i].CreatedAt.Equal(candidates[j].CreatedAt) {
			return candidates[i].UsableID < candidates[j].UsableID
		}
		return candidates[i].CreatedAt.Before(candidates[j].CreatedAt)
	})

	return candidates, nil
}

func (s *fifoService) orderClauses(custom []string) []string {
	if len(custom) > 0 {
		return custom
	}
	return s.defaultOrderBy
}