package processor

import (
	"context"
	"time"

	"github.com/google/uuid"

	"apskel-pos-be/internal/entities"
	"apskel-pos-be/internal/models"
	"apskel-pos-be/internal/repository"
)

// walletTraceMaxDepth bounds how far back a chain is followed. Every step is a
// transfer, exchange or refund of the step before, so real chains are short; the
// bound only guards against a loop in bad data.
const walletTraceMaxDepth = 100

// WalletTraceProcessor traces a ledger row lot by lot for the dashboard
// (docs/prd-point-coin.md F7, §8.1). It only reads.
type WalletTraceProcessor struct {
	repo repository.WalletTraceRepository
	now  func() time.Time
}

func NewWalletTraceProcessor(repo repository.WalletTraceRepository) *WalletTraceProcessor {
	return &WalletTraceProcessor{repo: repo, now: time.Now}
}

type walletTraceStart struct {
	lotID  uuid.UUID
	amount int64
}

// Trace follows a row of the organization's ledger: a debit through the lots it took
// from, a credit through the lots it created, and each lot back through origin_lot_id
// to the lot that first brought the balance in. A row of another organization is
// repository.ErrWalletTransactionNotFound.
func (p *WalletTraceProcessor) Trace(ctx context.Context, organizationID, transactionID uuid.UUID) (*models.AdminWalletTrace, error) {
	row, err := p.repo.GetTransaction(ctx, organizationID, transactionID)
	if err != nil {
		return nil, err
	}

	var starts []walletTraceStart
	if row.Amount < 0 {
		allocations, err := p.repo.ListAllocations(ctx, row.ID)
		if err != nil {
			return nil, err
		}
		for _, a := range allocations {
			starts = append(starts, walletTraceStart{lotID: a.LotID, amount: a.Amount})
		}
	} else {
		created, err := p.repo.ListLotsBySource(ctx, row.ID)
		if err != nil {
			return nil, err
		}
		for _, lot := range created {
			starts = append(starts, walletTraceStart{lotID: lot.ID, amount: lot.OriginalAmount})
		}
	}

	lots, err := p.loadChains(ctx, starts)
	if err != nil {
		return nil, err
	}

	sourceIDs := make([]uuid.UUID, 0, len(lots))
	customerIDs := []uuid.UUID{row.CustomerID}
	for _, lot := range lots {
		sourceIDs = append(sourceIDs, lot.SourceTransactionID)
		customerIDs = append(customerIDs, lot.CustomerID)
	}
	sourceRows, err := p.repo.GetTransactions(ctx, uniqueIDs(sourceIDs))
	if err != nil {
		return nil, err
	}
	sources := make(map[uuid.UUID]entities.WalletTransaction, len(sourceRows))
	for _, s := range sourceRows {
		sources[s.ID] = s
	}
	names, err := p.repo.CustomerNames(ctx, uniqueIDs(customerIDs))
	if err != nil {
		return nil, err
	}

	now := p.now()
	trace := &models.AdminWalletTrace{
		Transaction: walletTraceRow(*row, names),
		Lots:        make([]models.AdminWalletTraceLot, 0, len(starts)),
	}
	for _, start := range starts {
		item := models.AdminWalletTraceLot{Amount: start.amount, Chain: []models.AdminWalletTraceStep{}}
		visited := map[uuid.UUID]bool{}
		for id := &start.lotID; id != nil && !visited[*id]; {
			lot, ok := lots[*id]
			if !ok {
				break
			}
			visited[*id] = true
			item.Chain = append(item.Chain, models.AdminWalletTraceStep{
				Lot:    adminWalletLot(lot, now),
				Source: walletTraceRow(sources[lot.SourceTransactionID], names),
			})
			id = lot.OriginLotID
		}
		trace.Lots = append(trace.Lots, item)
	}
	return trace, nil
}

// loadChains loads the starting lots and every lot they were carried over from, one
// generation at a time.
func (p *WalletTraceProcessor) loadChains(ctx context.Context, starts []walletTraceStart) (map[uuid.UUID]entities.WalletLot, error) {
	lots := map[uuid.UUID]entities.WalletLot{}
	pending := make([]uuid.UUID, 0, len(starts))
	for _, s := range starts {
		pending = append(pending, s.lotID)
	}
	for depth := 0; len(pending) > 0 && depth < walletTraceMaxDepth; depth++ {
		found, err := p.repo.GetLots(ctx, uniqueIDs(pending))
		if err != nil {
			return nil, err
		}
		pending = pending[:0]
		for _, lot := range found {
			lots[lot.ID] = lot
		}
		for _, lot := range found {
			if lot.OriginLotID != nil {
				if _, seen := lots[*lot.OriginLotID]; !seen {
					pending = append(pending, *lot.OriginLotID)
				}
			}
		}
	}
	return lots, nil
}

func walletTraceRow(row entities.WalletTransaction, names map[uuid.UUID]string) models.AdminWalletTraceRow {
	return models.AdminWalletTraceRow{
		ID:            row.ID,
		Customer:      models.AdminWalletNamedRef{ID: row.CustomerID, Name: names[row.CustomerID]},
		Currency:      row.Currency,
		Type:          row.Type,
		Amount:        row.Amount,
		Description:   row.Description,
		ReferenceType: row.ReferenceType,
		ReferenceID:   row.ReferenceID,
		OutletID:      row.OutletID,
		CreatedAt:     row.CreatedAt,
	}
}

func adminWalletLot(lot entities.WalletLot, now time.Time) models.AdminWalletLot {
	return models.AdminWalletLot{
		ID:                  lot.ID,
		Currency:            lot.Currency,
		OriginalAmount:      lot.OriginalAmount,
		RemainingAmount:     lot.RemainingAmount,
		ExpiresAt:           lot.ExpiresAt,
		Expired:             lot.ExpiresAt != nil && !lot.ExpiresAt.After(now),
		SourceTransactionID: lot.SourceTransactionID,
		OriginLotID:         lot.OriginLotID,
		CreatedAt:           lot.CreatedAt,
	}
}

func uniqueIDs(ids []uuid.UUID) []uuid.UUID {
	seen := make(map[uuid.UUID]bool, len(ids))
	out := make([]uuid.UUID, 0, len(ids))
	for _, id := range ids {
		if !seen[id] {
			seen[id] = true
			out = append(out, id)
		}
	}
	return out
}