MoneyMap/MoneyMap.Core/Services/TransactionImporter.cs

using CsvHelper;
using CsvHelper.Configuration;
using Microsoft.EntityFrameworkCore;
using MoneyMap.Data;
using MoneyMap.Models;
using MoneyMap.Models.Import;
using System.Globalization;

namespace MoneyMap.Services
{
    /// <summary>
    /// Service for importing transactions from CSV files.
    /// </summary>
    public interface ITransactionImporter
    {
        Task<PreviewOperationResult> PreviewAsync(Stream csvStream, ImportContext context);
        Task<ImportOperationResult> ImportAsync(List<Transaction> transactions);
    }

    public class TransactionImporter : ITransactionImporter
    {
        private readonly MoneyMapContext _db;
        private readonly ICardResolver _cardResolver;

        public TransactionImporter(MoneyMapContext db, ICardResolver cardResolver)
        {
            _db = db;
            _cardResolver = cardResolver;
        }

        public async Task<PreviewOperationResult> PreviewAsync(Stream csvStream, ImportContext context)
        {
            var previewItems = new List<TransactionPreview>();
            var addedInThisBatch = new HashSet<TransactionKey>();

            // First pass: read CSV to get date range and all transactions
            var csvTransactions = new List<(TransactionCsvRow Row, Transaction Transaction, TransactionKey Key)>();
            DateTime? minDate = null;
            DateTime? maxDate = null;

            using (var reader = new StreamReader(csvStream))
            using (var csv = new CsvReader(reader, new CsvConfiguration(CultureInfo.InvariantCulture)
            {
                HasHeaderRecord = true,
                HeaderValidated = null,
                MissingFieldFound = null
            }))
            {
                csv.Read();
                csv.ReadHeader();
                var hasCategory = csv.HeaderRecord?.Any(h => h.Equals("Category", StringComparison.OrdinalIgnoreCase)) ?? false;
                csv.Context.RegisterClassMap(new TransactionCsvRowMap(hasCategory));

                while (csv.Read())
                {
                    var row = csv.GetRecord<TransactionCsvRow>();

                    var paymentResolution = await _cardResolver.ResolvePaymentAsync(row.Memo, context);
                    if (!paymentResolution.IsSuccess)
                        return PreviewOperationResult.Failure(paymentResolution.ErrorMessage!);

                    var transaction = MapToTransaction(row, paymentResolution);
                    var key = new TransactionKey(transaction);

                    csvTransactions.Add((row, transaction, key));

                    // Track date range
                    if (minDate == null || transaction.Date < minDate) minDate = transaction.Date;
                    if (maxDate == null || transaction.Date > maxDate) maxDate = transaction.Date;
                }
            }

            // Load existing transactions within the date range for fast duplicate checking
            HashSet<TransactionKey> existingTransactions;
            if (minDate.HasValue && maxDate.HasValue)
            {
                // Add a buffer of 1 day on each side to catch any edge cases
                var startDate = minDate.Value.AddDays(-1);
                var endDate = maxDate.Value.AddDays(1);

                existingTransactions = await _db.Transactions
                    .Where(t => t.Date >= startDate && t.Date <= endDate)
                    .Select(t => new TransactionKey(t.Date, t.Amount, t.Name, t.Memo, t.AccountId, t.CardId))
                    .ToHashSetAsync();
            }
            else
            {
                existingTransactions = new HashSet<TransactionKey>();
            }

            // Second pass: check for duplicates and build preview
            foreach (var (row, transaction, key) in csvTransactions)
            {
                // Fast in-memory duplicate checking
                bool isDuplicate = addedInThisBatch.Contains(key) || existingTransactions.Contains(key);

                previewItems.Add(new TransactionPreview
                {
                    Transaction = transaction,
                    IsDuplicate = isDuplicate,
                    PaymentMethodLabel = GetPaymentLabel(transaction, context)
                });

                addedInThisBatch.Add(key);
            }

            // Order by date descending (newest first)
            var orderedPreview = previewItems.OrderByDescending(p => p.Transaction.Date).ToList();

            return PreviewOperationResult.Success(orderedPreview);
        }

        public async Task<ImportOperationResult> ImportAsync(List<Transaction> transactions)
        {
            int inserted = 0;
            int skipped = 0;

            foreach (var transaction in transactions)
            {
                _db.Transactions.Add(transaction);
                inserted++;
            }

            await _db.SaveChangesAsync();

            var result = new ImportResult(
                transactions.Count,
                inserted,
                skipped,
                null
            );

            return ImportOperationResult.Success(result);
        }

        private static Transaction MapToTransaction(TransactionCsvRow row, PaymentResolutionResult paymentResolution)
        {
            return new Transaction
            {
                Date = row.Date,
                TransactionType = row.Transaction?.Trim() ?? "",
                Name = row.Name?.Trim() ?? "",
                Memo = row.Memo?.Trim() ?? "",
                Amount = row.Amount,
                Category = (row.Category ?? "").Trim(),
                Last4 = paymentResolution.Last4,
                CardId = paymentResolution.CardId,
                AccountId = paymentResolution.AccountId!.Value
            };
        }

        private string GetPaymentLabel(Transaction transaction, ImportContext context)
        {
            var account = context.AvailableAccounts.FirstOrDefault(a => a.Id == transaction.AccountId);
            var accountLabel = account?.DisplayLabel ?? $"Account ···· {transaction.Last4}";

            if (transaction.CardId.HasValue)
            {
                var card = context.AvailableCards.FirstOrDefault(c => c.Id == transaction.CardId);
                var cardLabel = card?.DisplayLabel ?? $"Card ···· {transaction.Last4}";
                return $"{cardLabel} → {accountLabel}";
            }

            return accountLabel;
        }
    }
}