PR F: precompute dictionaries in ViewService

Eliminate O(N*M) linear scans in view assembly.
GetTransactionViewList / GetBookingRuleViewList: precompute
accountsById and use dictionary lookups instead of accountList.Single
per row. GetAccountViewList: group transactions by
TargetAccountId/OriginAccountId once; per-account balance is now O(k)
instead of a full scan. Fold GetBalanceInAccountCurrency and
GetCurrentBalanceInCHF into a single ComputeBalance helper so the
account-currency balance is computed once per account instead of
twice.

Defer item 19 (async I/O): after PR D each XML file is loaded at most
once per circuit, and this app is single-user local Blazor Server, so
async would be ~40 files of churn for negligible benefit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

IMPROVEMENT_PLAN.md

@@ -100,5 +100,6 @@ Analysis and phased plan produced 2026-07-02. See conversation history for full
 - **PR C** — Phase 2 mechanical cleanup: items 11, 13, 15, 16, 17.
 - **PR D** — Phase 2 item 9: per-request unit of work / cache. Split off from PR C because it is invasive enough to warrant its own review.
 - **PR E** — Phase 3 item 22: ChartService cleanup. Promoted ahead of async because it fixes an active correctness bug — `GetAccountExpensesPerMonth(accountId, year)` was mutating `ISettingsService` on the singleton to hop years, with the year parameter always hardcoded to `DateTime.Now.Year`, so the Spendings-over-time chart discarded the user's header year selection.
-- **PR F** — Phase 3 items 19 + 21: async I/O across the repository stack, and precomputed dictionaries for view assembly.
+- **PR F** — Phase 3 item 21: precomputed dictionaries in `ViewService` to eliminate O(N·M) `Single(...)` scans and per-account transaction filtering.
-- **PR G** — Phase 4 (tests) — locks in behavior for PR F.
+- **Item 19 (async I/O)** — deferred. After PR D, each XML file is loaded at most once per SignalR circuit, and this app is single-user local Blazor Server. Converting every repository/service method to async would touch ~40 files for negligible user-visible benefit and real regression risk. Revisit if the app is ever hosted for multiple concurrent users.
+- **PR G** — Phase 4 (tests) — locks in behavior before further invasive changes.

Schaad.Accounting.Services/ViewService.cs

@@ -57,22 +57,43 @@ namespace Schaad.Accounting.Services
             var subClassNameByNumber = subclassRepository.GetSubClassList().ToDictionary(s => s.Number, s => s.Name);
             var settings = settingsService.GetSettings();
 
-            var accountList = accounts.Select(
+            // Group transactions by account so per-account balance is O(k) instead of O(M).
-                    a =>
+            var creditsByAccount = transactionList.GroupBy(t => t.TargetAccountId).ToDictionary(g => g.Key, g => g.ToList());
-                        new AccountDataset(
+            var debitsByAccount = transactionList.GroupBy(t => t.OriginAccountId).ToDictionary(g => g.Key, g => g.ToList());
+
+            var accountList = accounts.Select(a =>
+                {
+                    var balance = ComputeBalance(a, creditsByAccount, debitsByAccount);
+                    return new AccountDataset(
                         account: a,
-                            balance: GetBalanceInAccountCurrency(a, transactionList),
+                        balance: balance,
-                            balanceCHF: GetCurrentBalanceInCHF(a, transactionList),
+                        balanceCHF: fxService.ConvertCurrency(balance, a.Currency, "CHF", settings.FixerIoApiKey),
                         startBalanceCHF: fxService.ConvertCurrency(a.StartBalance, a.Currency, "CHF", settings.FixerIoApiKey),
                         className: subclassRepository.GetClass(a.Class),
-                            subClassName: subClassNameByNumber[a.SubClass]
+                        subClassName: subClassNameByNumber[a.SubClass]);
-                        )
+                })
-                )
                 .ToList();
 
             return accountList.OrderBy(a => a.Number).ToList();
         }
 
+        private static decimal ComputeBalance(
+            Account account,
+            Dictionary<string, List<TransactionDataset>> creditsByAccount,
+            Dictionary<string, List<TransactionDataset>> debitsByAccount)
+        {
+            var balance = account.StartBalance;
+            if (creditsByAccount.TryGetValue(account.Id, out var credits))
+            {
+                balance += credits.Sum(t => t.GetValue(account.IsFxAccount));
+            }
+            if (debitsByAccount.TryGetValue(account.Id, out var debits))
+            {
+                balance -= debits.Sum(t => t.GetValue(account.IsFxAccount));
+            }
+            return balance;
+        }
+
         public BalanceDataset GetBalanceView()
         {
             var accountList = GetAccountViewList();
@@ -107,40 +128,20 @@ namespace Schaad.Accounting.Services
             return balanceView;
         }
 
-        private decimal GetBalanceInAccountCurrency(Account a, List<TransactionDataset> transactionList)
-        {
-            var balance = a.StartBalance
-                            + transactionList.Where(t => t.TargetAccountId == a.Id).Sum(t => t.GetValue(a.IsFxAccount))
-                            - transactionList.Where(t => t.OriginAccountId == a.Id).Sum(t => t.GetValue(a.IsFxAccount));
-            return balance;
-        }
-
-        private decimal GetCurrentBalanceInCHF(Account a, List<TransactionDataset> transactionList)
-        {
-            var settings = settingsService.GetSettings();
-            var balanceInAccountCurrency = GetBalanceInAccountCurrency(a, transactionList);
-            var balanceInChf = fxService.ConvertCurrency(balanceInAccountCurrency, a.Currency, "CHF", settings.FixerIoApiKey);
-            return balanceInChf;
-        }
-
         /// <summary>
         /// Get transaction list with the origin and target account for each transaction
         /// </summary>
         public List<TransactionDataset> GetTransactionViewList()
         {
-            var accountList = accountRepository.GetAccountList();
+            var accountsById = accountRepository.GetAccountList().ToDictionary(a => a.Id);
             var transactionList = transactionRepository.GetTransactionList();
 
-            var transactionViewList = transactionList.Select(
+            return transactionList.Select(t =>
-                    t =>
                     new TransactionDataset(
                         t,
-                            accountList.Single(a => a.Id == t.OriginAccountId),
+                        accountsById[t.OriginAccountId],
-                            accountList.Single(a => a.Id == t.TargetAccountId))
+                        accountsById[t.TargetAccountId]))
-                )
                 .ToList();
-
-            return transactionViewList;
         }
 
         /// <summary>
@@ -148,53 +149,41 @@ namespace Schaad.Accounting.Services
         /// </summary>
         public List<TransactionDataset> GetTransactionViewList(string accountId)
         {
-            var accountList = accountRepository.GetAccountList();
+            var accountsById = accountRepository.GetAccountList().ToDictionary(a => a.Id);
-            var transactionList = transactionRepository.GetTransactionList().Where(t => t.OriginAccountId == accountId || t.TargetAccountId == accountId);
+            var transactionList = transactionRepository.GetTransactionList()
+                .Where(t => t.OriginAccountId == accountId || t.TargetAccountId == accountId);
 
-            var transactionViewList = transactionList.Select(
+            return transactionList.Select(t =>
-                    t =>
                     new TransactionDataset(
                         WithDisplaySign(t),
-                            accountList.Single(a => a.Id == t.OriginAccountId),
+                        accountsById[t.OriginAccountId],
-                            accountList.Single(a => a.Id == t.TargetAccountId)
+                        accountsById[t.TargetAccountId]))
-                        )
-                )
                 .ToList();
 
-            return transactionViewList;
-
             Transaction WithDisplaySign(Transaction t)
             {
-                var account = accountList.Single(a => a.Id == t.OriginAccountId);
+                var originAccount = accountsById[t.OriginAccountId];
-
+                if (originAccount.Class == ClassIds.Activa && accountId == t.OriginAccountId)
-                if (account.Class == ClassIds.Activa && accountId == t.OriginAccountId)
                 {
                     var copy = new Transaction();
                     t.Copy(copy);
                     copy.Value *= -1;
                     return copy;
                 }
-
                 return t;
             }
         }
 
-
         /// <summary>
         /// Get booking rules with their account
         /// </summary>
         public List<BookingRuleDataset> GetBookingRuleViewList()
         {
-            var accountList = accountRepository.GetAccountList();
+            var accountsById = accountRepository.GetAccountList().ToDictionary(a => a.Id);
-            var bookinRuleList = bookingRuleRepository.GetBookingRuleList();
+            var bookingRules = bookingRuleRepository.GetBookingRuleList();
 
-            return bookinRuleList.Select(
+            return bookingRules.Select(t =>
-                    t =>
+                    new BookingRuleDataset(t, accountsById[t.AccountId].Name))
-                        new BookingRuleDataset(
-                            t,
-                            accountList.Single(a => a.Id == t.AccountId).Name
-                        )
-                )
                 .ToList();
         }