Leftover penny issue FIX

src/store/addStore.ts

@@ -103,7 +103,7 @@ export const useAddExpenseStore = create<AddExpenseState>()((set) => ({
             s.participants,
             s.splitType,
             s.splitShares,
-            s.paidBy,
+            s.paidBy
           ),
         };
       }),
@@ -116,7 +116,7 @@ export const useAddExpenseStore = create<AddExpenseState>()((set) => ({
           state.participants,
           splitType,
           state.splitShares,
-          state.paidBy,
+          state.paidBy
         ),
       })),
     setSplitShare: (splitType, userId, share) =>
@@ -161,7 +161,7 @@ export const useAddExpenseStore = create<AddExpenseState>()((set) => ({
             participants,
             state.splitType,
             splitShares,
-            state.paidBy,
+            state.paidBy
           ),
         };
       }),
@@ -171,6 +171,32 @@ export const useAddExpenseStore = create<AddExpenseState>()((set) => ({
           res[p.id] = initSplitShares();
           return res;
         }, {});
+
+
+        const hasExistingAmounts = participants.some((p) => p.amount !== undefined && p.amount !== 0n);
+
+        if (hasExistingAmounts) {
+
+          if (splitType) {
+            calculateSplitShareBasedOnAmount(
+              state.amount,
+              participants,
+              splitType,
+              splitShares,
+              state.paidBy,
+            );
+          } else {
+            splitType = SplitType.EQUAL;
+          }
+          return {
+            splitType,
+            splitShares,
+            participants, 
+            canSplitScreenClosed: true,
+          };
+        }
+
+
         if (splitType) {
           calculateSplitShareBasedOnAmount(
             state.amount,
@@ -190,7 +216,7 @@ export const useAddExpenseStore = create<AddExpenseState>()((set) => ({
             participants,
             splitType,
             splitShares,
-            state.paidBy,
+            state.paidBy
           ),
         };
       }),
@@ -304,7 +330,7 @@ export function calculateParticipantSplit(
   participants: Participant[],
   splitType: SplitType,
   splitShares: SplitShares,
-  paidBy?: Participant,
+  paidBy?: Participant
 ) {
   let canSplitScreenClosed = true;
   if (0n === amount) {
@@ -399,7 +425,7 @@ export function calculateSplitShareBasedOnAmount(
   participants: Participant[],
   splitType: SplitType,
   splitShares: SplitShares,
-  paidBy?: User,
+  paidBy?: User
 ) {
   switch (splitType) {
     case SplitType.EQUAL:

src/tests/addStore.test.ts

@@ -5,6 +5,7 @@ import {
   calculateParticipantSplit,
   calculateSplitShareBasedOnAmount,
   initSplitShares,
+  useAddExpenseStore,
 } from '~/store/addStore';
 
 // Mock dependencies
@@ -985,3 +986,83 @@ describe('Function Reversibility Tests', () => {
     });
   });
 });
+
+describe('Penny distribution preservation', () => {
+  beforeEach(() => {
+    useAddExpenseStore.getState().actions.resetState();
+  });
+
+  it('should preserve different penny distributions when editing expenses', () => {
+    const store = useAddExpenseStore.getState();
+
+    store.actions.setCurrentUser(user1);
+    store.actions.setPaidBy(user1);
+    store.actions.setAmount(4n);
+
+    const distributionA: Participant[] = [
+      { ...user1, amount: 3n },
+      { ...user2, amount: -2n },
+      { ...user3, amount: -1n },
+    ];
+
+    store.actions.setParticipants(distributionA, SplitType.EQUAL);
+    const resultA = useAddExpenseStore.getState().participants;
+
+    store.actions.resetState();
+    store.actions.setCurrentUser(user1);
+    store.actions.setPaidBy(user1);
+    store.actions.setAmount(4n);
+
+    const distributionB: Participant[] = [
+      { ...user1, amount: 2n },
+      { ...user2, amount: -1n },
+      { ...user3, amount: -1n },
+    ];
+
+    store.actions.setParticipants(distributionB, SplitType.EQUAL);
+    const resultB = useAddExpenseStore.getState().participants;
+
+    expect(resultA[0]!.amount).toBe(3n);
+    expect(resultA[1]!.amount).toBe(-2n);
+    expect(resultA[2]!.amount).toBe(-1n);
+
+    expect(resultB[0]!.amount).toBe(2n);
+    expect(resultB[1]!.amount).toBe(-1n);
+    expect(resultB[2]!.amount).toBe(-1n);
+
+    expect(resultA[0]!.amount).not.toBe(resultB[0]!.amount);
+  });
+
+  it('should maintain amounts across multiple edits', () => {
+    const store = useAddExpenseStore.getState();
+
+    store.actions.setCurrentUser(user1);
+    store.actions.setPaidBy(user1);
+    store.actions.setAmount(4n);
+
+    const edit1: Participant[] = [
+      { ...user1, amount: 3n },
+      { ...user2, amount: -2n },
+      { ...user3, amount: -1n },
+    ];
+    store.actions.setParticipants(edit1, SplitType.EQUAL);
+    let result = useAddExpenseStore.getState().participants;
+    expect(result[0]!.amount).toBe(3n);
+    expect(result[1]!.amount).toBe(-2n);
+
+    const edit2: Participant[] = [
+      { ...user1, amount: 2n },
+      { ...user2, amount: -1n },
+      { ...user3, amount: -1n },
+    ];
+    store.actions.setParticipants(edit2, SplitType.EQUAL);
+    result = useAddExpenseStore.getState().participants;
+    expect(result[0]!.amount).toBe(2n);
+    expect(result[1]!.amount).toBe(-1n);
+
+    store.actions.setParticipants(edit1, SplitType.EQUAL);
+    result = useAddExpenseStore.getState().participants;
+    expect(result[0]!.amount).toBe(3n);
+    expect(result[1]!.amount).toBe(-2n);
+  });
+});