Helper function for calculating expenses

Assuming the following expenses:
    { resident: 'Person 1', amount: 100 },
    { resident: 'Person 2', amount: 40 },
    { resident: 'Person 3', amount: 60 },
    { resident: 'Person 4', amount: 0 },
then
Person 2 owes Person 1 $10
Person 4 owes Person 1 $40
Person 4 owes Person 3 $10

app/calculateExpenses.tsx

@@ -0,0 +1,43 @@
+type Expense = {
+    resident: string;
+    amount: number;
+};
+
+type Transaction = {
+    from: string;
+    to: string;
+    amount: number;
+};
+
+export const calculateExpenses = (expenses: Expense[]): Transaction[] => {
+    // Calculate total spent by each resident
+    const totalSpent: { [name: string]: number } = {};
+    expenses.forEach((expense) => {
+        totalSpent[expense.resident] = (totalSpent[expense.resident] || 0) + expense.amount;
+    });
+
+    // Calculate the average amount spent by all residents
+    const numResidents = Object.keys(totalSpent).length;
+    const totalAmountSpent = Object.values(totalSpent).reduce((total, amount) => total + amount, 0);
+    const averageAmount = totalAmountSpent / numResidents;
+
+    // Calculate who owes or is owed how much
+    const transactions: Transaction[] = [];
+    Object.keys(totalSpent).forEach((creditor) => {
+        let amountOwed = totalSpent[creditor] - averageAmount;
+
+        if (amountOwed > 0) {
+            Object.keys(totalSpent).forEach((debitor) => {
+                if (creditor !== debitor && totalSpent[debitor] < averageAmount) {
+                    const settleAmount = Math.min(amountOwed, averageAmount - totalSpent[debitor]);
+                    transactions.push({ to: creditor, from: debitor, amount: settleAmount });
+                    totalSpent[creditor] -= settleAmount;
+                    totalSpent[debitor] += settleAmount;
+                    amountOwed -= settleAmount;
+                }
+            });
+        }
+    });
+
+    return transactions;
+};