Fix/refactor expense calculation

src/app/groups/[groupId]/expenses/expense-form.tsx

@@ -318,7 +318,7 @@
 
   useEffect(() => {
     setManuallyEditedParticipants(new Set())
   }, [form.watch('splitMode'), form.watch('amount')])
 
   useEffect(() => {
     const splitMode = form.getValues().splitMode
@@ -368,10 +368,10 @@
       }
       form.setValue('paidFor', newPaidFor, { shouldValidate: true })
     }
   }, [
     manuallyEditedParticipants,
     form.watch('amount'),
     form.watch('splitMode'),
   ])
 
   const [usingCustomConversionRate, setUsingCustomConversionRate] = useState(
@@ -382,7 +382,7 @@
     if (!usingCustomConversionRate && exchangeRate.data) {
       form.setValue('conversionRate', exchangeRate.data)
     }
   }, [exchangeRate.data, usingCustomConversionRate])
 
   useEffect(() => {
     if (!form.getFieldState('originalAmount').isTouched) return
@@ -402,10 +402,10 @@
         form.setValue('amount', Number(v))
       }
     }
   }, [
     form.watch('originalAmount'),
     form.watch('conversionRate'),
     form.getFieldState('originalAmount').isTouched,
   ])
 
   let conversionRateMessage = ''
@@ -915,6 +915,9 @@
                                             Number(form.watch('amount')),
                                             groupCurrency,
                                           ), // Convert to cents
+                                          expenseDate:
+                                            form.watch('expenseDate') ??
+                                            new Date(),
                                           paidFor: field.value.map(
                                             ({ participant, shares }) => ({
                                               participant: {
@@ -940,6 +943,13 @@
                                           splitMode: form.watch('splitMode'),
                                           isReimbursement:
                                             form.watch('isReimbursement'),
+                                          paidBy: {
+                                            id:
+                                              form.watch('paidBy') ??
+                                              field.value[0]?.participant,
+                                            // name is required for the helper but irrelevant here
+                                            name: '',
+                                          },
                                         }),
                                         locale,
                                       )}

src/app/groups/[groupId]/expenses/export/csv/route.ts

@@ -1,4 +1,5 @@
 import { getCurrency } from '@/lib/currency'
+import { calculateShares } from '@/lib/totals'
 import { formatAmountAsDecimal, getCurrencyFromGroup } from '@/lib/utils'
 import { Parser } from '@json2csv/plainjs'
 import { PrismaClient } from '@prisma/client'
@@ -43,8 +44,13 @@ export async function GET(
           originalAmount: true,
           originalCurrency: true,
           conversionRate: true,
-          paidById: true,
-          paidFor: { select: { participantId: true, shares: true } },
+          paidBy: { select: { id: true, name: true } },
+          paidFor: {
+            select: {
+              participant: { select: { id: true, name: true } },
+              shares: true,
+            },
+          },
           isReimbursement: true,
           splitMode: true,
         },
@@ -102,50 +108,54 @@ export async function GET(
 
   const currency = getCurrencyFromGroup(group)
 
-  const expenses = group.expenses.map((expense) => ({
-    date: formatDate(expense.expenseDate),
-    title: expense.title,
-    categoryName: expense.category?.name || '',
-    currency: group.currencyCode ?? group.currency,
-    amount: formatAmountAsDecimal(expense.amount, currency),
-    originalAmount: expense.originalAmount
-      ? formatAmountAsDecimal(
-          expense.originalAmount,
-          getCurrency(expense.originalCurrency),
-        )
-      : null,
-    originalCurrency: expense.originalCurrency,
-    conversionRate: expense.conversionRate
-      ? expense.conversionRate.toString()
-      : null,
-    isReimbursement: expense.isReimbursement ? 'Yes' : 'No',
-    splitMode: splitModeLabel[expense.splitMode],
-    ...Object.fromEntries(
-      group.participants.map((participant) => {
-        const { totalShares, participantShare } = expense.paidFor.reduce(
-          (acc, { participantId, shares }) => {
-            acc.totalShares += shares
-            if (participantId === participant.id) {
-              acc.participantShare = shares
-            }
-            return acc
-          },
-          { totalShares: 0, participantShare: 0 },
-        )
-
-        const isPaidByParticipant = expense.paidById === participant.id
-        const participantAmountShare = +formatAmountAsDecimal(
-          (expense.amount / totalShares) * participantShare,
-          currency,
-        )
-
-        return [
-          participant.name,
-          participantAmountShare * (isPaidByParticipant ? 1 : -1),
-        ]
-      }),
-    ),
-  }))
+  const expenses = group.expenses.map((expense) => {
+    const shares = calculateShares({
+      amount: expense.amount,
+      paidFor: expense.paidFor,
+      splitMode: expense.splitMode,
+      isReimbursement: expense.isReimbursement,
+      paidBy: expense.paidBy,
+      expenseDate: expense.expenseDate,
+    })
+
+    const payerId =
+      expense.paidBy?.id ?? expense.paidFor[0]?.participant.id ?? null
+
+    return {
+      date: formatDate(expense.expenseDate),
+      title: expense.title,
+      categoryName: expense.category?.name || '',
+      currency: group.currencyCode ?? group.currency,
+      amount: formatAmountAsDecimal(expense.amount, currency),
+      originalAmount: expense.originalAmount
+        ? formatAmountAsDecimal(
+            expense.originalAmount,
+            getCurrency(expense.originalCurrency),
+          )
+        : null,
+      originalCurrency: expense.originalCurrency,
+      conversionRate: expense.conversionRate
+        ? expense.conversionRate.toString()
+        : null,
+      isReimbursement: expense.isReimbursement ? 'Yes' : 'No',
+      splitMode: splitModeLabel[expense.splitMode],
+      ...Object.fromEntries(
+        group.participants.map((participant) => {
+          const participantShare = shares[participant.id] ?? 0
+          const isPaidByParticipant = payerId === participant.id
+          const netAmount = isPaidByParticipant
+            ? expense.amount - participantShare
+            : -participantShare
+
+          return [
+            participant.name,
+            // keep as formatted string to preserve trailing zeros
+            formatAmountAsDecimal(netAmount, currency),
+          ]
+        }),
+      ),
+    }
+  })
 
   const json2csvParser = new Parser({ fields })
   const csv = json2csvParser.parse(expenses)

src/lib/balances.ts

@@ -1,6 +1,6 @@
 import { getGroupExpenses } from '@/lib/api'
+import { calculateShares } from '@/lib/totals'
 import { Participant } from '@prisma/client'
-import { match } from 'ts-pattern'
 
 export type Balances = Record<
   Participant['id'],
@@ -20,43 +20,23 @@ export function getBalances(
 
   for (const expense of expenses) {
     const paidBy = expense.paidBy.id
-    const paidFors = expense.paidFor
 
     if (!balances[paidBy]) balances[paidBy] = { paid: 0, paidFor: 0, total: 0 }
     balances[paidBy].paid += expense.amount
 
-    const totalPaidForShares = paidFors.reduce(
-      (sum, paidFor) => sum + paidFor.shares,
-      0,
-    )
-    let remaining = expense.amount
-    paidFors.forEach((paidFor, index) => {
-      if (!balances[paidFor.participant.id])
-        balances[paidFor.participant.id] = { paid: 0, paidFor: 0, total: 0 }
-
-      const isLast = index === paidFors.length - 1
-
-      const [shares, totalShares] = match(expense.splitMode)
-        .with('EVENLY', () => [1, paidFors.length])
-        .with('BY_SHARES', () => [paidFor.shares, totalPaidForShares])
-        .with('BY_PERCENTAGE', () => [paidFor.shares, totalPaidForShares])
-        .with('BY_AMOUNT', () => [paidFor.shares, totalPaidForShares])
-        .exhaustive()
-
-      const dividedAmount = isLast
-        ? remaining
-        : (expense.amount * shares) / totalShares
-      remaining -= dividedAmount
-      balances[paidFor.participant.id].paidFor += dividedAmount
-    })
+    const shares = calculateShares(expense)
+    for (const participantId in shares) {
+      if (!balances[participantId])
+        balances[participantId] = { paid: 0, paidFor: 0, total: 0 }
+      balances[participantId].paidFor += shares[participantId]
+    }
   }
 
-  // rounding and add total
+  // add totals (shares are already integers)
   for (const participantId in balances) {
-    // add +0 to avoid negative zeros
-    balances[participantId].paidFor =
-      Math.round(balances[participantId].paidFor) + 0
-    balances[participantId].paid = Math.round(balances[participantId].paid) + 0
+    // add +0 to avoid negative zeros (keep values as integers)
+    balances[participantId].paidFor = balances[participantId].paidFor + 0
+    balances[participantId].paid = balances[participantId].paid + 0
 
     balances[participantId].total =
       balances[participantId].paid - balances[participantId].paidFor