Update balances based on shares

src/lib/balances.ts

@@ -1,5 +1,6 @@
 import { getGroupExpenses } from '@/lib/api'
 import { Participant } from '@prisma/client'
+import { match } from 'ts-pattern'
 
 export type Balances = Record<
   Participant['id'],
@@ -19,34 +20,42 @@ export function getBalances(
 
   for (const expense of expenses) {
     const paidBy = expense.paidById
-    const paidFors = expense.paidFor.map((p) => p.participantId)
+    const paidFors = expense.paidFor
 
     if (!balances[paidBy]) balances[paidBy] = { paid: 0, paidFor: 0, total: 0 }
     balances[paidBy].paid += expense.amount
     balances[paidBy].total += expense.amount
-    paidFors.forEach((paidFor, index) => {
-      if (!balances[paidFor])
-        balances[paidFor] = { paid: 0, paidFor: 0, total: 0 }
 
-      const dividedAmount = divide(
-        expense.amount,
-        paidFors.length,
-        index === paidFors.length - 1,
-      )
-      balances[paidFor].paidFor += dividedAmount
-      balances[paidFor].total -= dividedAmount
+    const totalPaidForShares = paidFors.reduce(
+      (sum, paidFor) => sum + paidFor.shares,
+      0,
+    )
+    let remaining = expense.amount
+    paidFors.forEach((paidFor, index) => {
+      if (!balances[paidFor.participantId])
+        balances[paidFor.participantId] = { 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
+        : Math.floor((expense.amount * shares) / totalShares)
+      remaining -= dividedAmount
+      balances[paidFor.participantId].paidFor += dividedAmount
+      balances[paidFor.participantId].total -= dividedAmount
     })
   }
 
   return balances
 }
 
-function divide(total: number, count: number, isLast: boolean): number {
-  if (!isLast) return Math.floor(total / count)
-
-  return total - divide(total, count, false) * (count - 1)
-}
-
 export function getSuggestedReimbursements(
   balances: Balances,
 ): Reimbursement[] {