1 files changed, 42 insertions, 0 deletions
diff --git a/src/lib/balances.ts b/src/lib/balances.ts
new file mode 100644
index 0000000..fa00640
--- /dev/null
+++ b/src/lib/balances.ts
@@ -0,0 +1,42 @@
+import { getGroupExpenses } from '@/lib/api'
+import { Participant } from '@prisma/client'
+
+export type Balances = Record<
+  Participant['id'],
+  { paid: number; paidFor: number; total: number }
+>
+
+export function getBalances(
+  expenses: NonNullable<Awaited<ReturnType<typeof getGroupExpenses>>>,
+): Balances {
+  const balances: Balances = {}
+
+  for (const expense of expenses) {
+    const paidBy = expense.paidById
+    const paidFors = expense.paidFor.map((p) => p.participantId)
+
+    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
+    })
+  }
+
+  return balances
+}
+
+function divide(total: number, count: number, isLast: boolean): number {
+  if (!isLast) return Math.floor((total * 100) / count) / 100
+
+  return total - divide(total, count, false) * (count - 1)
+}