Balances

src/app/groups/[groupId]/balances-list.tsx

@@ -0,0 +1,63 @@
+import { Balances } from '@/lib/balances'
+import { cn } from '@/lib/utils'
+import { Participant } from '@prisma/client'
+
+type Props = {
+  balances: Balances
+  participants: Participant[]
+  currency: string
+}
+
+export function BalancesList({ balances, participants, currency }: Props) {
+  const maxBalance = Math.max(
+    ...Object.values(balances).map((b) => Math.abs(b.total)),
+  )
+
+  return (
+    <div className="text-sm">
+      {participants.map((participant) => (
+        <div
+          key={participant.id}
+          className={cn(
+            'flex',
+            balances[participant.id]?.total > 0 || 'flex-row-reverse',
+          )}
+        >
+          <div
+            className={cn(
+              'w-1/2 p-2',
+              balances[participant.id]?.total > 0 && 'text-right',
+            )}
+          >
+            {participant.name}
+          </div>
+          <div
+            className={cn(
+              'w-1/2 relative',
+              balances[participant.id]?.total > 0 || 'text-right',
+            )}
+          >
+            <div className="absolute inset-0 p-2 z-20">
+              {currency} {(balances[participant.id]?.total ?? 0).toFixed(2)}
+            </div>
+            <div
+              className={cn(
+                'absolute top-1 h-7 z-10',
+                balances[participant.id]?.total > 0
+                  ? 'bg-green-200 left-0 rounded-r-lg border border-green-300'
+                  : 'bg-red-200 right-0 rounded-l-lg border  border-red-300',
+              )}
+              style={{
+                width:
+                  (Math.abs(balances[participant.id]?.total ?? 0) /
+                    maxBalance) *
+                    100 +
+                  '%',
+              }}
+            ></div>
+          </div>
+        </div>
+      ))}
+    </div>
+  )
+}

src/app/groups/[groupId]/page.tsx

@@ -1,3 +1,4 @@
+import { BalancesList } from '@/app/groups/[groupId]/balances-list'
 import { SaveGroupLocally } from '@/app/groups/[groupId]/save-recent-group'
 import { Badge } from '@/components/ui/badge'
 import { Button } from '@/components/ui/button'
@@ -17,6 +18,7 @@ import {
   TableRow,
 } from '@/components/ui/table'
 import { getGroup, getGroupExpenses } from '@/lib/api'
+import { getBalances } from '@/lib/balances'
 import { ChevronRight, Plus } from 'lucide-react'
 import Link from 'next/link'
 import { notFound } from 'next/navigation'
@@ -30,6 +32,7 @@ export default async function GroupPage({
   if (!group) notFound()
 
   const expenses = await getGroupExpenses(groupId)
+  const balances = getBalances(expenses)
 
   return (
     <>
@@ -52,7 +55,7 @@ export default async function GroupPage({
 
         <CardContent className="p-0">
           {expenses.length > 0 ? (
-            <Table className="">
+            <Table>
               <TableHeader>
                 <TableRow>
                   <TableHead>Title</TableHead>
@@ -115,14 +118,17 @@ export default async function GroupPage({
 
       <Card className="mb-4">
         <CardHeader>
-          <CardTitle>Participants</CardTitle>
+          <CardTitle>Balances</CardTitle>
+          <CardDescription>
+            This is the amount that each participant paid or was paid for.
+          </CardDescription>
         </CardHeader>
         <CardContent>
-          <ul>
+          <BalancesList
-            {group.participants.map((participant) => (
+            balances={balances}
-              <li key={participant.id}>{participant.name}</li>
+            participants={group.participants}
-            ))}
+            currency={group.currency}
-          </ul>
+          />
         </CardContent>
       </Card>
       <SaveGroupLocally group={{ id: group.id, name: group.name }} />

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)
+}