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import { getGroupExpenses } from '@/lib/api'
import { Participant } from '@prisma/client'
export type Balances = Record<
Participant['id'],
{ paid: number; paidFor: number; total: number }
>
export type Reimbursement = {
from: Participant['id']
to: Participant['id']
amount: 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 / count)
return total - divide(total, count, false) * (count - 1)
}
export function getSuggestedReimbursements(
balances: Balances,
): Reimbursement[] {
const balancesArray = Object.entries(balances)
.map(([participantId, { total }]) => ({ participantId, total }))
.filter((b) => b.total !== 0)
balancesArray.sort((b1, b2) => b2.total - b1.total)
const reimbursements: Reimbursement[] = []
while (balancesArray.length > 1) {
const first = balancesArray[0]
const last = balancesArray[balancesArray.length - 1]
const amount = first.total + last.total
if (first.total > -last.total) {
reimbursements.push({
from: last.participantId,
to: first.participantId,
amount: -last.total,
})
first.total = amount
balancesArray.pop()
} else {
reimbursements.push({
from: last.participantId,
to: first.participantId,
amount: first.total,
})
last.total = amount
balancesArray.shift()
}
}
return reimbursements.filter(({ amount }) => amount !== 0)
}
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