64 lines
2.1 KiB
TypeScript
64 lines
2.1 KiB
TypeScript
import type { Expense, Participant, ParticipantId, Settlement } from '../types'
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import { computeSplit } from './split'
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export function calculateBalances(
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participants: Participant[],
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expenses: Expense[],
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): { netBalances: Record<ParticipantId, number>, settlements: Settlement[] } {
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const netBalances: Record<ParticipantId, number> = {}
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for (const participant of participants) {
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netBalances[participant.id] = 0
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}
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// A settlement payment (fromId pays toId) is folded in as a regular
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// expense: payerId = fromId, participantIds = [toId]. The math is
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// identical to a one-person split, so no separate handling is needed here.
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for (const expense of expenses) {
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netBalances[expense.payerId] = (netBalances[expense.payerId] ?? 0) + expense.amountCents
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const shares = computeSplit(expense.amountCents, expense.participantIds)
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for (const [participantId, share] of Object.entries(shares)) {
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const id = Number(participantId)
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netBalances[id] = (netBalances[id] ?? 0) - share
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}
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}
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const settlements = simplifyDebts(netBalances)
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return { netBalances, settlements }
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}
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function simplifyDebts(netBalances: Record<ParticipantId, number>): Settlement[] {
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const creditors: { id: ParticipantId, amount: number }[] = []
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const debtors: { id: ParticipantId, amount: number }[] = []
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for (const [id, amount] of Object.entries(netBalances)) {
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if (amount > 0) creditors.push({ id: Number(id), amount })
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else if (amount < 0) debtors.push({ id: Number(id), amount: -amount })
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}
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creditors.sort((a, b) => b.amount - a.amount)
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debtors.sort((a, b) => b.amount - a.amount)
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const settlements: Settlement[] = []
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let ci = 0
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let di = 0
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while (ci < creditors.length && di < debtors.length) {
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const creditor = creditors[ci]!
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const debtor = debtors[di]!
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const amount = Math.min(creditor.amount, debtor.amount)
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if (amount > 0) {
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settlements.push({ fromId: debtor.id, toId: creditor.id, amountCents: amount })
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}
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creditor.amount -= amount
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debtor.amount -= amount
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if (creditor.amount === 0) ci++
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if (debtor.amount === 0) di++
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}
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return settlements
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}
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