71 lines
2.2 KiB
TypeScript
71 lines
2.2 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] =
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(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(
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netBalances: Record<ParticipantId, number>,
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): 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({
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fromId: debtor.id,
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toId: creditor.id,
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amountCents: amount,
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});
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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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