server/tests/unit/services/budgetService.test.ts

import { describe, it, expect, vi, beforeEach } from 'vitest';

// ── DB mock setup ────────────────────────────────────────────────────────────

const mockDb = vi.hoisted(() => {
  return {
    db: {
      prepare: vi.fn(() => ({
        all: vi.fn(() => []),
        get: vi.fn(() => undefined),
        run: vi.fn(),
      })),
    },
    canAccessTrip: vi.fn(() => true),
  };
});

vi.mock('../../../src/db/database', () => mockDb);

import { calculateSettlement } from '../../../src/services/budgetService';
import type { BudgetItem, BudgetItemMember, BudgetItemPayer } from '../../../src/types';

// ── Helpers ──────────────────────────────────────────────────────────────────
// Who actually paid is recorded as explicit payers (budget_item_payers); members
// are only the equal-split participants.

function makeItem(id: number, total_price: number, trip_id = 1): BudgetItem {
  return { id, trip_id, name: `Item ${id}`, total_price, category: 'other' } as BudgetItem;
}

function makeMember(budget_item_id: number, user_id: number, username: string): BudgetItemMember & { budget_item_id: number } {
  return { budget_item_id, user_id, paid: 0, username, avatar: null } as BudgetItemMember & { budget_item_id: number };
}

function makePayer(budget_item_id: number, user_id: number, amount: number, username: string): BudgetItemPayer & { budget_item_id: number } {
  return { budget_item_id, user_id, amount, username, avatar: null } as BudgetItemPayer & { budget_item_id: number };
}

function setupDb(
  items: BudgetItem[],
  members: (BudgetItemMember & { budget_item_id: number })[],
  payers: (BudgetItemPayer & { budget_item_id: number })[] = [],
) {
  mockDb.db.prepare.mockImplementation((sql: string) => {
    if (sql.includes('SELECT * FROM budget_items')) {
      return { all: vi.fn(() => items), get: vi.fn(), run: vi.fn() };
    }
    if (sql.includes('budget_item_members')) {
      return { all: vi.fn(() => members), get: vi.fn(), run: vi.fn() };
    }
    if (sql.includes('budget_item_payers')) {
      return { all: vi.fn(() => payers), get: vi.fn(), run: vi.fn() };
    }
    // budget_settlements and anything else → empty
    return { all: vi.fn(() => []), get: vi.fn(), run: vi.fn() };
  });
}

beforeEach(() => {
  vi.clearAllMocks();
  setupDb([], [], []);
});

// ── calculateSettlement ──────────────────────────────────────────────────────

describe('calculateSettlement', () => {
  it('returns empty balances and flows when trip has no items', () => {
    setupDb([], [], []);
    const result = calculateSettlement(1);
    expect(result.balances).toEqual([]);
    expect(result.flows).toEqual([]);
  });

  it('returns no flows when there are items but no members', () => {
    setupDb([makeItem(1, 100)], [], [makePayer(1, 1, 100, 'alice')]);
    const result = calculateSettlement(1);
    expect(result.flows).toEqual([]);
  });

  it('returns no flows when no one has paid', () => {
    setupDb(
      [makeItem(1, 100)],
      [makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob')],
      [],
    );
    const result = calculateSettlement(1);
    expect(result.flows).toEqual([]);
  });

  it('2 members, 1 payer: payer is owed half, non-payer owes half', () => {
    // Item: $100. Alice paid all, [Alice, Bob] split. Each owes $50. Alice net: +$50. Bob: -$50.
    setupDb(
      [makeItem(1, 100)],
      [makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob')],
      [makePayer(1, 1, 100, 'alice')],
    );
    const result = calculateSettlement(1);
    const alice = result.balances.find(b => b.user_id === 1)!;
    const bob = result.balances.find(b => b.user_id === 2)!;
    expect(alice.balance).toBe(50);
    expect(bob.balance).toBe(-50);
    expect(result.flows).toHaveLength(1);
    expect(result.flows[0].from.user_id).toBe(2); // Bob owes
    expect(result.flows[0].to.user_id).toBe(1);   // Alice is owed
    expect(result.flows[0].amount).toBe(50);
  });

  it('3 members, 1 payer: correct 3-way split', () => {
    // Item: $90. Alice paid. Each of 3 owes $30. Alice net: +$60. Bob: -$30. Carol: -$30.
    setupDb(
      [makeItem(1, 90)],
      [makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob'), makeMember(1, 3, 'carol')],
      [makePayer(1, 1, 90, 'alice')],
    );
    const result = calculateSettlement(1);
    const alice = result.balances.find(b => b.user_id === 1)!;
    const bob = result.balances.find(b => b.user_id === 2)!;
    const carol = result.balances.find(b => b.user_id === 3)!;
    expect(alice.balance).toBe(60);
    expect(bob.balance).toBe(-30);
    expect(carol.balance).toBe(-30);
    expect(result.flows).toHaveLength(2);
  });

  it('all paid equally: all balances are zero, no flows', () => {
    // Item: $60. 3 members, each paid $20 and owes $20. Net: 0 for everyone.
    setupDb(
      [makeItem(1, 60)],
      [makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob'), makeMember(1, 3, 'carol')],
      [makePayer(1, 1, 20, 'alice'), makePayer(1, 2, 20, 'bob'), makePayer(1, 3, 20, 'carol')],
    );
    const result = calculateSettlement(1);
    for (const b of result.balances) {
      expect(Math.abs(b.balance)).toBeLessThanOrEqual(0.01);
    }
    expect(result.flows).toHaveLength(0);
  });

  it('flow direction: from is debtor (owes), to is creditor (is owed)', () => {
    // Alice paid $100 for 2 people. Bob owes Alice $50.
    setupDb(
      [makeItem(1, 100)],
      [makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob')],
      [makePayer(1, 1, 100, 'alice')],
    );
    const result = calculateSettlement(1);
    const flow = result.flows[0];
    expect(flow.from.username).toBe('bob');   // debtor
    expect(flow.to.username).toBe('alice');   // creditor
  });

  it('amounts are rounded to 2 decimal places', () => {
    // Item: $10. 3 members, 1 payer. Share = 3.333... Each rounded to 3.33.
    setupDb(
      [makeItem(1, 10)],
      [makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob'), makeMember(1, 3, 'carol')],
      [makePayer(1, 1, 10, 'alice')],
    );
    const result = calculateSettlement(1);
    for (const b of result.balances) {
      const str = b.balance.toString();
      const decimals = str.includes('.') ? str.split('.')[1].length : 0;
      expect(decimals).toBeLessThanOrEqual(2);
    }
    for (const flow of result.flows) {
      const str = flow.amount.toString();
      const decimals = str.includes('.') ? str.split('.')[1].length : 0;
      expect(decimals).toBeLessThanOrEqual(2);
    }
  });

  it('2 items with different payers: aggregates balances correctly', () => {
    // Item 1: $100, Alice paid, [Alice, Bob] (Alice net: +50, Bob: -50)
    // Item 2: $60, Bob paid, [Alice, Bob] (Bob net: +30, Alice: -30)
    // Final: Alice: +50 - 30 = +20, Bob: -50 + 30 = -20
    setupDb(
      [makeItem(1, 100), makeItem(2, 60)],
      [
        makeMember(1, 1, 'alice'), makeMember(1, 2, 'bob'),
        makeMember(2, 1, 'alice'), makeMember(2, 2, 'bob'),
      ],
      [makePayer(1, 1, 100, 'alice'), makePayer(2, 2, 60, 'bob')],
    );
    const result = calculateSettlement(1);
    const alice = result.balances.find(b => b.user_id === 1)!;
    const bob = result.balances.find(b => b.user_id === 2)!;
    expect(alice.balance).toBe(20);
    expect(bob.balance).toBe(-20);
    expect(result.flows).toHaveLength(1);
    expect(result.flows[0].amount).toBe(20);
  });
});