From e1ac6d9fac57b76800eb72fad7d908fa6bb431b4 Mon Sep 17 00:00:00 2001
From: Lucas Winther <lucwin@ece.au.dk>
Date: Tue, 1 Sep 2026 16:22:40 +0200
Subject: [PATCH] feat(domain): add domain types

---
 src/domain/types.test.ts | 12 +++++
 src/domain/types.ts      | 94 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 106 insertions(+)
 create mode 100644 src/domain/types.test.ts
 create mode 100644 src/domain/types.ts

diff --git a/src/domain/types.test.ts b/src/domain/types.test.ts
new file mode 100644
index 0000000..e5f3641
--- /dev/null
+++ b/src/domain/types.test.ts
@@ -0,0 +1,12 @@
+import { test, expect } from "bun:test";
+import { CURRENCIES, CURRENCY_DIGITS } from "./types.ts";
+
+test("supported currencies are exactly DKK, USD, EUR", () => {
+  expect(CURRENCIES).toEqual(["DKK", "USD", "EUR"]);
+});
+
+test("every supported currency has 2 decimal digits", () => {
+  for (const c of CURRENCIES) {
+    expect(CURRENCY_DIGITS[c]).toBe(2);
+  }
+});
diff --git a/src/domain/types.ts b/src/domain/types.ts
new file mode 100644
index 0000000..6a8e247
--- /dev/null
+++ b/src/domain/types.ts
@@ -0,0 +1,94 @@
+export const CURRENCIES = ["DKK", "USD", "EUR"] as const;
+export type Currency = (typeof CURRENCIES)[number];
+
+export const CURRENCY_DIGITS: Record<Currency, number> = {
+  DKK: 2,
+  USD: 2,
+  EUR: 2,
+};
+
+/** A float amount paired with its currency. See CLAUDE.md for rounding rules. */
+export interface Money {
+  amount: number;
+  currency: Currency;
+}
+
+/** "YYYY-MM". Never a Date. */
+export type MonthId = string;
+/** "YYYY-MM-DD". Never a Date. */
+export type IsoDate = string;
+export type PostId = string;
+export type PurchaseId = string;
+
+export type Rule =
+  | { kind: "fixed"; amount: Money }
+  | { kind: "percentOfIncome"; percent: number };
+
+export interface Post {
+  id: PostId;
+  name: string;
+  order: number;
+  archived: boolean;
+  /** Display currency for this post's own views only. All totals use base. */
+  currency: Currency;
+  standingRule: Rule;
+}
+
+export interface Month {
+  id: MonthId;
+  /** Always in Settings.baseCurrency. */
+  income: Money;
+  ruleOverrides: Record<PostId, Rule>;
+}
+
+export interface Split {
+  postId: PostId;
+  /** A percentage, or an amount in the purchase's currency, per Purchase.splitMode. */
+  value: number;
+  absorbsRemainder: boolean;
+}
+
+export interface ScheduleSlice {
+  month: MonthId;
+  amount: Money;
+}
+
+export interface Schedule {
+  slices: ScheduleSlice[];
+  /** Slices in this month AND every month after it are ignored by the fold. */
+  cancelledFromMonth?: MonthId;
+}
+
+export interface Purchase {
+  id: PurchaseId;
+  date: IsoDate;
+  description: string;
+  /** In the purchase's own currency. */
+  total: Money;
+  splitMode: "percent" | "fixed";
+  splits: Split[];
+  /** null means the whole total is charged in the month of `date`. */
+  schedule: Schedule | null;
+}
+
+export interface FxRate {
+  currency: Currency;
+  /** How many base units one unit of `currency` buys. Base has no row. */
+  baseUnitsPerOne: number;
+  updatedAt: string;
+  source: "manual" | "api";
+}
+
+export interface Settings {
+  baseCurrency: Currency;
+  foldStartMonth: MonthId;
+  schemaVersion: number;
+}
+
+export interface Dataset {
+  settings: Settings;
+  fxRates: FxRate[];
+  posts: Post[];
+  months: Month[];
+  purchases: Purchase[];
+}