Fix #178: Splitting $10.00 three ways loses a cent

src/money/cents.ts

@@ -29,22 +29,20 @@ export function parseCents(input: string): number {
 /**
  * Split an integer amount of cents evenly across member ids.
  *
- * SEEDED DEFECT (issue #2): this floors every share and drops the
- * remainder instead of handing the leftover cents to the first N members,
- * so the shares can sum to less than totalCents. `bun test` does not catch
- * this because the baseline tests only split amounts that divide evenly.
- * The correct remainder rule is documented in
+ * The first `remainder` members (in the order given) get one extra cent, so
+ * the shares always sum to exactly totalCents. See the remainder rule in
  * .claude/skills/handling-money/references/rules.md.
  */
 export function splitCents(totalCents: number, memberIds: string[]): Record<string, number> {
   if (memberIds.length === 0) {
     throw new Error("cannot split among zero members");
   }
-  const share = Math.floor(totalCents / memberIds.length);
+  const base = Math.floor(totalCents / memberIds.length);
+  const remainder = totalCents - base * memberIds.length;
   const shares: Record<string, number> = {};
-  for (const id of memberIds) {
-    shares[id] = share;
-  }
+  memberIds.forEach((id, i) => {
+    shares[id] = i < remainder ? base + 1 : base;
+  });
   return shares;
 }
 

tests/money.test.ts

@@ -36,4 +36,16 @@ describe("splitCents", () => {
     const shares = splitCents(2000, ["a", "b"]);
     expect(sumCents(Object.values(shares))).toBe(2000);
   });
+
+  test("gives the leftover cent to the first member (issue #178)", () => {
+    const shares = splitCents(1000, ["a", "b", "c"]);
+    expect(shares).toEqual({ a: 334, b: 333, c: 333 });
+    expect(sumCents(Object.values(shares))).toBe(1000);
+  });
+
+  test("gives leftover cents to the first N members in order", () => {
+    const shares = splitCents(1001, ["a", "b", "c"]);
+    expect(shares).toEqual({ a: 334, b: 334, c: 333 });
+    expect(sumCents(Object.values(shares))).toBe(1001);
+  });
 });

tests/routes.test.ts

@@ -116,9 +116,6 @@ describe("POST and DELETE /api/groups/:id/expenses", () => {
 
 describe("GET /api/groups/:id/balances", () => {
   test("transfers settle every balance to zero net", async () => {
-    // Flat 4B's seeded amounts all divide evenly by its 3 members, so this
-    // fixture is unaffected by the splitCents remainder bug (issue #2) and
-    // exercises settle-up on its own.
     const app = setupApp();
     const res = await app.request("/api/groups/flat-4b/balances", { headers: auth("flat-priya") });
     const body = (await res.json()) as { balances: { netCents: number }[]; transfers: unknown[] };