Splitting $10.00 three ways loses a cent (#26)

src/money/cents.ts

@@ -29,22 +29,21 @@ 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
- * .claude/skills/handling-money/references/rules.md.
+ * Remainder rule (.claude/skills/handling-money/references/rules.md): every
+ * member gets floor(totalCents / n), and the first `remainder` members, in
+ * the order passed in, get one extra cent, so the shares always sum to
+ * exactly totalCents.
  */
 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 +36,28 @@ describe("splitCents", () => {
     const shares = splitCents(2000, ["a", "b"]);
     expect(sumCents(Object.values(shares))).toBe(2000);
   });
+
+  test("gives the leftover cent to the first member when splitting 1000 three ways", () => {
+    const shares = splitCents(1000, ["a", "b", "c"]);
+    expect(shares).toEqual({ a: 334, b: 333, c: 333 });
+    expect(sumCents(Object.values(shares))).toBe(1000);
+  });
+
+  test("assigns leftover cents so the shares sum to the total", () => {
+    const shares = splitCents(101, ["a", "b", "c", "d", "e"]);
+    expect(shares).toEqual({ a: 21, b: 20, c: 20, d: 20, e: 20 });
+    expect(sumCents(Object.values(shares))).toBe(101);
+  });
+
+  test("gives leftover cents to the first members in member order", () => {
+    const shares = splitCents(1002, ["c", "a", "d", "b"]);
+    expect(shares).toEqual({ c: 251, a: 251, d: 250, b: 250 });
+    expect(sumCents(Object.values(shares))).toBe(1002);
+  });
+
+  test("splits a total smaller than the member count one cent at a time", () => {
+    const shares = splitCents(2, ["a", "b", "c", "d", "e"]);
+    expect(shares).toEqual({ a: 1, b: 1, c: 0, d: 0, e: 0 });
+    expect(sumCents(Object.values(shares))).toBe(2);
+  });
 });