From 573b06e245078673c7882ae40e37648fb857ff38 Mon Sep 17 00:00:00 2001
From: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>
Date: Wed, 12 Aug 2026 20:04:37 +0300
Subject: [PATCH] reject duplicate ids in addUnit

addUnit() overwrote unitsById unconditionally while its own addPhrase
calls are first-wins, so an extension reusing an existing id silently
corrupted the id->Unit table for anything using that id as a
baseUnitId afterwards. Throw instead.
---
 src/engine/engine.test.ts | 7 +++++++
 src/engine/index.ts       | 1 +
 2 files changed, 8 insertions(+)

diff --git a/src/engine/engine.test.ts b/src/engine/engine.test.ts
index 29f15e1..febef3a 100644
--- a/src/engine/engine.test.ts
+++ b/src/engine/engine.test.ts
@@ -335,6 +335,13 @@ describe("extension API", () => {
     eng.addUnit({ id: "horse", phrases: "horse, horses, hrs", baseUnitId: "meter", format: "hrs", ratio: 2.4 });
     expect(calc("2 horses in meters")).toBe("4.8 m");
   });
+  it("addUnit throws on id collision instead of silently corrupting unitsById", () => {
+    expect(() =>
+      eng.addUnit({ id: "horse", phrases: "nag", baseUnitId: "meter", format: "nag", ratio: 99 })
+    ).toThrow(/horse/);
+    // the original registration must survive the failed collision
+    expect(calc("2 horses in meters")).toBe("4.8 m");
+  });
   it("addFunction (zum from Sample.js)", () => {
     eng.addFunction({ id: "zum", phrases: "zum" }, (values) => ({ double: values[0].double + values[1].double }));
     expect(calc("zum(2;3)")).toBe("5");
diff --git a/src/engine/index.ts b/src/engine/index.ts
index e507dd9..5d6e82e 100644
--- a/src/engine/index.ts
+++ b/src/engine/index.ts
@@ -218,6 +218,7 @@ export class SumEngine {
   addUnit(spec: ExtensionUnitSpec): void {
     const base = this.reg.unitsById.get(spec.baseUnitId) ?? this.currencyBase(spec.baseUnitId);
     if (!base) throw new Error(`numi.addUnit: unknown baseUnitId ${spec.baseUnitId}`);
+    if (this.reg.unitsById.has(spec.id)) throw new Error(`numi.addUnit: id "${spec.id}" is already registered`);
     const unit: Unit = {
       id: spec.id,
       dimension: base.dimension,