From 423d22d46244494a086d79a1cbbba538b897f162 Mon Sep 17 00:00:00 2001
From: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>
Date: Sun, 9 Aug 2026 15:59:13 +0300
Subject: [PATCH] add evaluator quantity conversion tests

---
 src/engine/evaluator.test.ts | 90 ++++++++++++++++++++++++++++++++++++
 src/engine/evaluator.ts      |  2 +-
 2 files changed, 91 insertions(+), 1 deletion(-)
 create mode 100644 src/engine/evaluator.test.ts

diff --git a/src/engine/evaluator.test.ts b/src/engine/evaluator.test.ts
new file mode 100644
index 0000000..53fbfe6
--- /dev/null
+++ b/src/engine/evaluator.test.ts
@@ -0,0 +1,90 @@
+import { describe, it, expect } from "vitest";
+import { toBase, fromBase, convertQ, attachUnit } from "./evaluator";
+import { qty, pct, EvalError, Decimal } from "./types";
+import { buildRegistry } from "./registry";
+
+const reg = buildRegistry();
+const getUnit = (id: string) => {
+  const u = reg.unitsById.get(id);
+  if (!u) throw new Error(`unit ${id} not found`);
+  return u;
+};
+
+describe("quantity conversion", () => {
+  it("toBase works", () => {
+    // 1 km = 1000 m
+    const km = getUnit("kilo:meter");
+    const res1 = toBase(qty(2, km));
+    expect(res1.toNumber()).toBe(2000);
+
+    // 0 C = 273.15 K
+    const c = getUnit("celsius");
+    const res2 = toBase(qty(0, c));
+    expect(res2.toNumber()).toBe(273.15);
+
+    // 10 mpg
+    const mpg = getUnit("mpg");
+    const res3 = toBase(qty(10, mpg));
+    expect(res3.toNumber()).toBeCloseTo(23.5215, 4);
+
+    // error on zero for reciprocal
+    expect(() => toBase(qty(0, mpg))).toThrow(EvalError);
+  });
+
+  it("fromBase works", () => {
+    const km = getUnit("kilo:meter");
+    const res1 = fromBase(new Decimal(2000), km);
+    expect(res1.toNumber()).toBe(2);
+
+    const c = getUnit("celsius");
+    const res2 = fromBase(new Decimal(273.15), c);
+    expect(res2.toNumber()).toBe(0);
+
+    const mpg = getUnit("mpg");
+    const res3 = fromBase(new Decimal(23.5215), mpg);
+    expect(res3.toNumber()).toBeCloseTo(10, 4);
+
+    expect(() => fromBase(new Decimal(0), mpg)).toThrow(EvalError);
+  });
+
+  it("convertQ works", () => {
+    const m = getUnit("meter");
+    const km = getUnit("kilo:meter");
+
+    // 2000m -> km
+    const q1 = convertQ(qty(2000, m), km);
+    expect(q1.value.toNumber()).toBe(2);
+    expect(q1.unit?.id).toBe("kilo:meter");
+
+    // c -> f
+    const c = getUnit("celsius");
+    const f = getUnit("fahrenheit");
+    const q2 = convertQ(qty(0, c), f);
+    expect(q2.value.toNumber()).toBeCloseTo(32);
+
+    // fail on mismatched dimensions
+    const kg = getUnit("kilo:gram");
+    expect(() => convertQ(qty(1, m), kg)).toThrow(EvalError);
+  });
+
+  it("attachUnit works", () => {
+    const m = getUnit("meter");
+    const km = getUnit("kilo:meter");
+    const kg = getUnit("kilo:gram");
+
+    // attaching to dimensionless
+    const r1 = attachUnit(qty(2), m);
+    expect(r1).toMatchObject({ kind: "quantity", unit: { id: "meter" } });
+
+    // attaching matching dimension does conversion
+    const r2 = attachUnit(qty(2000, m), km);
+    expect((r2 as any).value.toNumber()).toBe(2);
+    expect((r2 as any).unit?.id).toBe("kilo:meter");
+
+    // attaching mismatched dimension throws
+    expect(() => attachUnit(qty(1, m), kg)).toThrow(EvalError);
+
+    // attaching to non-quantity throws
+    expect(() => attachUnit(pct(50), m)).toThrow(EvalError);
+  });
+});
diff --git a/src/engine/evaluator.ts b/src/engine/evaluator.ts
index 18d8d77..848ed72 100644
--- a/src/engine/evaluator.ts
+++ b/src/engine/evaluator.ts
@@ -80,7 +80,7 @@ function asScalar(v: Value): Decimal {
   throw new EvalError("expected a number");
 }
 
-function attachUnit(v: Value, unit: Unit): Value {
+export function attachUnit(v: Value, unit: Unit): Value {
   if (v.kind === "quantity") {
     if (v.unit && v.unit.dimension === unit.dimension) return convertQ(v, unit);
     if (v.unit) throw new EvalError("unit mismatch");