From f90c009d2888eb45f74637ec2b671ae5d7cd8f11 Mon Sep 17 00:00:00 2001
From: Braden MacDonald <mail@bradenm.com>
Date: Tue, 11 Oct 2022 14:14:31 -0700
Subject: [PATCH] fix: "remember" to use dimensionless units like "%" if
 originally used

---
 quantity.ts            | 15 ++++++++++++++-
 tests/quantity.test.ts | 12 ++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/quantity.ts b/quantity.ts
index f2ef490..a0a04b9 100644
--- a/quantity.ts
+++ b/quantity.ts
@@ -155,7 +155,7 @@ export class Quantity {
             }
             r += "±" + plusMinusString;
         }
-        if (!this.isDimensionless) {
+        if (serialized.units.length > 0) {
             r += " " + serialized.units;
         }
         return r;
@@ -280,6 +280,19 @@ export class Quantity {
             remainder = bestRemainder;
         }
 
+        // Special case to handle dimensionless units like "%" that we may actually want to use:
+        if (unitList.length === 1 && useUnits.length === 0) {
+            // We want "50 % ⋅ 50 %" to give "25 %"
+            // But we want "50 % ⋅ 400 g" to give "200 g" (not "20,000 g⋅%"!)
+            for (let unitIdx = 0; unitIdx < unitList.length; unitIdx++) {
+                if (unitArray[unitIdx].isDimensionless) {
+                    useUnits.push(unitIdx);
+                    useUnitsPower.push(1);
+                    break; // Only include up to one dimensionless unit like "%"
+                }
+            }
+        }
+
         // At this point the units to be used are in useUnits
         return useUnits.map((i, pi) => ({
             unit: unitList[i].unit,
diff --git a/tests/quantity.test.ts b/tests/quantity.test.ts
index d04933a..fb7616e 100644
--- a/tests/quantity.test.ts
+++ b/tests/quantity.test.ts
@@ -262,6 +262,18 @@ Deno.test("Multiplying quantities", async (t) => {
         assertEquals(z.magnitude, 15);
         assertEquals(z.dimensions, TWO_LENGTH_DIMENSIONS); // m²
     });
+    await t.step(`(50 %) * (50 %)`, () => {
+        const x = new Quantity(50, { units: "%" });
+        const y = new Quantity(50, { units: "%" });
+        const z = x.multiply(y);
+        assertEquals(z.toString(), "25 %");
+    });
+    await t.step(`(50 %) * (400 g)`, () => {
+        const x = new Quantity(50, { units: "%" });
+        const y = new Quantity(400, { units: "g" });
+        const z = x.multiply(y);
+        assertEquals(z.toString(), "200 g");
+    });
     await t.step(`(500 g) * (2 m/s^2)`, () => {
         const x = new Quantity(500, { units: "g" });
         const y = new Quantity(2, { units: "m/s^2" });