From 1375bb356fdb50832f7cf2f4774edbc3fcad84f3 Mon Sep 17 00:00:00 2001
From: Zoltan Czirkos <zoltan@vespertool.com>
Date: Tue, 12 May 2026 14:13:47 +0200
Subject: [PATCH] Inverting FactorUnitPart calculates the reciprocal of its
 ratio

---
 src/FactorUnitPart.php       |  8 +++++++
 tests/FactorUnitPartTest.php | 12 ++++++++++
 tests/UnitTest.php           | 46 ++++++++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+)

diff --git a/src/FactorUnitPart.php b/src/FactorUnitPart.php
index 388dc78..1078750 100644
--- a/src/FactorUnitPart.php
+++ b/src/FactorUnitPart.php
@@ -14,4 +14,12 @@ public function format(): string
     {
         return "{$this->getRatio()}";
     }
+
+    #[\Override]
+    public function invert(): self
+    {
+        // A FactorUnitPart always has power = 1. So when inverting it,
+        // we calculate the reciprocal of the ratio instead of negating the power.
+        return new self(1 / $this->getRatio());
+    }
 }
\ No newline at end of file
diff --git a/tests/FactorUnitPartTest.php b/tests/FactorUnitPartTest.php
index e8af6c5..6fd3cbf 100644
--- a/tests/FactorUnitPartTest.php
+++ b/tests/FactorUnitPartTest.php
@@ -18,4 +18,16 @@ public function test_stringifies()
     {
         $this->assertEquals('100', (string)new FactorUnitPart(100));
     }
+
+    public function test_inverts()
+    {
+        $factor = new FactorUnitPart(100);
+
+        $factorInverted = $factor->invert();
+
+        $this->assertEquals(FactorUnitPart::class, get_class($factorInverted));
+        $this->assertNull($factorInverted->getDimension());
+        $this->assertEquals(0.01, $factorInverted->getRatio());
+        $this->assertEquals(1, $factorInverted->getPower());
+    }
 }
diff --git a/tests/UnitTest.php b/tests/UnitTest.php
index 84ef554..2f40303 100644
--- a/tests/UnitTest.php
+++ b/tests/UnitTest.php
@@ -171,4 +171,50 @@ public function test_divide()
 
         $this->assertEquals($ms->getDimensions(), $meterDividedBySeconds->getDimensions());
     }
+
+    public function test_invert()
+    {
+        $hundredSquareMeters = new Unit(
+            new FactorUnitPart(100),
+            new UnitPart(1, Dimension::LENGTH, 2),
+        );
+
+        $inverted = $hundredSquareMeters->invert();
+
+        $this->assertEquals(-2, $inverted->getDimensions()[Dimension::LENGTH->name]);
+        $this->assertEquals(0.01, $inverted->getRatio());
+    }
+
+    public function test_invert_twice()
+    {
+        $hundredSquareMeters = new Unit(
+            new FactorUnitPart(100),
+            new UnitPart(1, Dimension::LENGTH, 2),
+        );
+
+        $invertedTwice = $hundredSquareMeters->invert()->invert();
+
+        $this->assertEquals($invertedTwice->getRatio(), $hundredSquareMeters->getRatio());
+        $this->assertEquals($invertedTwice->getDimensions(), $hundredSquareMeters->getDimensions());
+    }
+
+    public function test_unit_and_factor_mix()
+    {
+        // 100m^2
+        $hundredSquareMeters = new Unit(
+            new FactorUnitPart(100),
+            new UnitPart(1, Dimension::LENGTH, 2),
+        );
+
+        // (10m)^2
+        $tenMetersSquared = new Unit(
+            new UnitPart(10, Dimension::LENGTH, 2),
+        );
+
+        $this->assertEquals($hundredSquareMeters->getRatio(), $tenMetersSquared->getRatio());
+        $this->assertEquals($hundredSquareMeters->getDimensions(), $tenMetersSquared->getDimensions());
+
+        $this->assertEquals($hundredSquareMeters->invert()->getRatio(), $tenMetersSquared->invert()->getRatio());
+        $this->assertEquals($hundredSquareMeters->invert()->getDimensions(), $tenMetersSquared->invert()->getDimensions());
+    }
 }