Fix cosine similarity to handle division by zero

Author: Joseph Smith

common-lib/src/main/scala/com/gu/mediaservice/lib/VectorUtils.scala

@@ -10,6 +10,8 @@ object VectorUtils {
   }
 
   def cosineSimilarity(vectorOne: List[Double], vectorTwo: List[Double]): Double = {
-    dotProduct(vectorOne, vectorTwo) / (magnitude(vectorOne) * magnitude(vectorTwo))
+    val magnitudeProduct = magnitude(vectorOne) * magnitude(vectorTwo)
+    if (magnitudeProduct == 0.0) 0.0
+    else dotProduct(vectorOne, vectorTwo) / magnitudeProduct
   }
 }

common-lib/src/test/scala/com/gu/mediaservice/lib/VectorUtilsTest.scala

@@ -40,4 +40,16 @@ class VectorUtilsTest extends AnyFunSpec with Matchers {
   it ("should return a cosine similarity of -1 for opposite vectors") {
     VectorUtils.cosineSimilarity(List(1.0, 2.0, 3.0), List(-1.0, -2.0, -3.0)) shouldBe -1.0 +- 1e-9
   }
+
+  it ("should return a cosine similarity of 0 when the first vector has zero magnitude") {
+    VectorUtils.cosineSimilarity(List(0.0, 0.0, 0.0), List(1.0, 2.0, 3.0)) shouldBe 0.0 +- 1e-9
+  }
+
+  it ("should return a cosine similarity of 0 when the second vector has zero magnitude") {
+    VectorUtils.cosineSimilarity(List(1.0, 2.0, 3.0), List(0.0, 0.0, 0.0)) shouldBe 0.0 +- 1e-9
+  }
+
+  it ("should return a cosine similarity of 0 when both vectors have zero magnitude") {
+    VectorUtils.cosineSimilarity(List(0.0, 0.0, 0.0), List(0.0, 0.0, 0.0)) shouldBe 0.0 +- 1e-9
+  }
 }