CosineSimilarity protection from zeros; full coverage tests. (#392)

Added `max(sqrt(normA), sqrt(normB)), EPSILON)` to insulate from zero
vectors. Tested error results of embeddings of different sizes.

langchain4j-core/src/main/java/dev/langchain4j/store/embedding/CosineSimilarity.java

@@ -6,6 +6,7 @@
 import static dev.langchain4j.internal.ValidationUtils.ensureNotNull;
 
 public class CosineSimilarity {
+    public static final float EPSILON = 1e-8f;
 
     /**
      * Calculates cosine similarity between two vectors.
@@ -20,6 +21,9 @@ public class CosineSimilarity {
      * 1 indicates vectors are pointing in the same direction (but not necessarily of the same magnitude).
      * <p>
      * Not to be confused with cosine distance ([0..2]), which quantifies how different two vectors are.
+     * <p>
+     * Embeddings of all-zeros vectors are considered orthogonal to all other vectors;
+     * including other all-zeros vectors.
      *
      * @param embeddingA first embedding vector
      * @param embeddingB second embedding vector
@@ -47,7 +51,8 @@ public static double between(Embedding embeddingA, Embedding embeddingB) {
             normB += vectorB[i] * vectorB[i];
         }
 
-        return dotProduct / (Math.sqrt(normA) * Math.sqrt(normB));
+        // Avoid division by zero.
+        return dotProduct / Math.max(Math.sqrt(normA) * Math.sqrt(normB), EPSILON);
     }
 
     /**

langchain4j-core/src/test/java/dev/langchain4j/store/embedding/CosineSimilarityTest.java

@@ -1,17 +1,34 @@
 package dev.langchain4j.store.embedding;
 
 import dev.langchain4j.data.embedding.Embedding;
+import org.assertj.core.api.WithAssertions;
 import org.junit.jupiter.api.Test;
 
-import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.data.Percentage.withPercentage;
 
-class CosineSimilarityTest {
+class CosineSimilarityTest implements WithAssertions {
+    @Test
+    public void test_bad() {
+        Embedding embeddingA = Embedding.from(new float[]{1, 1, 1});
+        Embedding embeddingB = Embedding.from(new float[]{1, 1, 1, 1});
+
+        assertThatExceptionOfType(IllegalArgumentException.class)
+                .isThrownBy(() -> CosineSimilarity.between(embeddingA, embeddingB))
+                .withMessage("Length of vector a (3) must be equal to the length of vector b (4)");
+    }
+
+    @Test
+    public void test_zeros() {
+        Embedding embeddingA = Embedding.from(new float[]{0, 0, 0});
+        Embedding embeddingB = Embedding.from(new float[]{0, 0, 0});
+
+        assertThat(CosineSimilarity.between(embeddingA, embeddingB)).isCloseTo(0, withPercentage(1));
+    }
 
     @Test
     void should_calculate_cosine_similarity() {
-        Embedding embeddingA = Embedding.from(new float[]{1, 1, 1});
-        Embedding embeddingB = Embedding.from(new float[]{-1, -1, -1});
+        Embedding embeddingA = Embedding.from(new float[]{1, -1, 1});
+        Embedding embeddingB = Embedding.from(new float[]{-1, 1, -1});
 
         assertThat(CosineSimilarity.between(embeddingA, embeddingA)).isCloseTo(1, withPercentage(1));
         assertThat(CosineSimilarity.between(embeddingA, embeddingB)).isCloseTo(-1, withPercentage(1));