Ensure embeddings matrix is never initialized with zero dimensions

* Because we divide by 1.5; in the edge case were we have 1 example we'd
  end up rounding down to 0 which will cause problems when we try to initialize a matrix to that size.
Fix #248

app/pkg/learn/in_memory.go

@@ -209,12 +209,8 @@ func (db *InMemoryExampleDB) loadExamples(ctx context.Context) error {
 			db.examples = make([]*v1alpha1.Example, 0, len(matches))
 		}
 
-		// We intentionally initialize an initial matrix which is too small so that during the initial load
-		// grow will be triggered. Since we grow by a factor of two we should end up with an overallocated matrix
-		// This means that by default the matrix should contain extra rows that haven't been populated with examples
-		// yet. This way we can verify that doesn't trip up rag
 		if db.embeddings == nil {
-			db.embeddings = mat.NewDense(int(float32(len(matches))/1.5), oai.SmallEmbeddingsDims, nil)
+			db.embeddings = mat.NewDense(initialNumberOfRows(len(matches)), oai.SmallEmbeddingsDims, nil)
 		}
 
 		// Load the examples.
@@ -229,6 +225,19 @@ func (db *InMemoryExampleDB) loadExamples(ctx context.Context) error {
 	return nil
 }
 
+func initialNumberOfRows(numExamples int) int {
+	// We intentionally initialize an initial matrix which is too small so that during the initial load
+	// grow will be triggered. Since we grow by a factor of two we should end up with an overallocated matrix
+	// This means that by default the matrix should contain extra rows that haven't been populated with examples
+	// yet. This way we can verify that doesn't trip up rag
+	size := int(float32(numExamples) / 1.5)
+	// If size is < 1 we end up initializing an empty matrix which will cause a panic
+	if size < 1 {
+		size = 1
+	}
+	return size
+}
+
 func (db *InMemoryExampleDB) Shutdown(ctx context.Context) error {
 	log := logs.FromContext(ctx)
 

app/pkg/learn/in_memory_test.go

@@ -254,3 +254,38 @@ func Test_UpdateExample(t *testing.T) {
 		})
 	}
 }
+
+func Test_initialNumberOfRows(t *testing.T) {
+	type testCase struct {
+		name        string
+		numExamples int
+		expected    int
+	}
+
+	cases := []testCase{
+		{
+			name:        "edge-case-1",
+			numExamples: 1,
+			expected:    1,
+		},
+		{
+			name:        "small",
+			numExamples: 10,
+			expected:    6,
+		},
+		{
+			name:        "large",
+			numExamples: 100,
+			expected:    66,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			actual := initialNumberOfRows(c.numExamples)
+			if actual != c.expected {
+				t.Errorf("Expected %v but got %v", c.expected, actual)
+			}
+		})
+	}
+}