make nils comparable

src/metabase/feature_extraction/comparison.clj

@@ -75,6 +75,10 @@
   [a b]
   {:difference 1})
 
+(defmethod difference [nil nil]
+  [a b]
+  {:difference 0})
+
 (defn chi-squared-distance
   "Chi-squared distane between empirical probability distributions `p` and `q`.
    http://www.aip.de/groups/soe/local/numres/bookcpdf/c14-3.pdf"

test/metabase/feature_extraction/comparison_test.clj

@@ -28,7 +28,8 @@
    1
    1
    0
-   0.25]
+   0.25
+   0]
   (mapv :difference [(difference 1 2.0)
                      (difference 2.0 2.0)
                      (difference 2.0 nil)
@@ -37,7 +38,8 @@
                      (difference true false)
                      (difference false true)
                      (difference false false)
-                     (difference [1 0 1] [0 1 1])]))
+                     (difference [1 0 1] [0 1 1])
+                     (difference nil nil)]))
 
 (expect
   true