assocs.extras: Add a histogram-diff for fun.

diff --git a/extra/assocs/extras/extras.factor b/extra/assocs/extras/extras.factor
index 9a6ba7553e8ec9ccca0e9ae628edc450ec675043..69a2c3cbce77811544fadc58ee916446fd22f11a 100644 (file)
--- a/extra/assocs/extras/extras.factor
+++ b/extra/assocs/extras/extras.factor
@@ -230,3 +230,7 @@ PRIVATE>
 : collect-value-by ( ... seq quot: ( ... obj -- ... key ) -- ... assoc )
     [ keep swap ] curry H{ } clone
     [ '[ @ [ second ] dip _ push-at ] each ] keep ; inline
+
+: histogram-diff ( hashtable1 hashtable2 -- hashtable3 )
+    [ neg swap pick at+ ] assoc-each
+    [ 0 > ] filter-values ;