Add duplicates word: finds elements which appear more than once

core/sets/sets-docs.factor

@@ -8,6 +8,7 @@ $nl
 { $subsection prune }
 "Test for duplicates:"
 { $subsection all-unique? }
+{ $subsection duplicates }
 "Set operations on sequences:"
 { $subsection diff }
 { $subsection intersect }
@@ -64,6 +65,13 @@ HELP: prune
     { $example "USING: sets prettyprint ;" "{ 1 1 t 3 t } prune ." "V{ 1 t 3 }" }
 } ;
 
+HELP: duplicates
+{ $values { "seq" "a sequence" } { "newseq" "a sequence" } }
+{ $description "Outputs a new sequence consisting of elements which occur more than once in " { $snippet "seq" } "." }
+{ $examples
+    { $example "USING: sets prettyprint ;" "{ 1 2 3 1 2 1 } duplicates ." "{ 1 2 1 }" }
+} ;
+
 HELP: all-unique?
 { $values { "seq" sequence } { "?" "a boolean" } }
 { $description "Tests whether a sequence contains any repeated elements." }

core/sets/sets.factor

@@ -16,6 +16,9 @@ IN: sets
     [ ] [ length <hashtable> ] [ length <vector> ] tri
     [ [ (prune) ] 2curry each ] keep ;
 
+: duplicates ( seq -- newseq )
+    H{ } clone [ [ key? ] [ conjoin ] 2bi ] curry filter ;
+
 : gather ( seq quot -- newseq )
     map concat prune ; inline