Add Array#intersection which is new in Ruby2.7.

mrbgems/mruby-array-ext/mrblib/array.rb

@@ -173,6 +173,24 @@ class Array
     array
   end
 
+  ##
+  # call-seq:
+  #    ary.intersection(other_ary,...)  -> new_ary
+  #
+  # Set Intersection---Returns a new array containing elements common to
+  # this array and <i>other_ary</i>, removing duplicates.
+  #
+  #    ["a", "b", "c"].union(["c", "d", "a"], ["a", "c", "e"])
+  #           #=> ["a", "b", "c", "d", "e"]
+  #
+  def intersection(*args)
+    ary = self
+    args.each do |x|
+      ary = ary & x
+    end
+    ary
+  end
+
   ##
   # call-seq:
   #    ary.flatten -> new_ary

mrbgems/mruby-array-ext/test/array.rb

@@ -111,6 +111,14 @@ assert("Array#&") do
   assert_equal [1, 2, 3, 1], a
 end
 
+assert("Array#intersection") do
+  a = [1, 2, 3, 1, 8, 6, 7, 8]
+  b = [1, 4, 6, 8]
+  c = [1, 5, 7, 8]
+
+  assert_equal [1, 8], a.intersection(b,c)
+end
+
 assert("Array#flatten") do
   assert_equal [1, 2, "3", {4=>5}, :'6'],    [1, 2, "3", {4=>5}, :'6'].flatten
   assert_equal [1, 2, 3, 4, 5, 6], [1, 2,    [3, 4, 5], 6].flatten