Implement `Enumerable` tally from Ruby2.7.

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

@@ -839,4 +839,21 @@ module Enumerable
   end
 
   alias filter select
+
+  ##
+  # call-seq:
+  #   enum.tally -> a_hash
+  #
+  # Tallys the collection.  Returns a hash where the keys are the
+  # elements and the values are numbers of elements in the collection
+  # that correspond to the key.
+  #
+  #    ["a", "b", "c", "b"].tally #=> {"a"=>1, "b"=>2, "c"=>1}
+  def tally
+    hash = {}
+    self.each do |x|
+      hash[x] = (hash[x]||0)+1
+    end
+    hash
+  end
 end

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

@@ -189,7 +189,10 @@ assert("Enumerable#to_h") do
   assert_equal({1=>4,3=>8}, c.new.to_h{|k,v|[k,v*2]})
 end
 
-
 assert("Enumerable#filter_map") do
   assert_equal [4, 8, 12, 16, 20], (1..10).filter_map{|i| i * 2 if i%2==0}
 end
+
+assert("Enumerable#tally") do
+  assert_equal({"a"=>1, "b"=>2, "c"=>1}, ["a", "b", "c", "b"].tally)
+end