A look at how to tally a collection in Ruby, from Ruby 1.9 to 2.7.

tally.rb

# Ruby 1.9
Hash[collection.group_by { |n| n }.map { |k, v| [k, v.size] }]

# Ruby 2.0 Array#to_h
collection.group_by { |n| n }.map { |k, v| [k, v.size] }.to_h

# Ruby 2.2 Object#itself
collection.group_by(&:itself).map { |k, v| [k, v.size] }.to_h

# Ruby 2.4 Hash#transform_values
collection.group_by(&:itself).transform_values(&:size)

# Ruby 2.7 Enumerable#tally
collection.tally