Sorting an Array by Frequency of Appearance

enumerable.rb

module Enumerable
  def to_histogram
    inject(Hash.new(0)) { | h, x | h[x] += 1; h }
  end
  
  def sort_by_frequency
    histogram = inject(Hash.new(0)) { | hash, x | hash[x] += 1; hash }
    sort_by { | x | [histogram[x], x] }
  end

  def sort_by_frequency_faster
    histogram = inject(Hash.new(0)) { | hash, x | hash[x] += 1; hash }
    sort_by { | x | histogram[x] }
  end

  def sort_by_frequency_descending
    histogram = inject(Hash.new(0)) { | hash, x | hash[x] += 1; hash }
    sort_by { | x | [histogram[x] * -1, x] }
  end

  def sort_distinct_by_frequency
    histogram = inject(Hash.new(0)) { | hash, x | hash[x] += 1; hash }
    histogram.keys.sort_by { | x | [histogram[x], x] }
  end
end

Source: Ruby Cookbook https://www.safaribooksonline.com/library/view/ruby-cookbook/0596523696/ch04s09.html