Enumerable#sequenced_group_by

module Enumerable
  # like general <tt>group_by</tt>, but keeps order of original collection
  def sequenced_group_by
    groups, hash = [], {}
    each do |element|
      key = yield(element)
      groups << [key, hash[key]=[]] unless hash.has_key?(key)
      hash[key] << element
    end
    groups
  end
end

Example

  def test_sequenced_group_by
    input = ["a2", "c1", "b1", "f1", "a1"]
    result = [["a", ["a2", "a1"]], ["c", ["c1"]], ["b", ["b1"]], ["f", ["f1"]]]
    assert_not_equal result, input.group_by{|x| x.at(0)}
    assert_equal result, input.sequenced_group_by{|x| x.at(0)}
  end