A fast version of Enumerable#group_by.

gistfile1.rb

# Basis for fast_group_by gem: http://github.com/Narnach/fast_group_by
module Enumerable
  # An unordered group_by implementation that is a gazillion times faster than
  # ActiveSupport's ordered version, when used with large data sets.
  # ActiveSupport uses OrderedHash, which are nested Arrays. Array#assoc is
  # way slower than Hash#[] because it does a linear search vs a tree search.
  # For small data sets, the difference is not that obvious, but in the range
  # of 50k+ items, the difference can be 60 seconds vs less than 1 second.
  def fast_group_by(&block)
    res = Hash.new { |hash, key| hash[key] = [] }
    each do |e|
      res[block.call(e)] << e
    end
    res
  end
end