Is sort in Ruby stable?

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Both MRI‘s sort and sort_by are unstable. Some time ago there was a request to make them stable, but it was rejected. The reason: Ruby uses an in-place quicksort algorithm, which performs better if stability is not required. Note that you can still implement stable methods from unstable ones:

module Enumerable
  def stable_sort
    sort_by.with_index { |x, idx| [x, idx] }
  end

  def stable_sort_by
    sort_by.with_index { |x, idx| [yield(x), idx] }
  end
end

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