stable_sort_by (Ruby)

stable_sort_by.rb

class Array
  def stable_sort_by
    n = 0
    sort_by { |x| [yield(x), n += 1] }
  end
end

p [[83, 'annette'], [82, 'kate'], [82, 'lucy']].sort_by { |x| x[0] }
p [[83, 'annette'], [82, 'kate'], [82, 'lucy']].stable_sort_by { |x| x[0] }
# p [[83, 'annette'], [82, 'kate'], [82, 'lucy']].stable_sort_by #=> no block given (yield) (LocalJumpError)

# ref. http://ideone.com/x3jSc
#      http://moserei.de/2008/09/18/stable-array-sorting-in-ruby.html
#      http://d.hatena.ne.jp/troter/20070514/1179073399

Also: sort_by.with_index { |x, idx| [yield(x), idx] }