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# frozen_string_literal: true
module Enumerable
# Returns duplicate elements from +self+.
#
# If a block is given, it will use the return value of the
# given block for comparison.
#
# @return [Array<Array>]
# @example
# [1, 2, 3].non_uniq
# #=> []
#
# [1, 1, 2, 3].non_uniq
# #=> [[1, 1]]
#
# [1, 1, 1, 2, 3].non_uniq
# #=> [[1, 1, 1]]
#
# [1, 2, 3, 4].non_uniq { |x| x * 0 }
# #=> [[1, 2, 3, 4]]
#
def non_uniq(&block)
block ||= proc { |item| item }
group_by(&block).select { |_, occurrences| occurrences.count > 1 }.values
end
end
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