34 lines
597 B
Ruby
34 lines
597 B
Ruby
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def pentagonal?(n)
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((Math.sqrt((24 * n) + 1) + 1) / 6) % 1 == 0
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end
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def nth_pentagonal(n)
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n * (3 * n - 1) / 2
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end
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def pentagonal
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pentagonal_enum = Enumerator.new do |y|
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n = 1
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loop do
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x = n * (3 * n - 1) / 2
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n += 1
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y << x
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end
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end
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pentagonal_enum
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end
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def sum_and_difference_pentagonal_up_to(n)
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x_list = pentagonal.take_while { |x| x < n }
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y_list = pentagonal.take_while { |y| y < n }
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solutions = []
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x_list.each do |x|
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y_list.each do |y|
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solutions << [x,y] if pentagonal?(x + y) && pentagonal?((x - y).abs)
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end
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end
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solutions
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end
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