31 lines
873 B
Elixir
31 lines
873 B
Elixir
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defmodule Prime do
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@doc """
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Generates the nth prime.
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"""
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@spec nth(non_neg_integer) :: non_neg_integer
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def nth(count) when count >= 1 do
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0
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|> Stream.iterate(&(&1 + 1))
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|> Stream.filter(&prime?/1)
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|> Stream.take(count + 1)
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|> Enum.to_list()
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|> List.last()
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end
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@spec prime?(non_neg_integer) :: boolean
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defp prime?(number) when number in 1..3, do: true
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defp prime?(number) when rem(number, 2) == 0, do: false
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defp prime?(number) when number > 3, do: check_prime(number)
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defp check_prime(number, prime \\ 3)
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defp check_prime(number, number), do: true
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defp check_prime(number, prime) do
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if prime >= trunc(number ** 0.5) + 1,
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do: true,
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else: rem(number, prime) != 0 && check_prime(number, prime + 2)
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end
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# defp check_prime(number, prime), do: rem(number, prime) != 0 && check_prime(number, prime + 2)
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end
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