Created
July 3, 2019 17:08
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| defmodule Prime do | |
| @doc """ | |
| Generates the nth prime. | |
| """ | |
| @spec nth(non_neg_integer) :: non_neg_integer | |
| def nth(n) when n < 1, do: raise :error | |
| def nth(n) do | |
| Stream.iterate(2, &(&1 + 1)) | |
| |> Stream.filter(&prime?/1) | |
| |> Enum.at(n - 1) | |
| end | |
| @spec prime?(non_neg_integer) :: boolean | |
| defp prime?(number), do: number |> factors |> Enum.empty? | |
| defguardp is_divisible(x, y) when rem(x, y) == 0 | |
| @spec factors(non_neg_integer, non_neg_integer, [non_neg_integer]) :: [non_neg_integer] | |
| defp factors(number, i \\ 2, acc \\ []) | |
| defp factors(number, i, acc) when number < i * i, do: acc | |
| defp factors(number, i, acc) when is_divisible(number, i), do: factors(number, i + 1, [i, div(number, i) | acc]) | |
| defp factors(number, i, acc), do: factors(number, i + 1, acc) | |
| end |
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