48 lines
1.6 KiB
Plaintext
48 lines
1.6 KiB
Plaintext
defmodule Prime do
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defp left_truncatable?(n, prime) do
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func = fn i when i<=9 -> 0
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i -> to_string(i) |> String.slice(1..-1) |> String.to_integer end
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truncatable?(n, prime, func)
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end
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defp right_truncatable?(n, prime) do
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truncatable?(n, prime, fn i -> div(i, 10) end)
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end
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defp truncatable?(n, prime, trunc_func) do
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if to_string(n) |> String.match?(~r/0/),
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do: false,
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else: trunc_loop(trunc_func.(n), prime, trunc_func)
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end
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defp trunc_loop(0, _prime, _trunc_func), do: true
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defp trunc_loop(n, prime, trunc_func) do
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if elem(prime,n), do: trunc_loop(trunc_func.(n), prime, trunc_func), else: false
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end
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def eratosthenes(limit) do # descending order
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Enum.to_list(2..limit) |> sieve(:math.sqrt(limit), [])
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end
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defp sieve([h|_]=list, max, sieved) when h>max, do: Enum.reverse(list, sieved)
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defp sieve([h | t], max, sieved) do
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list = for x <- t, rem(x,h)>0, do: x
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sieve(list, max, [h | sieved])
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end
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defp prime_table(_, [], list), do: [false, false | list]
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defp prime_table(n, [n|t], list), do: prime_table(n-1, t, [true|list])
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defp prime_table(n, prime, list), do: prime_table(n-1, prime, [false|list])
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def task(limit \\ 1000000) do
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prime = eratosthenes(limit)
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prime_tuple = prime_table(limit, prime, []) |> List.to_tuple
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left = Enum.find(prime, fn n -> left_truncatable?(n, prime_tuple) end)
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IO.puts "Largest left-truncatable prime : #{left}"
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right = Enum.find(prime, fn n -> right_truncatable?(n, prime_tuple) end)
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IO.puts "Largest right-truncatable prime: #{right}"
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end
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end
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Prime.task
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