38 lines
830 B
Elixir
38 lines
830 B
Elixir
defmodule AmicableNumbers do
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def divisors(n) do
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[ 1 |
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(for i <- 2..floor(:math.sqrt(n))//1,
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j = div(n, i),
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j == n / i do
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if i !== j, do: [i,j], else: [i]
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end |> Enum.flat_map(&(&1))) ]
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end
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end
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defmodule Abundance do
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def is_abundant?(n) do
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cache = Process.get(:abundance, %{})
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case cache |> Map.get(n) do
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nil ->
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sum = AmicableNumbers.divisors(n) |> Enum.sum()
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is? = sum > n
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Process.put(:abundance, Map.put(cache, n, is?))
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is?
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is? -> is?
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end
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end
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end
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# sum of all numbers that can be expressed as sum of two abundant numbers
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sum = for n <- 1..28123,
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mid = div(n, 2),
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Enum.any?(1..mid, &(Abundance.is_abundant?(&1) and Abundance.is_abundant?(n - &1))),
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reduce: 0
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do
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s -> s + n
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end
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(((28123*28124)/2) - sum)
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|> IO.puts()
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