39 lines
1.0 KiB
Plaintext
39 lines
1.0 KiB
Plaintext
use Bitwise, skip_operators: true
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defmodule Partition do
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def init(), do:
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:ets.new :pN, [:set, :named_table, :private]
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def gpentagonals(), do: Stream.unfold {1, 0}, &next/1
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defp next({m, n}) do
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a = case rem m, 2 do
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0 -> div m, 2
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1 -> m
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end
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{n, {m + 1, n + a}}
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end
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def p(0), do: 1
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def p(n) do
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case :ets.lookup :pN, n do
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[{^n, val}] -> val
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[] ->
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{val, _} = gpentagonals()
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|> Stream.drop(1)
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|> Stream.take_while(fn m -> m <= n end)
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|> Stream.map(fn g -> p(n - g) end)
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|> Enum.reduce({0, 0},
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fn n, {a, sgn} -> {
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a + (if sgn < 2, do: n, else: -n),
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band(sgn + 1, 3)
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}
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end)
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:ets.insert :pN, {n, val}
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val
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end
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end
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end
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Partition.init
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IO.puts Partition.p 6666
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