41 lines
1.1 KiB
Plaintext
41 lines
1.1 KiB
Plaintext
defmodule Permutation do
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def by_swap(n) do
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p = Enum.to_list(0..-n) |> List.to_tuple
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by_swap(n, p, 1)
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end
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defp by_swap(n, p, s) do
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IO.puts "Perm: #{inspect for i <- 1..n, do: abs(elem(p,i))} Sign: #{s}"
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k = 0 |> step_up(n, p) |> step_down(n, p)
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if k > 0 do
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pk = elem(p,k)
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i = if pk>0, do: k+1, else: k-1
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p = Enum.reduce(1..n, p, fn i,acc ->
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if abs(elem(p,i)) > abs(pk), do: put_elem(acc, i, -elem(acc,i)), else: acc
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end)
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pi = elem(p,i)
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p = put_elem(p,i,pk) |> put_elem(k,pi) # swap
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by_swap(n, p, -s)
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end
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end
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defp step_up(k, n, p) do
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Enum.reduce(2..n, k, fn i,acc ->
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if elem(p,i)<0 and abs(elem(p,i))>abs(elem(p,i-1)) and abs(elem(p,i))>abs(elem(p,acc)),
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do: i, else: acc
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end)
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end
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defp step_down(k, n, p) do
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Enum.reduce(1..n-1, k, fn i,acc ->
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if elem(p,i)>0 and abs(elem(p,i))>abs(elem(p,i+1)) and abs(elem(p,i))>abs(elem(p,acc)),
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do: i, else: acc
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end)
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end
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end
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Enum.each(3..4, fn n ->
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Permutation.by_swap(n)
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IO.puts ""
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end)
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