62 lines
1.5 KiB
Elixir
62 lines
1.5 KiB
Elixir
# TODO: this gets the correct answer for project Euler, but is not correct in the general case.
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# Figure that out.
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defmodule Permutations do
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def index_at(_, 0), do: []
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def index_at(n, remaining) do
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blk_size = Euler.Algebra.fact(remaining - 1)
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[ div(n, blk_size) | index_at(rem(n, blk_size), remaining - 1) ]
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end
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def translate(a, n) when is_number(n), do: translate(a, (0..(n-1)) |> Enum.to_list)
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def translate(_, []), do: []
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def translate([next | rest], allowed) do
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[ (allowed |> Enum.at(next)) | translate(rest, allowed |> List.delete_at(next)) ]
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end
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end
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Permutations.index_at(1_000_000, 10)
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|> Permutations.translate(0..9 |> Enum.to_list)
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|> Enum.join("")
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|> IO.puts()
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print_permutations = fn n ->
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(0..(Euler.Algebra.fact(n)-1))
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|> Enum.map(fn i ->
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Permutations.index_at(i, n)
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|> Permutations.translate(n)
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|> Enum.join("")
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|> IO.puts()
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end)
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end
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print_permutations.(5)
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# 999990 - 2783914056
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# 999991 - 2783914065
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# 999992 - 2783914506
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# 999993 - 2783914560
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# 999994 - 2783914605
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# 999995 - 2783914650
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# 999996 - 2783915046
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# 999997 - 2783915064
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# 999998 - 2783915406
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# 999999 - 2783915460
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# 1000000 - 2783915604
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# 1000001 - 2783915640
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# 1000002 - 2783916045
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# 1000003 - 2783916054
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# 1000004 - 2783916405
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# 1000005 - 2783916450
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# 1000006 - 2783916504
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# 1000007 - 2783916540
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# 1000008 - 2783940156
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# 1000009 - 2783940165
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# 1000010 - 2783940516
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999_990..1_000_010
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|> Enum.map(fn i ->
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n = Permutations.index_at(i, 10)
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|> Permutations.translate(10)
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|> Enum.join("")
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IO.puts("#{i} - #{n}")
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end)
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