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lib/utils/permutations.ex
defmodule Permutations do
@moduledoc """
Helper to calculate all the permutations of the list given.
The maximal allowed number of permutations is limited to 12.
"""
Enum.each(2..12, fn n ->
@n n
defmacrop unquote(:"do_#{@n}!")(l) do
clause = fn i -> {:<-, [], [{:"i#{i}", [], Elixir}, l]} end
return = Enum.map(1..@n, fn i -> {:"i#{i}", [], Elixir} end)
Enum.reduce(1..@n, return, fn i, acc ->
{:for, [], [clause.(i), [do: acc]]}
end)
end
end)
@doc "Calculates all permutations of an empty list, returning an empty list"
@spec do_0 :: []
def do_0(), do: []
@doc "Calculates all permutations of an empty list, returning an empty list"
@spec do_0([]) :: []
def do_0([]), do: []
@doc """
Calculates all permutations of a list having a single keyword element,
returning the keyword wrapped into a list.
"""
@spec do_1(kw :: keyword()) :: [keyword()]
def do_1(kw), do: [kw]
Enum.each(2..12, fn n ->
@n n
@doc "Calculates the permutations of `#{n}` for the list given"
def unquote(:"do_#{@n}")(list),
do: Enum.flat_map(unquote(:"do_#{@n}!")(list), & &1)
end)
end