Permutations

Author: Ben Schmeckpeper

permutations.exs

@@ -0,0 +1,29 @@
+defmodule Permutations do
+  def of([]), do: [[]]
+  def of(set) do
+    Enum.flat_map(set, fn(x) ->
+      subset = of(List.delete(set, x))
+      Enum.map(subset, fn(p) -> [x | p] end)
+    end)
+  end
+end
+
+ExUnit.start
+
+defmodule PermutationTests do
+  use ExUnit.Case, async: false
+
+  test "permutations of an empty list is an empty list" do
+    assert Permutations.of([]) == [[]]
+  end
+
+  test "permutations of a single item list is the item" do
+    assert Permutations.of([1]) == [[1]]
+  end
+  
+  test "it computes permutations" do
+     assert Permutations.of([1, 2, 3]) == [[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]
+  end
+end
+
+