Unique pairs

Author: bschmeck

exercise_2.40.exs

@@ -0,0 +1,57 @@
+defmodule UniquePairs do
+  def of(n) when n < 2, do: []
+  
+  def of(n) do
+    Enum.flat_map(2..n, fn(i) ->
+      Enum.map(1..(i-1), fn(j) -> [i, j] end)
+    end)
+  end
+end
+
+defmodule PrimeSumPairs do
+  def of(n) when n < 2, do: []
+
+  def of(n) do
+    Enum.filter(UniquePairs.of(n), &prime_sum?/1)
+  end
+
+  defp prime_sum?(pair) do
+    i = hd pair
+    j = hd tl pair
+    PrimeChecker.is_prime?(i + j)
+  end
+end
+
+defmodule PrimeChecker do
+  def is_prime?(n), do: n == smallest(n)
+
+  defp smallest(n), do: find(n, 2)
+
+  defp find(n, test) when test * test > n, do: n
+  defp find(n, test) when rem(n, test) == 0, do: test
+  defp find(n, test), do: find(n, test + 1)
+end
+
+
+ExUnit.start
+
+defmodule UniquePairsTests do
+  use ExUnit.Case, async: true
+
+  test "no pairs when n is 1" do
+    assert UniquePairs.of(1) == []
+  end
+
+  test "generates all pairs" do
+    assert UniquePairs.of(3) == [[2, 1], [3, 1], [3, 2]]
+  end
+end
+
+defmodule PrimeSumPairsTests do
+  use ExUnit.Case, async: true
+
+  test "it generates the expected pairs" do
+    assert PrimeSumPairs.of(6) == [[2, 1], [3, 2], [4, 1], [4, 3], [5, 2], [6, 1], [6, 5]]
+  end
+end
+