luhn redo

Author: neenjaw

luhn/lib/luhn.ex

@@ -4,5 +4,34 @@ defmodule Luhn do
   """
   @spec valid?(String.t()) :: boolean
   def valid?(number) do
+    number =
+      number
+      |> String.replace(" ", "")
+      |> String.graphemes()
+
+    state = %{
+      digit_count: 0,
+      sum: 0,
+      double_digit?: length(number) |> rem(2) == 0
+    }
+
+    with %{digit_count: c, sum: s} <- Enum.reduce_while(number, state, &reduce_luhn/2) do
+      c > 1 and rem(s, 10) == 0
+    else
+      _ -> false
+    end
+  end
+
+  def reduce_luhn(digit, state) do
+    with {digit, ""} <- Integer.parse(digit),
+         digit <- (if state.double_digit?, do: digit * 2, else: digit),
+         digit <- (if digit > 9, do: digit - 9, else: digit),
+         sum <- state.sum + digit,
+         digit_count <- state.digit_count + 1,
+         double_digit? <- not state.double_digit? do
+      {:cont, %{digit_count: digit_count, sum: sum, double_digit?: double_digit?}}
+    else
+      _ -> {:halt, {:error, "invalid number"}}
+    end
   end
 end

luhn/luhn_test.exs

@@ -5,62 +5,50 @@ defmodule LuhnTest do
     refute Luhn.valid?("1")
   end
 
-  @tag :pending
   test "A single zero is invalid" do
     refute Luhn.valid?("0")
   end
 
-  @tag :pending
   test "a simple valid SIN that remains valid if reversed" do
     assert Luhn.valid?("059")
   end
 
-  @tag :pending
   test "a simple valid SIN that becomes invalid if reversed" do
     assert Luhn.valid?("59")
   end
 
-  @tag :pending
   test "a valid Canadian SIN" do
     assert Luhn.valid?("055 444 285")
   end
 
-  @tag :pending
   test "invalid Canadian SIN" do
     refute Luhn.valid?("055 444 286")
   end
 
-  @tag :pending
   test "invalid credit card" do
     refute Luhn.valid?("8273 1232 7352 0569")
   end
 
-  @tag :pending
   test "valid strings with a non-digit included become invalid" do
     refute Luhn.valid?("055a 444 285")
   end
 
-  @tag :pending
   test "valid strings with punctuation included become invalid" do
     refute Luhn.valid?("055-444-285")
   end
 
-  @tag :pending
   test "valid strings with symbols included become invalid" do
     refute Luhn.valid?("055£ 444$ 285")
   end
 
-  @tag :pending
   test "single zero with space is invalid" do
     refute Luhn.valid?(" 0")
   end
 
-  @tag :pending
   test "more than a single zero is valid" do
     assert Luhn.valid?("0000 0")
   end
 
-  @tag :pending
   test "input digit 9 is correctly converted to output digit 9" do
     assert Luhn.valid?("091")
   end