diff --git a/.gitignore b/.gitignore
index b6012c7..4f7a773 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,3 +18,6 @@ erl_crash.dump
 
 # Also ignore archive artifacts (built via "mix archive.build").
 *.ez
+
+# asdf-vm version file
+/.tool-versions
diff --git a/lib/ex_junk.ex b/lib/ex_junk.ex
index de89021..1a66923 100644
--- a/lib/ex_junk.ex
+++ b/lib/ex_junk.ex
@@ -35,6 +35,21 @@ defmodule Junk do
     post_op(output, opts)
   end
 
+  def junk(:long_npi, opts) do
+    opts = construct_opts(opts)
+    |> Map.put(:size, 8)
+    # grab 8 digits, and put a 1 or 2 on the front
+    junk(Integer, opts)
+    |> Integer.digits
+    |> (fn digits -> [8,0,8,4,0] ++ [Enum.random(1..2)] ++ digits end).()
+    |> Integer.undigits
+    |> Junk.luhn
+  end
+
+  def junk(:npi, opts) do
+    junk(:long_npi, opts) - trunc(80840 * :math.pow(10, 10))
+  end
+
   def junk(f, opts) when is_function(f) do
     opts = construct_opts(opts)
     apply(f, opts.parameters)
@@ -49,11 +64,38 @@ defmodule Junk do
     end
   end
 
+  def luhn(number) do
+    base_digits = Integer.digits(number)
+    check_sum = base_digits
+    |> Enum.reverse
+    |> Junk.map_every(2, fn(n) ->Integer.digits(n*2) end)
+    |> List.flatten
+    |> Enum.sum
+
+    # calcs the check digit
+    check_digit = case Kernel.rem(check_sum, 10) do
+      0 -> 0
+      n -> 10 - n
+    end
+
+    (base_digits ++ [check_digit]) |> Integer.undigits
+  end
+
+  def map_every(enumerable, nth, mapper) do
+    {res, _acc} = Enum.map_reduce(enumerable, 0, fn(x, i) -> if (rem(i, nth) == 0) do
+        {mapper.(x), i+1}
+      else
+        {x, i+1}
+      end
+    end)
+    res
+  end
+
   defp construct_opts(opts) do
     Enum.into(opts, Map.from_struct(%Junk{}))
   end
 
   defp post_op(output, opts) do
-    output = if opts.prefix, do: "#{opts.prefix}-#{output}", else: output
+    if opts.prefix, do: "#{opts.prefix}-#{output}", else: output
   end
 end
diff --git a/test/ex_junk_test.exs b/test/ex_junk_test.exs
index 3078cc6..418893a 100644
--- a/test/ex_junk_test.exs
+++ b/test/ex_junk_test.exs
@@ -23,6 +23,13 @@ defmodule JunkTest do
     assert output != Junk.junk(String, byte_size: 5)
   end
 
+  test "unnecessary options are ignored" do
+    string = Junk.junk(String, size: 10)
+    integer = Junk.junk(Integer, byte_size: "asdf")
+    assert is_binary(string) == true
+    assert integer |> is_integer == true
+  end
+
   test "returns unique Strings" do
     output = Junk.junk(String)
     assert is_binary(output) == true
@@ -79,6 +86,23 @@ defmodule JunkTest do
     assert "123-45-6789" = output
   end
 
+  test ":npi returns a short NPI" do
+    digits = Junk.junk(:npi) |> Integer.digits
+    assert Kernel.length(digits) == 10
+    assert Enum.member?([1,2], List.first(digits))
+  end
+
+  test ":long_npi returns a long NPI" do
+    digits = Junk.junk(:long_npi) |> Integer.digits
+    assert Kernel.length(digits) == 15
+    first_6 = digits |> Enum.take(6) |> Integer.undigits
+    assert Enum.member?([808401,808402], first_6)
+  end
+
+  test "luhn returns a luhn'd number" do
+    assert Junk.luhn(80840268496713) == 808402684967138
+  end
+
   def ssn do
     "123-45-6789"
   end