match different casings

Intermediate steps not committed - addresses_as_list, because long stack
trace in output. turned out 'nil' case needs to be explicit now. I
prefer the small functions though instead of whole implementation in
case statement.

lib/basic_signup_whitelist.ex

@@ -1,10 +1,18 @@
 defmodule BasicSignupWhitelist do
   @moduledoc """
-  Documentation for `BasicSignupWhitelist`.
+  Checks if an email address is allowed based on the SIGNUP_ALLOWED_EMAILS env variable.
   """
 
   @doc """
-  false
+  Checks if an email address is allowed based on the SIGNUP_ALLOWED_EMAILS env variable.
+
+  Rules:
+    - Returns true if ALLOWED_EMAILS == "*" (all emails allowed)
+    - Returns true if email matches any entry in the comma-separated list
+    - Returns false if:
+        * Environment variable doesn't exist
+        * Environment variable is empty string
+        * Email not found in whitelist
 
   ## Examples
 

test/fun_core/basic_signup_whitelist_test.exs

@@ -0,0 +1,57 @@
+defmodule FunCore.BasicSignupWhitelistTest do
+  use ExUnit.Case
+
+  defp normalize(email) do
+    email |> String.trim() |> String.downcase()
+  end
+
+  defp addresses_as_list(addresses_str) do
+    addresses_str
+    |> String.split(",")
+    |> Enum.map(&normalize/1)
+  end
+
+  def mail_whitelisted_fun(signups_allowed, email_received) do
+    case signups_allowed do
+      nil -> false
+      "*" -> true
+      list_str -> normalize(email_received) in addresses_as_list(list_str)
+    end
+  end
+
+  test "addresses_as_list" do
+    assert addresses_as_list("joe@example.com, jane@example.com") == [
+             "joe@example.com",
+             "jane@example.com"
+           ]
+  end
+
+  test "jane in list" do
+    lst = addresses_as_list("joe@example.com, jane@example.com")
+    assert "jane@example.com" in lst
+  end
+
+  describe "Not whitelisted when allowed list is" do
+    test "not set" do
+      refute(mail_whitelisted_fun(nil, "joe@example.com"))
+    end
+
+    test "empty" do
+      refute(mail_whitelisted_fun("j", "joe@example.com"))
+    end
+  end
+
+  describe "Whitelisted when" do
+    test "*" do
+      assert(mail_whitelisted_fun("*", "jane@example.com"))
+    end
+
+    test "Multiple set and one match" do
+      assert(mail_whitelisted_fun("joe@example.com, jane@example.com", "jane@example.com"))
+    end
+
+    test "Matches with different casings" do
+      assert(mail_whitelisted_fun("joe@Example.com, jane@example.com", "Joe@example.com"))
+    end
+  end
+end