Follow Credo refactor recommendations

lib/gc_index_relay_web/controllers/filter_controller.ex

@@ -74,10 +74,10 @@ defmodule GcIndexRelayWeb.FilterController do
 
   @spec validate_required_params(map()) :: {:ok, map()} | {:error, String.t()}
   def validate_required_params(params) do
-    if not Map.has_key?(params, "limit") do
-      {:error, "The filter must specify a limit."}
-    else
+    if Map.has_key?(params, "limit") do
       {:ok, params}
+    else
+      {:error, "The filter must specify a limit."}
     end
   end
 
@@ -141,11 +141,7 @@ defmodule GcIndexRelayWeb.FilterController do
     |> Enum.reduce_while({:ok, %{}}, fn {key, value}, {:ok, acc} ->
       case parse_param(key, value) do
         {:ok, parsed_value} ->
-          out_key =
-            if byte_size(key) == 1 and
-                 ((key >= "a" and key <= "z") or (key >= "A" and key <= "Z")),
-               do: "#" <> key,
-               else: key
+          out_key = parse_tag(key)
 
           {:cont, {:ok, Map.put(acc, out_key, parsed_value)}}
 
@@ -155,6 +151,16 @@ defmodule GcIndexRelayWeb.FilterController do
     end)
   end
 
+  # Parse filter keys that represent tags. Note that only single-letter keys are treated as tags;
+  # all other keys are passed through unchanged.
+  @spec parse_tag(String.t()) :: String.t()
+  defp parse_tag(key) do
+    if byte_size(key) == 1 and
+         ((key >= "a" and key <= "z") or (key >= "A" and key <= "Z")),
+       do: "#" <> key,
+       else: key
+  end
+
   # Parse individual parameter based on its key
   @spec parse_param(String.t(), String.t()) :: {:ok, any()} | {:error, String.t()}
   defp parse_param("ids", value), do: {:ok, String.split(value, ",")}