fix: remove more String.to_atom calls

lib/crontab/cron_expression/parser.ex

@@ -276,8 +276,7 @@ defmodule Crontab.CronExpression.Parser do
     error_message = "Can't parse #{value} as month."
 
     result =
-      case {Map.fetch(@month_values, String.to_atom(String.upcase(value))),
-            Integer.parse(value, 10)} do
+      case {fetch_month_value(String.upcase(value)), Integer.parse(value, 10)} do
         # No valid month string or integer
         {:error, :error} -> {:error, error_message}
         # Month specified as string
@@ -356,8 +355,7 @@ defmodule Crontab.CronExpression.Parser do
   defp parse_week_day(value) do
     error_message = "Can't parse #{value} as day of week."
 
-    case {Map.fetch(@weekday_values, String.to_atom(String.upcase(value))),
-          Integer.parse(value, 10)} do
+    case {fetch_weekday_value(String.upcase(value)), Integer.parse(value, 10)} do
       {:error, :error} -> {:error, error_message}
       {{:ok, number}, :error} -> {:ok, number}
       {:error, {number, ""}} -> {:ok, number}
@@ -400,6 +398,24 @@ defmodule Crontab.CronExpression.Parser do
     end
   end
 
+  @spec fetch_weekday_value(binary) :: {:ok, integer} | :error
+  Enum.map(@weekday_values, fn {weekday, weekday_number} ->
+    defp fetch_weekday_value(unquote(to_string(weekday))) do
+      {:ok, unquote(weekday_number)}
+    end
+  end)
+
+  defp fetch_weekday_value(_), do: :error
+
+  @spec fetch_month_value(binary) :: {:ok, integer} | :error
+  Enum.map(@month_values, fn {month, month_number} ->
+    defp fetch_month_value(unquote(to_string(month))) do
+      {:ok, unquote(month_number)}
+    end
+  end)
+
+  defp fetch_month_value(_), do: :error
+
   @spec special(binary) :: result
   Enum.map(@specials, fn {special, special_value} ->
     defp special(unquote(to_string(special))) do