Startup returns a unique_ptr instead of using a static

lib/src/Startup.cpp

@@ -12,19 +12,25 @@
 
 namespace aocpp {
 
-auto Startup(int argc, char ** argv) -> std::istream& {
-  if (std::getenv("DOCTEST")) {
-    exit(doctest::Context{argc, argv}.run());
-  }
+auto Startup(int argc, char ** argv) -> std::unique_ptr<std::istream, ConditionalDeleter> {
+  bool should_delete;
+  std::istream* result_ptr;
 
-  static std::ifstream fin;
   switch (argc) {
-  case 2: fin = std::ifstream{argv[1]}; return fin;
-  case 1: return std::cin;
-  default:
-    std::cerr << "bad arguments\n";
-    exit(EXIT_FAILURE);
+    case 2:
+      should_delete = true;
+      result_ptr = new std::ifstream{argv[1]};
+      break;
+    case 1:
+      should_delete = false;
+      result_ptr = &std::cin;
+      break;
+    default:
+      std::cerr << "bad arguments\n";
+      exit(EXIT_FAILURE);
   }
+
+  return {result_ptr, should_delete};
 }
 
 }