add brute-force search command for game seeds that result in rare enemies

src/FileContentsCache.cc

@@ -74,3 +74,25 @@ FileContentsCache::GetResult FileContentsCache::get(const char* name,
     std::function<std::string(const std::string&)> generate) {
   return this->get(string(name), generate);
 }
+
+shared_ptr<const string> ThreadSafeFileCache::get(
+    const string& name, std::function<shared_ptr<const string>(const std::string&)> generate) {
+  try {
+    shared_lock g(this->lock);
+    auto ret = this->name_to_file.at(name);
+    if (!ret) {
+      throw cannot_open_file(name);
+    }
+    return ret;
+  } catch (const out_of_range&) {
+    unique_lock g(this->lock);
+    auto it = this->name_to_file.find(name);
+    if (it == this->name_to_file.end()) {
+      it = this->name_to_file.emplace(name, generate(name)).first;
+    }
+    if (!it->second) {
+      throw cannot_open_file(name);
+    }
+    return it->second;
+  }
+}