add class dictionary to avoid storing strings all the time

src/style/class_dictionary.cpp

@@ -0,0 +1,29 @@
+#include <llmr/style/class_dictionary.hpp>
+
+namespace llmr {
+
+ClassID ClassDictionary::Lookup(const std::string &class_name) {
+    auto it = store.find(class_name);
+    if (it == store.end()) {
+        // Insert the class name into the store.
+        ClassID id = ClassID(uint32_t(ClassID::Named) + offset++);
+        store.emplace(class_name, id);
+        return id;
+    } else {
+        return it->second;
+    }
+}
+
+ClassID ClassDictionary::Normalize(ClassID id) {
+    if (id >= ClassID::Named) {
+        return ClassID::Named;
+    } else {
+        return id;
+    }
+}
+
+
+std::unordered_map<std::string, ClassID> ClassDictionary::store = { { "", ClassID::Default } };
+uint32_t ClassDictionary::offset = 0;
+
+}