add class dictionary to avoid storing strings all the time

include/llmr/style/class_dictionary.hpp

@@ -0,0 +1,32 @@
+#ifndef LLMR_STYLE_CLASS_DICTIONARY
+#define LLMR_STYLE_CLASS_DICTIONARY
+
+#include <cstdint>
+#include <string>
+#include <unordered_map>
+
+namespace llmr {
+
+enum class ClassID : uint32_t {
+    Fallback = 0, // These values are from the fallback properties
+    Default = 1, // These values are from the default style for a layer
+    Named = 2 // These values (and all subsequent IDs) are from a named style from the layer
+};
+
+class ClassDictionary {
+public:
+    // Returns an ID for a class name. If the class name does not yet have an ID, one is
+    // auto-generated and stored for future reference.
+    static ClassID Lookup(const std::string &class_name);
+
+    // Returns either Fallback, Default or Named, depending on the type of the class id.
+    static ClassID Normalize(ClassID id);
+
+private:
+    static std::unordered_map<std::string, ClassID> store;
+    static uint32_t offset;
+};
+
+}
+
+#endif

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;
+
+}