Add ProjectIndex and basic symbol lookup implementation (#274)

include/Index/ProjectIndex.h

@@ -0,0 +1,57 @@
+#pragma once
+
+#include "TUIndex.h"
+
+namespace clice::index {
+
+struct PathPool {
+    llvm::BumpPtrAllocator allocator;
+
+    std::vector<llvm::StringRef> paths;
+
+    llvm::DenseMap<llvm::StringRef, std::uint32_t> cache;
+
+    llvm::StringRef save(llvm::StringRef s) {
+        auto data = allocator.Allocate<char>(s.size() + 1);
+        std::ranges::copy(s, data);
+        data[s.size()] = '\0';
+        return llvm::StringRef(data, s.size());
+    }
+
+    auto path_id(llvm::StringRef path) {
+        assert(!path.empty());
+        auto [it, success] = cache.try_emplace(path, paths.size());
+        if(!success) {
+            return it->second;
+        }
+
+        auto& [k, v] = *it;
+        k = save(path);
+        paths.emplace_back(k);
+        return it->second;
+    }
+
+    llvm::StringRef path(std::uint32_t id) {
+        return paths[id];
+    }
+};
+
+struct FileInfo {
+    std::int64_t mtime;
+};
+
+struct ProjectIndex {
+    PathPool path_pool;
+
+    llvm::DenseMap<std::uint32_t, std::uint32_t> indices;
+
+    SymbolTable symbols;
+
+    void merge(this ProjectIndex& self, TUIndex& index);
+
+    void serialize(this ProjectIndex& self, llvm::raw_ostream& os);
+
+    static ProjectIndex from(const void* data);
+};
+
+}  // namespace clice::index