1 files changed, 121 insertions, 0 deletions
diff --git a/src/libexpr/eval-cache.hh b/src/libexpr/eval-cache.hh
new file mode 100644
index 000000000..afee85fa9
--- /dev/null
+++ b/src/libexpr/eval-cache.hh
@@ -0,0 +1,121 @@
+#pragma once
+
+#include "sync.hh"
+#include "hash.hh"
+#include "eval.hh"
+
+#include <functional>
+#include <variant>
+
+namespace nix::eval_cache {
+
+class AttrDb;
+class AttrCursor;
+
+class EvalCache : public std::enable_shared_from_this<EvalCache>
+{
+    friend class AttrCursor;
+
+    std::shared_ptr<AttrDb> db;
+    EvalState & state;
+    typedef std::function<Value *()> RootLoader;
+    RootLoader rootLoader;
+    RootValue value;
+
+    Value * getRootValue();
+
+public:
+
+    EvalCache(
+        std::optional<std::reference_wrapper<const Hash>> useCache,
+        EvalState & state,
+        RootLoader rootLoader);
+
+    std::shared_ptr<AttrCursor> getRoot();
+};
+
+enum AttrType {
+    Placeholder = 0,
+    FullAttrs = 1,
+    String = 2,
+    Missing = 3,
+    Misc = 4,
+    Failed = 5,
+    Bool = 6,
+};
+
+struct placeholder_t {};
+struct missing_t {};
+struct misc_t {};
+struct failed_t {};
+typedef uint64_t AttrId;
+typedef std::pair<AttrId, Symbol> AttrKey;
+typedef std::pair<std::string, std::vector<std::pair<Path, std::string>>> string_t;
+
+typedef std::variant<
+    std::vector<Symbol>,
+    string_t,
+    placeholder_t,
+    missing_t,
+    misc_t,
+    failed_t,
+    bool
+    > AttrValue;
+
+class AttrCursor : public std::enable_shared_from_this<AttrCursor>
+{
+    friend class EvalCache;
+
+    ref<EvalCache> root;
+    typedef std::optional<std::pair<std::shared_ptr<AttrCursor>, Symbol>> Parent;
+    Parent parent;
+    RootValue _value;
+    std::optional<std::pair<AttrId, AttrValue>> cachedValue;
+
+    AttrKey getKey();
+
+    Value & getValue();
+
+public:
+
+    AttrCursor(
+        ref<EvalCache> root,
+        Parent parent,
+        Value * value = nullptr,
+        std::optional<std::pair<AttrId, AttrValue>> && cachedValue = {});
+
+    std::vector<Symbol> getAttrPath() const;
+
+    std::vector<Symbol> getAttrPath(Symbol name) const;
+
+    std::string getAttrPathStr() const;
+
+    std::string getAttrPathStr(Symbol name) const;
+
+    std::shared_ptr<AttrCursor> maybeGetAttr(Symbol name);
+
+    std::shared_ptr<AttrCursor> maybeGetAttr(std::string_view name);
+
+    std::shared_ptr<AttrCursor> getAttr(Symbol name);
+
+    std::shared_ptr<AttrCursor> getAttr(std::string_view name);
+
+    std::shared_ptr<AttrCursor> findAlongAttrPath(const std::vector<Symbol> & attrPath);
+
+    std::string getString();
+
+    string_t getStringWithContext();
+
+    bool getBool();
+
+    std::vector<Symbol> getAttrs();
+
+    bool isDerivation();
+
+    Value & forceValue();
+
+    /* Force creation of the .drv file in the Nix store. */
+    StorePath forceDerivation();
+};
+
+}