1 files changed, 69 insertions, 0 deletions
diff --git a/third_party/nix/src/libexpr/symbol-table.hh b/third_party/nix/src/libexpr/symbol-table.hh
new file mode 100644
index 000000000000..c2599658859f
--- /dev/null
+++ b/third_party/nix/src/libexpr/symbol-table.hh
@@ -0,0 +1,69 @@
+#pragma once
+
+#include <absl/container/node_hash_set.h>
+#include <absl/strings/string_view.h>
+
+namespace nix {  // TODO(tazjin): ::expr
+
+// TODO(tazjin): Replace with a simpler struct, or get rid of.
+class Symbol {
+ private:
+  const std::string* s;  // pointer into SymbolTable
+  Symbol(const std::string* s) : s(s){};
+  friend class SymbolTable;
+
+ public:
+  bool operator==(const Symbol& s2) const { return s == s2.s; }
+
+  bool operator!=(const Symbol& s2) const { return s != s2.s; }
+
+  bool operator<(const Symbol& s2) const { return *s < *s2.s; }
+
+  operator const std::string&() const { return *s; }
+
+  bool set() const { return s; }
+
+  bool empty() const { return s->empty(); }
+
+  friend std::ostream& operator<<(std::ostream& str, const Symbol& sym);
+
+  template <typename H>
+  friend H AbslHashValue(H h, const Symbol& c) {
+    return H::combine(std::move(h), c.s);
+  }
+};
+
+// SymbolTable is a hash-set based symbol-interning mechanism.
+//
+// TODO(tazjin): Figure out which things use this. AttrSets, ...?
+// Is it possible this only exists because AttrSet wasn't a map?
+//
+// Original comment:
+//
+// Symbol table used by the parser and evaluator to represent and look
+// up identifiers and attributes efficiently. SymbolTable::create()
+// converts a string into a symbol. Symbols have the property that
+// they can be compared efficiently (using a pointer equality test),
+// because the symbol table stores only one copy of each string.
+class SymbolTable {
+ public:
+  // Create a new symbol in this table by emplacing the provided
+  // string into it.
+  //
+  // The symbol will reference an existing symbol if the symbol is
+  // already interned.
+  Symbol Create(absl::string_view sym);
+
+  // Return the number of symbols interned.
+  size_t Size() const;
+
+  // Return the total size (in bytes)
+  size_t TotalSize() const;
+
+ private:
+  // flat_hash_set does not retain pointer stability on rehashing,
+  // hence "interned" strings/symbols are stored on the heap.
+  absl::node_hash_set<std::string> symbols_;
+};
+
+}  // namespace nix