refactor(3p/nix/libexpr): Use absl::node_hash_set in SymbolTable r/794

This replaces the previous use of std::unordered_set with
absl::node_hash_set.

This type was chosen because the current implementation requires
pointer stability.

This does not yet touch the 'Attr' struct.

As a bonus, the implementation of the SymbolTable struct is now
consolidated into a single header/implementation file pair.

1 files changed, 24 insertions, 0 deletions

diff --git a/third_party/nix/src/libexpr/symbol-table.cc b/third_party/nix/src/libexpr/symbol-table.cc
new file mode 100644
index 000000000000..5b9fb27d59aa
--- /dev/null
+++ b/third_party/nix/src/libexpr/symbol-table.cc
@@ -0,0 +1,24 @@
+#include "symbol-table.hh"
+
+#include <absl/container/node_hash_set.h>
+#include <absl/strings/string_view.h>
+
+namespace nix {
+
+Symbol SymbolTable::Create(absl::string_view sym) {
+  auto it = symbols_.emplace(sym);
+  const string* ptr = &(*it.first);
+  return Symbol(ptr);
+}
+
+size_t SymbolTable::Size() const { return symbols_.size(); }
+
+size_t SymbolTable::TotalSize() const {
+  size_t n = 0;
+  for (auto& i : symbols_) {
+    n += i.size();
+  }
+  return n;
+}
+
+}  // namespace nix