refactor(3p/nix/libexpr): Backport upstream VectorBindings merge sort r/1289

Since one of the two implementations essentially uses the same shape
as the upstream Bindings, we backport their merge sort implementation
to ensure that we're doing the same thing semantically.

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Paired-With: Perry Lorier <isomer@tvl.fyi>
Change-Id: I0d865897991eec0c4dd84d9bd0415cd1ca437792
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1162
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

1 files changed, 28 insertions, 17 deletions

diff --git a/third_party/nix/src/libexpr/attr-set.cc b/third_party/nix/src/libexpr/attr-set.cc
index 31b5bdcef3..ddfdbef361 100644
--- a/third_party/nix/src/libexpr/attr-set.cc
+++ b/third_party/nix/src/libexpr/attr-set.cc
@@ -158,39 +158,51 @@ void VectorBindings::merge(Bindings& other) {
   AttributeVector new_attributes;
   new_attributes.reserve(size() + other.size());
 
-  auto m_it = attributes_.begin();
-  auto other_it = other.begin();
-
-  while (other_it != other.end() && m_it != attributes_.end()) {
-    if (other_it->first < m_it->first) {
-      new_attributes.push_back(*(m_it++));
+  auto lhs = attributes_.begin();
+  auto rhs = other.begin();
+
+  while (lhs != attributes_.end() && rhs != other.end()) {
+    if (lhs->first == rhs->first) {
+      new_attributes.push_back(*rhs);
+      ++lhs;
+      ++rhs;
+    } else if (lhs->first < rhs->first) {
+      new_attributes.push_back(*lhs++);
     } else {
-      if (m_it->first == other_it->first) {
-        ++m_it;
-      }
-      new_attributes.push_back(*(other_it++));
+      new_attributes.push_back(*rhs++);
     }
   }
 
-  if (m_it != attributes_.end()) {
-    std::copy(m_it, attributes_.end(), std::back_inserter(new_attributes));
+  while (lhs != attributes_.end()) {
+    new_attributes.push_back(*lhs++);
   }
 
-  if (other_it != other.end()) {
-    std::copy(other_it, other.end(), std::back_inserter(new_attributes));
+  while (rhs != other.end()) {
+    new_attributes.push_back(*rhs++);
   }
 
   new_attributes.shrink_to_fit();
-
   attributes_ = new_attributes;
 }
 
+// Insert or assign (i.e. replace) a value in the attribute set.
 void VectorBindings::push_back(const Attr& attr) {
+  for (auto it = attributes_.begin(); it != attributes_.end(); ++it) {
+    if (it->first == attr.name) {
+      it->second = attr;
+      return;
+    } else if (attr.name < it->first) {
+      attributes_.emplace(it, attr.name, attr);
+      return;
+    }
+  }
+
   attributes_.emplace_back(attr.name, attr);
 }
 
 std::vector<const Attr*> VectorBindings::lexicographicOrder() {
-  std::vector<const Attr*> result(attributes_.size());
+  std::vector<const Attr*> result;
+  result.reserve(attributes_.size());
 
   for (auto& [_, attr] : attributes_) {
     result.push_back(&attr);
@@ -240,5 +252,4 @@ Value* EvalState::allocAttr(Value& vAttrs, const Symbol& name) {
   return v;
 }
 
-
 }  // namespace nix