From 9732e193ea86c3311f9a62e433dd983a958e4dde Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jonatan=20K=C5=82osko?= <jonatanklosko@gmail.com>
Date: Mon, 9 Feb 2026 17:46:19 +0100
Subject: [PATCH] Ensure unique nodes in query and query_by_id

---
 c_src/lazy_html.cpp     | 43 ++++++++++++++++++++---------
 test/lazy_html_test.exs | 60 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+), 13 deletions(-)

diff --git a/c_src/lazy_html.cpp b/c_src/lazy_html.cpp
index bc34a1c..6bfda8f 100644
--- a/c_src/lazy_html.cpp
+++ b/c_src/lazy_html.cpp
@@ -697,17 +697,27 @@ ExLazyHTML query(ErlNifEnv *env, ExLazyHTML ex_lazy_html,
                                        LXB_SELECTORS_OPT_MATCH_ROOT));
 
   auto nodes = std::vector<lxb_dom_node_t *>();
+  auto inserted_nodes = std::unordered_set<lxb_dom_node_t *>();
+
+  struct FindCtx {
+    std::vector<lxb_dom_node_t *> *nodes;
+    std::unordered_set<lxb_dom_node_t *> *inserted_nodes;
+  };
+
+  auto ctx = FindCtx{&nodes, &inserted_nodes};
 
   for (auto node : ex_lazy_html.resource->nodes) {
     status = lxb_selectors_find(
         selectors, node, css_selector_list,
         [](lxb_dom_node_t *node, lxb_css_selector_specificity_t spec,
            void *ctx) -> lxb_status_t {
-          auto nodes_ptr = static_cast<std::vector<lxb_dom_node_t *> *>(ctx);
-          nodes_ptr->push_back(node);
+          auto find_ctx = static_cast<FindCtx *>(ctx);
+          if (find_ctx->inserted_nodes->insert(node).second) {
+            find_ctx->nodes->push_back(node);
+          }
           return LXB_STATUS_OK;
         },
-        &nodes);
+        &ctx);
     if (status != LXB_STATUS_OK) {
       throw std::runtime_error("failed to run find");
     }
@@ -789,22 +799,31 @@ bool matches_id(lxb_dom_node_t *node, ErlNifBinary *id) {
 ExLazyHTML query_by_id(ErlNifEnv *env, ExLazyHTML ex_lazy_html,
                        ErlNifBinary id) {
   auto nodes = std::vector<lxb_dom_node_t *>();
+  auto seen = std::unordered_set<lxb_dom_node_t *>();
 
-  auto ctx = std::make_tuple(&nodes, &id);
+  struct WalkCtx {
+    std::vector<lxb_dom_node_t *> *nodes;
+    std::unordered_set<lxb_dom_node_t *> *seen;
+    ErlNifBinary *id;
+  };
+
+  auto ctx = WalkCtx{&nodes, &seen, &id};
 
   for (auto node : ex_lazy_html.resource->nodes) {
     if (matches_id(node, &id)) {
-      nodes.push_back(node);
+      if (seen.insert(node).second) {
+        nodes.push_back(node);
+      }
     }
 
     lxb_dom_node_simple_walk(
         node,
         [](lxb_dom_node_t *node, void *ctx) -> lexbor_action_t {
-          auto [nodes_ptr, id_ptr] = *static_cast<
-              std::tuple<std::vector<lxb_dom_node_t *> *, ErlNifBinary *> *>(
-              ctx);
-          if (matches_id(node, id_ptr)) {
-            nodes_ptr->push_back(node);
+          auto walk_ctx = static_cast<WalkCtx *>(ctx);
+          if (matches_id(node, walk_ctx->id)) {
+            if (walk_ctx->seen->insert(node).second) {
+              walk_ctx->nodes->push_back(node);
+            }
           }
 
           return LEXBOR_ACTION_OK;
@@ -843,9 +862,7 @@ ExLazyHTML parent_node(ErlNifEnv *env, ExLazyHTML ex_lazy_html) {
     auto parent = lxb_dom_node_parent(node);
     if (parent != NULL && parent->type == LXB_DOM_NODE_TYPE_ELEMENT &&
         (is_document || !lxb_html_tree_node_is(parent, LXB_TAG_HTML))) {
-      auto inserted_node = inserted_nodes.find(parent);
-      if (inserted_node == inserted_nodes.end()) {
-        inserted_nodes.insert(parent);
+      if (inserted_nodes.insert(parent).second) {
         nodes.push_back(parent);
       }
     }
diff --git a/test/lazy_html_test.exs b/test/lazy_html_test.exs
index 84bee63..59c04fd 100644
--- a/test/lazy_html_test.exs
+++ b/test/lazy_html_test.exs
@@ -248,6 +248,36 @@ defmodule LazyHTMLTest do
         LazyHTML.query(lazy_html, "hover:")
       end
     end
+
+    test "does not include duplicated elements in the result set" do
+      fragment =
+        LazyHTML.from_fragment(~S"""
+        <div>
+          <div>1</div>
+          <div>2</div>
+        </div>
+        """)
+
+      result = fragment |> LazyHTML.query("div") |> LazyHTML.query("div")
+
+      # If nodes were not deduplicated, the second query would inflate
+      # the result to 5 nodes. We expect only 3 unique nodes.
+
+      assert inspect(result) == """
+             #LazyHTML<
+               3 nodes (from selector)
+               #1
+               <div>
+                 <div>1</div>
+                 <div>2</div>
+               </div>
+               #2
+               <div>1</div>
+               #3
+               <div>2</div>
+             >\
+             """
+    end
   end
 
   describe "parent_node/1" do
@@ -379,6 +409,36 @@ defmodule LazyHTMLTest do
       result = LazyHTML.query_by_id(lazy_html, "root")
       assert Enum.count(result) == 1
     end
+
+    test "does not include duplicated elements in the result set" do
+      # A proper HTML document should not have duplicated ids, but it
+      # can be the case.
+      fragment =
+        LazyHTML.from_fragment(~S"""
+        <div id="1">
+          <div id="1">1</div>
+          <div>2</div>
+        </div>
+        """)
+
+      result = fragment |> LazyHTML.query_by_id("1") |> LazyHTML.query_by_id("1")
+
+      # If nodes were not deduplicated, the second query would inflate
+      # the result to 3 nodes. We expect only 2 unique nodes.
+
+      assert inspect(result) == """
+             #LazyHTML<
+               2 nodes (from selector)
+               #1
+               <div id=\"1\">
+                 <div id=\"1\">1</div>
+                 <div>2</div>
+               </div>
+               #2
+               <div id=\"1\">1</div>
+             >\
+             """
+    end
   end
 
   describe "text/1" do