From 81cd7fe2fbe2fbd5e363f2f4edc4fac388cb06ca Mon Sep 17 00:00:00 2001
From: cong1920 <wu.cong@gmail.com>
Date: Thu, 26 Mar 2026 01:31:59 -0700
Subject: [PATCH] [http_server] Cache parameter child pointer for O(1) lookup
 fallback

In find(), when a static child lookup misses, the code previously did a
linear scan of ALL children to find a {{param}} child. This is O(n) per
tree level for every parameterized route lookup.

Fix: Cache param_child_ pointer during find_or_create(), so find() can
fall back to it in O(1) instead of scanning.

Benchmark results (median, 25 iterations, WSL2 g++ -O2):

                 Before        After         Change
  insert:        313.3 ns/op   299.2 ns/op   -4.5%
  lookup hit:     80.7 ns/op    72.7 ns/op   -9.9%
  lookup miss:    45.0 ns/op    36.6 ns/op  -18.7%
---
 .../http_server/dynamic_routing_table.hh      | 22 +++++++++----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/libraries/http_server/http_server/dynamic_routing_table.hh b/libraries/http_server/http_server/dynamic_routing_table.hh
index 7448371..579d310 100644
--- a/libraries/http_server/http_server/dynamic_routing_table.hh
+++ b/libraries/http_server/http_server/dynamic_routing_table.hh
@@ -53,7 +53,12 @@ template <typename V> struct drt_node {
     std::string_view k = r.substr(s, c - s);
 
     if (children_.find(k) == children_.end()) {
-      children_[k] = pool_.allocate(pool_);
+      auto* child = pool_.allocate(pool_);
+      children_[k] = child;
+      // Cache the parameter child pointer for O(1) lookup in find()
+      if (k.size() > 4 and k[0] == '{' and k[1] == '{' and
+          k[k.size() - 2] == '}' and k[k.size() - 1] == '}')
+        param_child_ = child;
     }
     return children_[k]->find_or_create(r, c);
   }
@@ -100,20 +105,15 @@ template <typename V> struct drt_node {
         return it2;
     }
 
-    {
-      // if one child is a url param {{param_name}}, choose it
-      for (const auto& kv : children_) {
-        auto name = kv.first;
-        if (name.size() > 4 and name[0] == '{' and name[1] == '{' and
-            name[name.size() - 2] == '}' and name[name.size() - 1] == '}')
-          return kv.second->find(r, c);
-      }
-      return end();
-    }
+    // O(1) fallback to cached parameter child instead of O(n) linear scan
+    if (param_child_)
+      return param_child_->find(r, c);
+    return end();
   }
 
   V v_;
   std::unordered_map<std::string_view, drt_node*> children_;
+  drt_node* param_child_ = nullptr; // cached {{param}} child for O(1) lookup
   drt_node_pool<drt_node>& pool_;
 };