From 7729398b197c56487241f68851cfcf81d559a43f Mon Sep 17 00:00:00 2001
From: Jorge Rocamora <33847633+aeroyorch@users.noreply.github.com>
Date: Thu, 4 Jun 2026 21:53:12 +0200
Subject: [PATCH] Use std::less<> for Labels to enable heterogeneous lookup on
 C++14+

---
 core/benchmarks/CMakeLists.txt   |  1 +
 core/benchmarks/labels_bench.cc  | 67 ++++++++++++++++++++++++++++++++
 core/include/prometheus/labels.h | 10 +++++
 3 files changed, 78 insertions(+)
 create mode 100644 core/benchmarks/labels_bench.cc

diff --git a/core/benchmarks/CMakeLists.txt b/core/benchmarks/CMakeLists.txt
index c1294d5ad..74120e17c 100644
--- a/core/benchmarks/CMakeLists.txt
+++ b/core/benchmarks/CMakeLists.txt
@@ -7,6 +7,7 @@ add_executable(benchmarks
   gauge_bench.cc
   histogram_bench.cc
   info_bench.cc
+  labels_bench.cc
   registry_bench.cc
   summary_bench.cc
 )
diff --git a/core/benchmarks/labels_bench.cc b/core/benchmarks/labels_bench.cc
new file mode 100644
index 000000000..a9c8d4612
--- /dev/null
+++ b/core/benchmarks/labels_bench.cc
@@ -0,0 +1,67 @@
+#include <benchmark/benchmark.h>
+
+#include <functional>
+#include <map>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "benchmark_helpers.h"
+
+#if defined(__cpp_lib_string_view) && defined(__cpp_lib_generic_associative_lookup)
+#include <string_view>
+
+using Plain = std::map<std::string, std::string>;
+using Transparent = std::map<std::string, std::string, std::less<>>;
+
+template <typename Map>
+static Map MakeMap(std::size_t size, std::size_t key_length,
+                   std::vector<std::string>& keys) {
+  Map map;
+  keys.reserve(size);
+  for (std::size_t i = 0; i < size; ++i) {
+    auto key = GenerateRandomString(key_length);
+    map.emplace(key, "value");
+    keys.push_back(std::move(key));
+  }
+  return map;
+}
+
+template <typename Map, bool Heterogeneous>
+static void BM_Labels_LookupView(benchmark::State& state) {
+  std::vector<std::string> keys;
+  const auto map = MakeMap<Map>(state.range(0), state.range(1), keys);
+  std::size_t i = 0;
+  while (state.KeepRunning()) {
+    const std::string_view key = keys[i];
+    if (++i == keys.size()) i = 0;
+    if constexpr (Heterogeneous) {
+      benchmark::DoNotOptimize(map.find(key));
+    } else {
+      benchmark::DoNotOptimize(map.find(std::string(key)));
+    }
+  }
+}
+
+template <typename Map>
+static void BM_Labels_LookupString(benchmark::State& state) {
+  std::vector<std::string> keys;
+  const auto map = MakeMap<Map>(state.range(0), state.range(1), keys);
+  std::size_t i = 0;
+  while (state.KeepRunning()) {
+    const std::string& key = keys[i];
+    if (++i == keys.size()) i = 0;
+    benchmark::DoNotOptimize(map.find(key));
+  }
+}
+
+BENCHMARK_TEMPLATE(BM_Labels_LookupView, Plain, false)
+    ->ArgsProduct({{4, 16}, {8, 32}});
+BENCHMARK_TEMPLATE(BM_Labels_LookupView, Transparent, true)
+    ->ArgsProduct({{4, 16}, {8, 32}});
+BENCHMARK_TEMPLATE(BM_Labels_LookupString, Plain)
+    ->ArgsProduct({{4, 16}, {8, 32}});
+BENCHMARK_TEMPLATE(BM_Labels_LookupString, Transparent)
+    ->ArgsProduct({{4, 16}, {8, 32}});
+
+#endif
diff --git a/core/include/prometheus/labels.h b/core/include/prometheus/labels.h
index d6fce0ff4..9790c0121 100644
--- a/core/include/prometheus/labels.h
+++ b/core/include/prometheus/labels.h
@@ -1,12 +1,22 @@
 #pragma once
 
+#if defined(__cpp_lib_generic_associative_lookup)
+#include <functional>
+#endif
 #include <map>
 #include <string>
 
 namespace prometheus {
 
 /// \brief Multiple labels and their value.
+///
+/// Uses the transparent comparator std::less<> on C++14 and newer to allow
+/// heterogeneous lookup, falling back to the default comparator on C++11.
+#if defined(__cpp_lib_generic_associative_lookup)
+using Labels = std::map<std::string, std::string, std::less<>>;
+#else
 using Labels = std::map<std::string, std::string>;
+#endif
 
 /// \brief Single label and its value.
 using Label = Labels::value_type;