Change iteration order to avoid temporary map

src/arena_allocator.hpp

@@ -9,6 +9,7 @@
 #include <iostream>
 #include <limits>
 #include <new>
+#include <span>
 #include <type_traits>
 #include <typeinfo>
 #include <utility>
@@ -639,6 +640,12 @@ template <typename T> struct ArenaVector {
 
   void clear() { size_ = 0; }
 
+  // Implicit conversion to std::span
+  operator std::span<T>() { return std::span<T>(data_, size_); }
+  operator std::span<const T>() const {
+    return std::span<const T>(data_, size_);
+  }
+
   // Iterator support for range-based for loops
   T *begin() { return data_; }
   const T *begin() const { return data_; }

src/metric.cpp

@@ -13,6 +13,7 @@
 #include <functional>
 #include <map>
 #include <mutex>
+#include <set>
 #include <string>
 #include <thread>
 #include <type_traits>
@@ -927,11 +928,83 @@ union MetricValue {
   uint64_t as_uint64;
 };
 
+// Label sets for each family type, built once and reused
+struct LabelSets {
+  std::vector<
+      std::set<LabelsKey, std::less<LabelsKey>, ArenaStlAllocator<LabelsKey>>>
+      counter_labels;
+  std::vector<
+      std::set<LabelsKey, std::less<LabelsKey>, ArenaStlAllocator<LabelsKey>>>
+      gauge_labels;
+  std::vector<
+      std::set<LabelsKey, std::less<LabelsKey>, ArenaStlAllocator<LabelsKey>>>
+      histogram_labels;
+};
+
+// Build label sets once for reuse in both phases
+static LabelSets build_label_sets(ArenaAllocator &arena) {
+  LabelSets label_sets;
+
+  // Build counter label sets
+  for (const auto &[name, family] : Metric::get_counter_families()) {
+    std::set<LabelsKey, std::less<LabelsKey>, ArenaStlAllocator<LabelsKey>>
+        all_labels{ArenaStlAllocator<LabelsKey>(&arena)};
+
+    for (const auto &[thread_id, per_thread] : family->per_thread_state) {
+      for (const auto &[labels_key, instance] : per_thread.instances) {
+        all_labels.insert(labels_key);
+      }
+    }
+    for (const auto &[labels_key, global_state] :
+         family->global_accumulated_values) {
+      if (global_state) {
+        all_labels.insert(labels_key);
+      }
+    }
+
+    label_sets.counter_labels.push_back(std::move(all_labels));
+  }
+
+  // Build gauge label sets (none needed - gauges iterate directly over
+  // instances)
+  for (const auto &[name, family] : Metric::get_gauge_families()) {
+    (void)name;
+    (void)family; // Suppress unused variable warnings
+    std::set<LabelsKey, std::less<LabelsKey>, ArenaStlAllocator<LabelsKey>>
+        empty_set{ArenaStlAllocator<LabelsKey>(&arena)};
+    label_sets.gauge_labels.push_back(std::move(empty_set));
+  }
+
+  // Build histogram label sets
+  for (const auto &[name, family] : Metric::get_histogram_families()) {
+    std::set<LabelsKey, std::less<LabelsKey>, ArenaStlAllocator<LabelsKey>>
+        all_labels{ArenaStlAllocator<LabelsKey>(&arena)};
+
+    for (const auto &[thread_id, per_thread] : family->per_thread_state) {
+      for (const auto &[labels_key, instance] : per_thread.instances) {
+        all_labels.insert(labels_key);
+      }
+    }
+    for (const auto &[labels_key, global_state] :
+         family->global_accumulated_values) {
+      if (global_state) {
+        all_labels.insert(labels_key);
+      }
+    }
+
+    label_sets.histogram_labels.push_back(std::move(all_labels));
+  }
+
+  return label_sets;
+}
+
 // Phase 1: Compute all metric values in deterministic order
-static ArenaVector<MetricValue> compute_metric_values(ArenaAllocator &arena) {
+static ArenaVector<MetricValue>
+compute_metric_values(ArenaAllocator &arena, const LabelSets &label_sets) {
   ArenaVector<MetricValue> values(&arena);
 
   // Compute counter values - ITERATION ORDER MUST MATCH FORMAT PHASE
+  size_t counter_family_idx = 0;
   for (const auto &[name, family] : Metric::get_counter_families()) {
     // Callback values
     for (const auto &[labels_key, callback] : family->callbacks) {
@@ -939,32 +1012,31 @@ static ArenaVector<MetricValue> compute_metric_values(ArenaAllocator &arena) {
       values.push_back({.as_double = value});
     }
 
-    // Aggregate all counter values (thread-local + global accumulated)
+    // Use pre-built label sets
-    std::map<LabelsKey, double, std::less<LabelsKey>,
+    const auto &all_labels = label_sets.counter_labels[counter_family_idx++];
-             ArenaStlAllocator<std::pair<const LabelsKey, double>>>
-        aggregated_values{
-            ArenaStlAllocator<std::pair<const LabelsKey, double>>(&arena)};
 
-    // First, add thread-local values
+    // Iterate by label, lookup per thread (O(1) unordered_map lookup)
+    for (const auto &labels_key : all_labels) {
+      double total_value = 0.0;
+
+      // Sum thread-local values for this label set
       for (const auto &[thread_id, per_thread] : family->per_thread_state) {
-      for (const auto &[labels_key, instance] : per_thread.instances) {
+        auto it = per_thread.instances.find(labels_key);
+        if (it != per_thread.instances.end()) {
           // Atomic read to match atomic store in Counter::inc()
           double value;
-        __atomic_load(&instance->value, &value, __ATOMIC_RELAXED);
+          __atomic_load(&it->second->value, &value, __ATOMIC_RELAXED);
-        aggregated_values[labels_key] += value;
+          total_value += value;
         }
       }
 
-    // Then, add globally accumulated values from destroyed threads
+      // Add global accumulated value for this label set
-    for (const auto &[labels_key, global_state] :
+      auto global_it = family->global_accumulated_values.find(labels_key);
-         family->global_accumulated_values) {
+      if (global_it != family->global_accumulated_values.end() &&
-      if (global_state) {
+          global_it->second) {
-        aggregated_values[labels_key] += global_state->value;
+        total_value += global_it->second->value;
-      }
       }
 
-    // Store aggregated counter values
-    for (const auto &[labels_key, total_value] : aggregated_values) {
       values.push_back({.as_double = total_value});
     }
   }
@@ -977,7 +1049,7 @@ static ArenaVector<MetricValue> compute_metric_values(ArenaAllocator &arena) {
       values.push_back({.as_double = value});
     }
 
-    // Instance values
+    // Instance values (gauges don't aggregate, just direct values)
     for (const auto &[labels_key, instance] : family->instances) {
       auto value = std::bit_cast<double>(
           instance->value.load(std::memory_order_relaxed));
@@ -986,38 +1058,37 @@ static ArenaVector<MetricValue> compute_metric_values(ArenaAllocator &arena) {
   }
 
   // Compute histogram values - ITERATION ORDER MUST MATCH FORMAT PHASE
+  size_t histogram_family_idx = 0;
   for (const auto &[name, family] : Metric::get_histogram_families()) {
-    // Aggregate all histogram values (thread-local + global accumulated)
+    // Use pre-built label sets
-    struct AggregatedHistogram {
+    const auto &all_labels =
-      ArenaVector<double> thresholds;
+        label_sets.histogram_labels[histogram_family_idx++];
-      ArenaVector<uint64_t> counts;
-      double sum;
-      uint64_t observations;
 
-      AggregatedHistogram(ArenaAllocator &arena)
+    // Iterate by label, lookup per thread (O(1) unordered_map lookup)
-          : thresholds(&arena), counts(&arena), sum(0.0), observations(0) {}
+    for (const auto &labels_key : all_labels) {
-    };
+      // Get bucket count from family config or first instance
-    std::map<
+      size_t bucket_count = family->buckets.size();
-        LabelsKey, AggregatedHistogram *, std::less<LabelsKey>,
-        ArenaStlAllocator<std::pair<const LabelsKey, AggregatedHistogram *>>>
-        aggregated_histograms{ArenaStlAllocator<
-            std::pair<const LabelsKey, AggregatedHistogram *>>(&arena)};
 
-    // First, collect thread-local histogram data
+      ArenaVector<uint64_t> total_counts(&arena);
+      for (size_t i = 0; i < bucket_count; ++i) {
+        total_counts.push_back(0);
+      }
+      double total_sum = 0.0;
+      uint64_t total_observations = 0;
+
+      // Sum thread-local values for this label set
       for (const auto &[thread_id, per_thread] : family->per_thread_state) {
-      for (const auto &[labels_key, instance] : per_thread.instances) {
+        auto it = per_thread.instances.find(labels_key);
+        if (it != per_thread.instances.end()) {
+          auto *instance = it->second;
+
           // Extract data under lock - minimize critical section
-        ArenaVector<double> thresholds_snapshot(&arena);
           ArenaVector<uint64_t> counts_snapshot(&arena);
           double sum_snapshot;
           uint64_t observations_snapshot;
 
-        // Copy data with minimal critical section
           {
             std::lock_guard<std::mutex> lock(instance->mutex);
-          for (size_t i = 0; i < instance->thresholds.size(); ++i) {
-            thresholds_snapshot.push_back(instance->thresholds[i]);
-          }
             for (size_t i = 0; i < instance->counts.size(); ++i) {
               counts_snapshot.push_back(instance->counts[i]);
             }
@@ -1025,77 +1096,38 @@ static ArenaVector<MetricValue> compute_metric_values(ArenaAllocator &arena) {
             observations_snapshot = instance->observations;
           }
 
-        // Initialize or aggregate into aggregated_histograms
+          // Add to totals
-        auto it = aggregated_histograms.find(labels_key);
-        if (it == aggregated_histograms.end()) {
-          // Create new entry
-          auto *agg_hist = new (arena.allocate_raw(
-              sizeof(AggregatedHistogram), alignof(AggregatedHistogram)))
-              AggregatedHistogram(arena);
-          for (size_t i = 0; i < thresholds_snapshot.size(); ++i) {
-            agg_hist->thresholds.push_back(thresholds_snapshot[i]);
-          }
           for (size_t i = 0; i < counts_snapshot.size(); ++i) {
-            agg_hist->counts.push_back(counts_snapshot[i]);
+            total_counts[i] += counts_snapshot[i];
-          }
-          agg_hist->sum = sum_snapshot;
-          agg_hist->observations = observations_snapshot;
-          aggregated_histograms[labels_key] = agg_hist;
-        } else {
-          // Aggregate with existing entry
-          auto *agg_hist = it->second;
-          for (size_t i = 0; i < counts_snapshot.size(); ++i) {
-            agg_hist->counts[i] += counts_snapshot[i];
-          }
-          agg_hist->sum += sum_snapshot;
-          agg_hist->observations += observations_snapshot;
           }
+          total_sum += sum_snapshot;
+          total_observations += observations_snapshot;
         }
       }
 
-    // Then, add globally accumulated values from destroyed threads
+      // Add global accumulated value for this label set
-    for (const auto &[labels_key, global_state] :
+      auto global_it = family->global_accumulated_values.find(labels_key);
-         family->global_accumulated_values) {
+      if (global_it != family->global_accumulated_values.end() &&
-      if (global_state) {
+          global_it->second) {
-        auto it = aggregated_histograms.find(labels_key);
+        auto *global_state = global_it->second;
-        if (it == aggregated_histograms.end()) {
-          // Create new entry from global state
-          auto *agg_hist = new (arena.allocate_raw(
-              sizeof(AggregatedHistogram), alignof(AggregatedHistogram)))
-              AggregatedHistogram(arena);
-          for (size_t i = 0; i < global_state->thresholds.size(); ++i) {
-            agg_hist->thresholds.push_back(global_state->thresholds[i]);
-          }
         for (size_t i = 0; i < global_state->counts.size(); ++i) {
-            agg_hist->counts.push_back(global_state->counts[i]);
+          total_counts[i] += global_state->counts[i];
-          }
-          agg_hist->sum = global_state->sum;
-          agg_hist->observations = global_state->observations;
-          aggregated_histograms[labels_key] = agg_hist;
-        } else {
-          // Add global accumulated values to existing entry
-          auto *agg_hist = it->second;
-          for (size_t i = 0; i < global_state->counts.size(); ++i) {
-            agg_hist->counts[i] += global_state->counts[i];
-          }
-          agg_hist->sum += global_state->sum;
-          agg_hist->observations += global_state->observations;
-        }
         }
+        total_sum += global_state->sum;
+        total_observations += global_state->observations;
       }
 
       // Store histogram values
-    for (const auto &[labels_key, agg_hist] : aggregated_histograms) {
       // Store explicit bucket counts
-      for (size_t i = 0; i < agg_hist->thresholds.size(); ++i) {
+      for (size_t i = 0; i < total_counts.size(); ++i) {
-        values.push_back({.as_uint64 = agg_hist->counts[i]});
+        values.push_back({.as_uint64 = total_counts[i]});
       }
       // Store +Inf bucket (total observations)
-      values.push_back({.as_uint64 = agg_hist->observations});
+      values.push_back({.as_uint64 = total_observations});
       // Store sum
-      values.push_back({.as_double = agg_hist->sum});
+      values.push_back({.as_double = total_sum});
       // Store count
-      values.push_back({.as_uint64 = agg_hist->observations});
+      values.push_back({.as_uint64 = total_observations});
     }
   }
 
@@ -1107,8 +1139,12 @@ std::span<std::string_view> render(ArenaAllocator &arena) {
   // Hold lock throughout both phases to prevent registry changes
   std::unique_lock<std::mutex> _{Metric::mutex};
 
+  // Build label sets once for both phases
+  LabelSets label_sets = build_label_sets(arena);
+
   // Phase 1: Compute all metric values
-  ArenaVector<MetricValue> metric_values = compute_metric_values(arena);
+  ArenaVector<MetricValue> metric_values =
+      compute_metric_values(arena, label_sets);
   const MetricValue *next_value = metric_values.data();
 
   ArenaVector<std::string_view> output(&arena);
@@ -1170,6 +1206,7 @@ std::span<std::string_view> render(ArenaAllocator &arena) {
   };
 
   // Format counters - ITERATION ORDER MUST MATCH COMPUTE PHASE
+  size_t counter_family_idx = 0;
   for (const auto &[name, family] : Metric::get_counter_families()) {
     output.push_back(format(arena, "# HELP %.*s %.*s\n",
                             static_cast<int>(name.length()), name.data(),
@@ -1195,28 +1232,11 @@ std::span<std::string_view> render(ArenaAllocator &arena) {
                               value));
     }
 
-    // Recreate aggregated values map for iteration order
+    // Use pre-built label sets (same as compute phase)
-    std::map<LabelsKey, double, std::less<LabelsKey>,
+    const auto &all_labels = label_sets.counter_labels[counter_family_idx++];
-             ArenaStlAllocator<std::pair<const LabelsKey, double>>>
-        aggregated_values{
-            ArenaStlAllocator<std::pair<const LabelsKey, double>>(&arena)};
 
-    // Populate map to get same iteration order (values ignored, using
+    // Format counter values using pre-computed values
-    // pre-computed)
+    for (const auto &labels_key : all_labels) {
-    for (const auto &[thread_id, per_thread] : family->per_thread_state) {
-      for (const auto &[labels_key, instance] : per_thread.instances) {
-        aggregated_values[labels_key] = 0.0; // Placeholder
-      }
-    }
-    for (const auto &[labels_key, global_state] :
-         family->global_accumulated_values) {
-      if (global_state) {
-        aggregated_values[labels_key] = 0.0; // Placeholder
-      }
-    }
-
-    // Format aggregated counter values
-    for (const auto &[labels_key, ignored_value] : aggregated_values) {
       auto total_value = next_value++->as_double;
       labels_sv.clear();
       for (size_t i = 0; i < labels_key.labels.size(); ++i) {
@@ -1272,6 +1292,7 @@ std::span<std::string_view> render(ArenaAllocator &arena) {
   }
 
   // Format histograms - ITERATION ORDER MUST MATCH COMPUTE PHASE
+  size_t histogram_family_idx = 0;
   for (const auto &[name, family] : Metric::get_histogram_families()) {
     output.push_back(format(arena, "# HELP %.*s %.*s\n",
                             static_cast<int>(name.length()), name.data(),
@@ -1280,71 +1301,27 @@ std::span<std::string_view> render(ArenaAllocator &arena) {
     output.push_back(format(arena, "# TYPE %.*s histogram\n",
                             static_cast<int>(name.length()), name.data()));
 
-    // Recreate aggregated histograms map for iteration order
+    // Use pre-built label sets (same as compute phase)
-    struct AggregatedHistogram {
+    const auto &all_labels =
-      ArenaVector<double> thresholds;
+        label_sets.histogram_labels[histogram_family_idx++];
-      ArenaVector<uint64_t> counts;
-      double sum;
-      uint64_t observations;
-
-      AggregatedHistogram(ArenaAllocator &arena)
-          : thresholds(&arena), counts(&arena), sum(0.0), observations(0) {}
-    };
-    std::map<
-        LabelsKey, AggregatedHistogram *, std::less<LabelsKey>,
-        ArenaStlAllocator<std::pair<const LabelsKey, AggregatedHistogram *>>>
-        aggregated_histograms{ArenaStlAllocator<
-            std::pair<const LabelsKey, AggregatedHistogram *>>(&arena)};
-
-    // Recreate map structure for iteration order (recompute thresholds for
-    // formatting)
-    for (const auto &[thread_id, per_thread] : family->per_thread_state) {
-      for (const auto &[labels_key, instance] : per_thread.instances) {
-        auto it = aggregated_histograms.find(labels_key);
-        if (it == aggregated_histograms.end()) {
-          auto *agg_hist = new (arena.allocate_raw(
-              sizeof(AggregatedHistogram), alignof(AggregatedHistogram)))
-              AggregatedHistogram(arena);
-          // Copy thresholds for le= formatting
-          std::lock_guard<std::mutex> lock(instance->mutex);
-          for (size_t i = 0; i < instance->thresholds.size(); ++i) {
-            agg_hist->thresholds.push_back(instance->thresholds[i]);
-          }
-          aggregated_histograms[labels_key] = agg_hist;
-        }
-      }
-    }
-    for (const auto &[labels_key, global_state] :
-         family->global_accumulated_values) {
-      if (global_state) {
-        auto it = aggregated_histograms.find(labels_key);
-        if (it == aggregated_histograms.end()) {
-          auto *agg_hist = new (arena.allocate_raw(
-              sizeof(AggregatedHistogram), alignof(AggregatedHistogram)))
-              AggregatedHistogram(arena);
-          // Copy thresholds for le= formatting
-          for (size_t i = 0; i < global_state->thresholds.size(); ++i) {
-            agg_hist->thresholds.push_back(global_state->thresholds[i]);
-          }
-          aggregated_histograms[labels_key] = agg_hist;
-        }
-      }
-    }
 
     ArenaVector<std::pair<std::string_view, std::string_view>> bucket_labels_sv(
         &arena);
 
     // Format histogram data using pre-computed values
-    for (const auto &[labels_key, agg_hist] : aggregated_histograms) {
+    for (const auto &labels_key : all_labels) {
+      // Get bucket thresholds from family config
+      size_t bucket_count = family->buckets.size();
+
       // Format explicit bucket counts
-      for (size_t i = 0; i < agg_hist->thresholds.size(); ++i) {
+      for (size_t i = 0; i < bucket_count; ++i) {
         auto count = next_value++->as_uint64;
         bucket_labels_sv.clear();
         for (size_t j = 0; j < labels_key.labels.size(); ++j) {
           bucket_labels_sv.push_back(labels_key.labels[j]);
         }
         bucket_labels_sv.push_back(
-            {"le", static_format(arena, agg_hist->thresholds[i])});
+            {"le", static_format(arena, family->buckets[i])});
         auto labels = format_labels(bucket_labels_sv);
         output.push_back(format(
             arena, "%.*s_bucket%.*s %llu\n", static_cast<int>(name.length()),
@@ -1385,9 +1362,7 @@ std::span<std::string_view> render(ArenaAllocator &arena) {
     }
   }
 
-  auto result = arena.allocate<std::string_view>(output.size());
+  return output;
-  std::copy(output.data(), output.data() + output.size(), result);
-  return std::span<std::string_view>(result, output.size());
 }
 
 // Template specialization implementations for register_callback