From d0f2b6550aee9fd7cf8fa5179cf17a3a15c80c61 Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Thu, 28 Aug 2025 17:32:34 -0400
Subject: [PATCH] More scaffolding

---
 src/metric.cpp | 110 ++++++++++++++++++++++++++++++++++++++++++++-----
 src/metric.hpp |   3 +-
 2 files changed, 100 insertions(+), 13 deletions(-)

diff --git a/src/metric.cpp b/src/metric.cpp
index 4fa03ad..1977ff3 100644
--- a/src/metric.cpp
+++ b/src/metric.cpp
@@ -1,9 +1,15 @@
 #include "metric.hpp"
+
+#include <atomic>
+#include <cassert>
+#include <cstdint>
 #include <mutex>
 #include <string_view>
 #include <thread>
+#include <type_traits>
 #include <unordered_map>
 #include <vector>
+
 namespace metric {
 struct MetricKey {
   std::string_view name;
@@ -33,9 +39,24 @@ template <> struct hash<metric::MetricKey> {
 
 namespace metric {
 
-struct Counter::State {};
-struct Gauge::State {};
-struct Histogram::State {};
+using AtomicWord = std::atomic<uint64_t>;
+
+struct Counter::State {
+  AtomicWord value;
+};
+
+struct Gauge::State {
+  std::mutex mutex;
+  double value;
+};
+
+struct Histogram::State {
+  std::vector<double> thresholds;
+  std::vector<AtomicWord> counts;
+  AtomicWord sum;
+  AtomicWord observations;
+};
+
 struct Metric {
 
   static std::mutex mutex;
@@ -47,6 +68,8 @@ struct Metric {
 
   static std::unordered_map<std::thread::id, PerThreadState> perThreadState;
 
+  static std::unordered_map<MetricKey, Gauge::State> gauges;
+
   struct ThreadInit {
     ThreadInit() {
       std::unique_lock<std::mutex> _{mutex};
@@ -58,23 +81,88 @@ struct Metric {
     }
   };
   static thread_local ThreadInit thread_init;
+
+  static Counter
+  create_counter(std::string_view name,
+                 std::vector<std::pair<std::string, std::string>> labels) {
+    std::unique_lock<std::mutex> _{mutex};
+    Counter result;
+    result.p = &perThreadState[std::this_thread::get_id()]
+                    .counters[MetricKey{name, labels}];
+    return result;
+  }
+
+  static Gauge
+  create_gauge(std::string_view name,
+               std::vector<std::pair<std::string, std::string>> labels) {
+    std::unique_lock<std::mutex> _{mutex};
+    Gauge result;
+    result.p = &gauges[MetricKey{name, labels}];
+    return result;
+  }
+
+  static Histogram
+  create_histogram(std::string_view name,
+                   std::vector<std::pair<std::string, std::string>> labels) {
+    std::unique_lock<std::mutex> _{mutex};
+    Histogram result;
+    result.p = &perThreadState[std::this_thread::get_id()]
+                    .histograms[MetricKey{name, labels}];
+    return result;
+  }
 };
 
-void Counter::inc(double x) {}
-void Gauge::inc(double x) {}
-void Gauge::dec(double x) {}
-void Gauge::set(double x) {}
-void Histogram::observe(double x) {}
+template <class U, class V> U reinterpret(V v) {
+  static_assert(sizeof(U) == sizeof(V));
+  static_assert(std::is_arithmetic_v<U>);
+  static_assert(std::is_arithmetic_v<V>);
+  U u;
+  memcpy(&u, &v, sizeof(u));
+  return u;
+}
+
+void Counter::inc(double x) {
+  assert(x >= 0);
+  p->value.fetch_add(x, std::memory_order_relaxed);
+}
+void Gauge::inc(double x) {
+  std::unique_lock<std::mutex> _{p->mutex};
+  p->value += x;
+}
+void Gauge::dec(double x) {
+  std::unique_lock<std::mutex> _{p->mutex};
+  p->value -= x;
+}
+void Gauge::set(double x) {
+  std::unique_lock<std::mutex> _{p->mutex};
+  p->value = x;
+}
+void Histogram::observe(double x) {
+  assert(p->thresholds.size() == p->counts.size());
+  for (size_t i = 0; i < p->thresholds.size(); ++i) {
+    p->counts[i].fetch_add(x <= p->thresholds[i], std::memory_order_relaxed);
+  }
+  auto sum = reinterpret<double>(p->sum.load(std::memory_order_relaxed));
+  sum += x;
+  p->sum.store(reinterpret<decltype(p->sum.load())>(sum));
+  p->observations.fetch_add(1, std::memory_order_relaxed);
+}
 
 template <>
 Counter Family<Counter>::create(
-    std::initializer_list<std::pair<std::string_view, std::string_view>>) {}
+    std::vector<std::pair<std::string, std::string>> labels) {
+  return Metric::create_counter(name, labels);
+}
 template <>
 Gauge Family<Gauge>::create(
-    std::initializer_list<std::pair<std::string_view, std::string_view>>) {}
+    std::vector<std::pair<std::string, std::string>> labels) {
+  return Metric::create_gauge(name, labels);
+}
 template <>
 Histogram Family<Histogram>::create(
-    std::initializer_list<std::pair<std::string_view, std::string_view>>) {}
+    std::vector<std::pair<std::string, std::string>> labels) {
+  return Metric::create_histogram(name, labels);
+}
 
 Family<Counter> create_counter(std::string_view name, std::string_view help) {}
 Family<Gauge> create_gauge(std::string_view name, std::string_view help) {}
diff --git a/src/metric.hpp b/src/metric.hpp
index 3be5949..ee62dcd 100644
--- a/src/metric.hpp
+++ b/src/metric.hpp
@@ -45,8 +45,7 @@ private:
 template <class T> struct Family {
   static_assert(std::is_same_v<T, Counter> || std::is_same_v<T, Gauge> ||
                 std::is_same_v<T, Histogram>);
-  T create(
-      std::initializer_list<std::pair<std::string_view, std::string_view>>);
+  T create(std::vector<std::pair<std::string, std::string>>);
 
 private:
   friend struct Metric;