From 7a91597392c64ec8bd2b3cdfc44df98b928f41d3 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Wed, 8 Nov 2023 00:14:42 +0100
Subject: latch: add simple latch based on a mutex and cv

---
 Makefile      |  1 +
 latch.h       | 41 +++++++++++++++++++++++++++++++++++++++++
 test/latch.cc | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 79 insertions(+)
 create mode 100644 latch.h
 create mode 100644 test/latch.cc

diff --git a/Makefile b/Makefile
index 9b14354..fe046c9 100644
--- a/Makefile
+++ b/Makefile
@@ -3,6 +3,7 @@ TEST += option
 TEST += timer
 TEST += log
 TEST += owning_mutex
+TEST += latch
 
 # -- INTERNALS -----------------------------------------------------------------
 
diff --git a/latch.h b/latch.h
new file mode 100644
index 0000000..ec2006e
--- /dev/null
+++ b/latch.h
@@ -0,0 +1,41 @@
+#ifndef UTILS_TIMER_H
+#define UTILS_TIMER_H
+
+#include <condition_variable>
+#include <mutex>
+
+#include <cassert>
+
+/// latch
+///
+/// A simple one-shot latch based on a mutex and condition variable.
+///
+/// The latch is initialized with a number N of threads to wait for. Different
+/// threads can call arrive_and_wait() on the latch. Those threads enter a wait
+/// state until the Nth thread calls arrive_and_wait(), which then releases all
+/// threads at the same time.
+struct latch {
+  explicit latch(std::size_t cnt) : m_cnt{cnt} {}
+  ~latch() {
+    assert(m_cnt == 0);
+  }
+
+  void arrive_and_wait() {
+    std::unique_lock<std::mutex> lk(m_mtx);
+    m_cnt -= 1;
+
+    if (m_cnt != 0) {
+      m_cv.wait(lk, [this]() { return m_cnt == 0; });
+    } else {
+      m_cv.notify_all();
+    }
+  }
+
+ private:
+  std::size_t m_cnt;
+
+  std::mutex m_mtx;
+  std::condition_variable m_cv;
+};
+
+#endif
diff --git a/test/latch.cc b/test/latch.cc
new file mode 100644
index 0000000..567e3db
--- /dev/null
+++ b/test/latch.cc
@@ -0,0 +1,37 @@
+#include <latch.h>
+
+#include <chrono>
+#include <thread>
+#include <vector>
+
+#include <cassert>
+#include <cstdio>
+
+constexpr unsigned kNumThreads = 16;
+
+int main() {
+  latch gate(kNumThreads);
+
+  std::vector<std::thread> threads;
+  threads.reserve(kNumThreads);
+
+  for (unsigned t = 0; t < kNumThreads; ++t) {
+    threads.emplace_back([&gate, t]() {
+      if (t % 2 == 0) {
+        unsigned sec = t / 2;
+        std::printf("th%02u sleep %us\n", t, sec);
+        std::this_thread::sleep_for(std::chrono::seconds(sec));
+      }
+
+      std::printf("th%02u at gate\n", t);
+      gate.arrive_and_wait();
+      std::printf("th%02u finished\n", t);
+    });
+  }
+
+  for (auto& th : threads) {
+    th.join();
+  }
+
+  return 0;
+}
-- 
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