From 0aac4f3291d65ebc1b0bbee744fca347ecef7022 Mon Sep 17 00:00:00 2001
From: Mikaël Capelle <capelle.mikael@gmail.com>
Date: Sun, 24 May 2020 23:32:47 +0200
Subject: Add MemoizedLocked.

---
 src/thread_utils.h | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

(limited to 'src')

diff --git a/src/thread_utils.h b/src/thread_utils.h
index 2cb87a8d..6c044138 100644
--- a/src/thread_utils.h
+++ b/src/thread_utils.h
@@ -6,6 +6,49 @@
 
 namespace MOShared {
 
+/**
+ * Class that can be used to perform thread-safe memoization.
+ *
+ * Each instance hold a flag indicating if the current value is up-to-date
+ * or not. This flag can be reset using `invalidate()`. When the value is queried,
+ * the flag is checked, and if it is not up-to-date, the given callback is used
+ * to compute the value.
+ *
+ * The computation and update of the value is locked to avoid concurrent modifications.
+ *
+ * @tparam T Type of value ot memoized.
+ * @tparam Fn Type of the callback.
+ */
+template <class T, class Fn>
+struct MemoizedLocked {
+
+  MemoizedLocked(Fn callback, T value = {}) :
+    m_Fn{ callback }, m_Value{ std::move(value) } { }
+  
+  template <class... Args>
+  T& value(Args&&... args) const {
+    if (m_NeedUpdating) {
+      std::scoped_lock lock(m_Mutex);
+      if (m_NeedUpdating) {
+        m_Value = std::invoke(m_Fn, std::forward<Args>(args)... );
+        m_NeedUpdating.store(false);
+      }
+    }
+    return m_Value;
+  }
+
+  void invalidate() const {
+    m_NeedUpdating.store(true);
+  }
+
+private:
+  mutable std::mutex m_Mutex;
+  mutable std::atomic<bool> m_NeedUpdating{ true };
+
+  Fn m_Fn;
+  mutable T m_Value;
+};
+
 /**
  * @brief Apply the given callable to each element between the two given iterators
  *     in a parallel way.
-- 
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