Add topic about "Lazy Initiliazation" pattern to Design patterns (#790)

New topic about Lazy initialization pattern and how it can be implemented using oneapi::tbb::collaborative_call_once has been added.

Signed-off-by: Ilya Isaev <ilya.isaev@intel.com>

doc/main/tbb_userguide/design_patterns/Design_Patterns.rst

@@ -51,6 +51,7 @@ expressions into equivalent C++03 code.
    ../../tbb_userguide/design_patterns/Divide_and_Conquer
    ../../tbb_userguide/design_patterns/GUI_Thread
    ../../tbb_userguide/design_patterns/Non-Preemptive_Priorities
+   ../../tbb_userguide/design_patterns/Lazy_Initialization
    ../../tbb_userguide/design_patterns/Local_Serializer
    ../../tbb_userguide/design_patterns/Fenced_Data_Transfer
    ../../tbb_userguide/design_patterns/Reference_Counting

doc/main/tbb_userguide/design_patterns/Lazy_Initialization.rst

@@ -0,0 +1,109 @@
+.. _Lazy_Initialization:
+
+Lazy Initialization
+==================
+
+
+.. container:: section
+
+
+   .. rubric:: Problem
+      :class: sectiontitle
+
+   Delay the creation of an object, potentially expensive, until it is accessed.
+   In parallel programming, initialization must also be guarded against race conditions.
+
+
+.. container:: section
+
+
+   .. rubric:: Context
+      :class: sectiontitle
+
+   The cost of operations that take place during the initialization
+   of the object may be considerably high. In that case, the object
+   should be initialized only when needed. Lazy initialization is
+   the common tactic that allows implementing such an approach.
+
+
+.. container:: section
+
+
+   .. rubric:: Solution
+      :class: sectiontitle
+
+   Using ``oneapi::tbb::collaborative_call_once`` with ``oneapi::tbb::collaborative_once_flag``
+   helps to implement thread-safe lazy initialization for a user object.
+
+
+   In addition, ``collaborative_call_once`` allows other thread blocked on
+   the same ``collaborative_once_flag`` to join other |short_name|
+   parallel constructions called within the initializing function.
+
+
+.. container:: section
+
+
+   .. rubric:: Example
+      :class: sectiontitle
+
+   This example illustrates the implementation of lazy initialization
+   for the calculation of the Fibonacci numbers. Here is a graphical
+   representation of the Fibonacci recursion tree for N=4.
+
+
+   |image0|
+
+
+   As seen in the diagram, some elements are recalculated more than once. These operations are redundant,
+   so the "lazy initialized" Fibonacci numbers are relevant here.
+
+
+   An implementation without the use of lazy initialization would have *O(2^N)* time complexity due to
+   the full recursion tree traversal and recalculation of values. Since all the nodes are traversed once,
+   the tree becomes a list, making the time complexity *O(N)*.
+
+
+   |image1|
+
+
+   Here you can see the code for the implementation. Already calculated values are stored in a buffer
+   paired with ``collaborative_once_flag`` and will not be recalculated when ``collaborative_call_once``
+   is invoked when initialization has already been done.
+
+
+   ::
+
+
+      using FibBuffer = std::vector<std::pair<oneapi::tbb::collaborative_once_flag, std::uint64_t>>;
+
+      std::uint64_t LazyFibHelper(int n, FibBuffer& buffer) {
+         // Base case
+         if (n <= 1) {
+            return n;
+         }
+         // Calculate nth value only once and store it in the buffer.
+         // Other threads won't be blocked on already taken collaborative_once_flag
+         // but join parallelism inside functor
+         oneapi::tbb::collaborative_call_once(buffer[n].first, [&]() {
+            std::uint64_t a, b;
+            oneapi::tbb::parallel_invoke([&] { a = LazyFibHelper(n - 2, buffer); },
+                                         [&] { b = LazyFibHelper(n - 1, buffer); });
+            buffer[n].second = a + b;
+         });
+
+         return buffer[n].second;
+      }
+
+      std::uint64_t Fib(int n) {
+         FibBuffer buffer(n+1);
+         return LazyFibHelper(n, buffer);
+      }
+
+
+.. |image0| image:: Images/image008a.jpg
+   :width: 744px
+   :height: 367px
+.. |image1| image:: Images/image009a.jpg
+   :width: 744px
+   :height: 367px