add original source

source/listing_8.5.cpp

@@ -0,0 +1,20 @@
+template<typename Iterator,typename T>
+T parallel_accumulate(Iterator first,Iterator last,T init)
+{
+    unsigned long const length=std::distance(first,last);
+    unsigned long const max_chunk_size=25;
+    if(length<=max_chunk_size)
+    {
+        return std::accumulate(first,last,init);
+    }
+    else
+    {
+        Iterator mid_point=first;
+        std::advance(mid_point,length/2);
+        std::future<T> first_half_result=
+            std::async(parallel_accumulate<Iterator,T>,
+                       first,mid_point,init);
+        T second_half_result=parallel_accumulate(mid_point,last,T());
+        return first_half_result.get()+second_half_result;
+    }
+}