chapter 2 code

ch2/accumulate.cpp

@@ -0,0 +1,69 @@
+#include <algorithm>
+#include <functional>
+#include <iostream>
+#include <vector>
+#include <thread>
+#include <numeric> // std::accumulate
+
+template <typename Iterator, typename T>
+struct accumulate_block
+{
+    void operator()(Iterator first, Iterator last, T& result)
+    {
+        result = std::accumulate(first,last,result);
+    }
+};
+
+template <typename Iterator, typename T>
+T parallel_accumulate(Iterator first, Iterator last, T init)
+{
+    unsigned long const length = std::distance(first,last); 
+
+    if (!length) return init;
+
+    unsigned long const min_per_thread = 25;
+    unsigned long const max_threads = 
+        (length+min_per_thread-1)/min_per_thread;
+
+    unsigned long const hardware_threads = 
+        std::thread::hardware_concurrency();
+
+    unsigned long const num_threads = 
+        std::min(hardware_threads != 0 ? hardware_threads:2, max_threads);
+
+    unsigned long const block_size = length / num_threads;
+
+    std::vector<T> results(num_threads);
+    std::vector<std::thread> threads(num_threads-1);
+
+    Iterator block_start = first;
+    for (unsigned long i = 0; i < (num_threads-1); ++i) 
+    {
+        Iterator block_end = block_start;
+        std::advance(block_end,block_size);
+        threads[i] = std::thread(
+                accumulate_block<Iterator,T>(),
+                block_start,block_end,std::ref(results[i]));
+        block_start = block_end;
+    }
+    accumulate_block<std::vector<int>::iterator,int>()(
+            block_start,last,results[num_threads-1]);
+
+    std::for_each(threads.begin(),threads.end(),
+            std::mem_fn(&std::thread::join));
+
+    return std::accumulate(results.begin(),results.end(),init);
+}
+
+int main()
+{
+    std::vector<int> numbers(50);
+    for (int i = 0; i < 50; ++i) {
+        numbers[i] = i;
+    }
+    int result = parallel_accumulate<std::vector<int>::iterator,int>
+        (numbers.begin(),numbers.end(),0);
+
+    std::cout << "result: " << result << std::endl;
+}
+