Added an example of parallel pi.

Makefile

@@ -12,7 +12,7 @@ CPPFLAGS=-Wall -g -std=c++0x -pthread
 # CXX=clang++
 # CPPFLAGS=-Wall -g -std=c++0x -stdlib=libc++ -L/opt/clang/lib -pthread
 
-all: auto constexpr ilists init init_new lambda loops regex1 regex2 thread thread2 tuple
+all: auto constexpr ilists init init_new lambda loops pi regex1 regex2 thread thread2 tuple
 
 run: all
 	./auto
@@ -30,7 +30,7 @@ run: all
 
 
 clean:
-	@rm -vf auto constexpr core* ilists init init_new lambda loops regex1 regex2 thread thread2 tuple
+	@rm -vf auto constexpr core* ilists init init_new lambda loops pi regex1 regex2 thread thread2 tuple
 
 stress: thread2
 	zsh stress.zsh

pi.cc

@@ -0,0 +1,58 @@
+#include <iostream>
+#include <thread>
+#include <atomic>
+#include <sys/time.h>
+#include <ctime>
+#include <cmath>
+#include <random>
+
+static const int num_threads = 8;
+static const int max_ = 100000;
+
+void slice_of_pi(int tries, int my_id, std::atomic<int> & in_circle) {
+    struct timeval tv;
+    gettimeofday(&tv, NULL);
+    srand(tv.tv_usec + my_id);
+
+    std::uniform_int_distribution<int> distribution {0, max_};
+    std::mt19937 engine; // Mersenne twister MT19937
+    auto generator = std::bind(distribution, engine);
+
+    double x, y, d;
+    int _in_circle = 0;
+    for(int i = 0; i < tries; i++) {
+        x = (float)generator() / (float)max_;
+        y = (float)generator() / (float)max_;
+        d = sqrt(x * x + y * y);
+        if(d < 1.0)
+            _in_circle++;
+    }
+    in_circle += _in_circle;
+}
+
+int main(int argc, char * argv[]) {
+
+    if(argc != 2) {
+        std::cerr << "pi <number of attempts>" << std::endl;
+        return 1;
+    }
+
+    std::atomic<int> successes {0};
+    int attempts = atoi(argv[1]);
+
+    std::thread t[num_threads];
+
+    for (int i = 0; i < num_threads; ++i) {
+        t[i] = std::thread(slice_of_pi, attempts, i, std::ref(successes));
+    }
+
+
+    for (int i = 0; i < num_threads; ++i) {
+        t[i].join();
+    }
+
+    float pi = 4 * float(successes) / float(attempts * num_threads);
+    printf("%0.8f\n", pi);
+
+    return 0;
+}