rng: use process-global generators

src/include/rng.hxx

@@ -6,27 +6,51 @@
 #include <random>
 #include <chrono>
 #include <functional>
+#include <memory>
+#include "mpi.h"
 
 namespace rng
 {
 
+namespace detail
+{
+using Generator = std::default_random_engine;
+using ProcessGeneratorPtr = std::shared_ptr<Generator>;
+ProcessGeneratorPtr process_generator;
+
+int get_process_seed()
+{
+  int rank;
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+  return 10000 * (rank + 1);
+}
+
+ProcessGeneratorPtr get_process_generator()
+{
+  if (!process_generator) {
+    process_generator = std::make_shared<Generator>(get_process_seed());
+  }
+  return process_generator;
+}
+
+} // namespace detail
+
 // ======================================================================
 // Uniform
 
 template <typename Real>
 struct Uniform
 {
   Uniform(Real min, Real max)
-    : dist(min, max),
-      gen(std::chrono::system_clock::now().time_since_epoch().count())
+    : dist(min, max), gen(detail::get_process_generator())
   {}
 
   Uniform() : Uniform(0, 1) {}
 
-  Real get() { return dist(gen); }
+  Real get() { return dist(*gen); }
 
 private:
-  std::default_random_engine gen;
+  detail::ProcessGeneratorPtr gen;
   std::uniform_real_distribution<Real> dist;
 };
 
@@ -37,21 +61,20 @@ template <typename Real>
 struct Normal
 {
   Normal(Real mean, Real stdev)
-    : dist(mean, stdev),
-      gen(std::chrono::system_clock::now().time_since_epoch().count())
+    : dist(mean, stdev), gen(detail::get_process_generator())
   {}
   Normal() : Normal(0, 1) {}
 
-  Real get() { return dist(gen); }
+  Real get() { return dist(*gen); }
   // FIXME remove me, or make standalone func
   Real get(Real mean, Real stdev)
   {
     // should be possible to pass params to existing dist
-    return std::normal_distribution<Real>(mean, stdev)(gen);
+    return std::normal_distribution<Real>(mean, stdev)(*gen);
   }
 
 private:
-  std::default_random_engine gen;
+  detail::ProcessGeneratorPtr gen;
   std::normal_distribution<Real> dist;
 };