Fixing random number generator function parser rand_seed

source/base/mu_parser_internal.cc

@@ -308,8 +308,10 @@ namespace internal
           parser.DefineFun("log", mu_log, true);
           parser.DefineFun("pow", mu_pow, true);
           parser.DefineFun("erfc", mu_erfc, true);
-          parser.DefineFun("rand_seed", mu_rand_seed, true);
-          parser.DefineFun("rand", mu_rand, true);
+          // Disable optimizations (by passing false) that assume the functions
+          // will always return the same value:
+          parser.DefineFun("rand_seed", mu_rand_seed, false);
+          parser.DefineFun("rand", mu_rand, false);
 
           try
             {

tests/base/function_parser_12.cc

@@ -0,0 +1,67 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2005 - 2020 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, redistribute
+// it, and/or modify it under the terms of the GNU Lesser General
+// Public License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+
+
+// Document a bug that rand_seed() in the FunctionParser always
+// returns the same value.
+
+#include <deal.II/base/function_parser.h>
+#include <deal.II/base/point.h>
+
+#include <deal.II/lac/vector.h>
+
+#include <iostream>
+#include <map>
+#include <random>
+
+#include "../tests.h"
+
+
+// This is copied from the FunctionParser implementation
+double
+mu_rand_seed(double seed)
+{
+  std::uniform_real_distribution<> uniform_distribution(0., 1.);
+
+  static std::map<double, std::mt19937> rng_map;
+
+  if (rng_map.find(seed) == rng_map.end())
+    rng_map[seed] = std::mt19937(static_cast<unsigned int>(seed));
+
+  return uniform_distribution(rng_map[seed]);
+}
+
+
+int
+main()
+{
+  initlog();
+  double                        seed      = 1;
+  std::string                   variables = "x,y";
+  std::map<std::string, double> constants;
+
+  FunctionParser<2> fp(1);
+  fp.initialize(variables, "rand_seed(1)", constants);
+  Point<2> p;
+
+  for (int i = 0; i < 10; ++i)
+    {
+      const double manual  = mu_rand_seed(seed);
+      const double from_fp = fp.value(p);
+      AssertThrow(abs(manual - from_fp) < 1e-16, ExcInternalError());
+    }
+
+  deallog << "OK" << std::endl;
+}

tests/base/function_parser_12.with_muparser=true.output

@@ -0,0 +1,2 @@
+
+DEAL::OK