PCG random number generator (#369)

Add support for Permuted Congruential Generator (PCG) for random number generation. This is an optional drop-in replacement for C++'s Mersenne Twister. See https://www.pcg-random.org/ for more information. The new engine must be enabled at compile time with `ENABLE_PCG=on`.
mlund · May 27, 2021 · 4cc9e5a · 4cc9e5a
1 parent 3d35805
commit 4cc9e5a
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Showing 6 changed files with 38 additions and 4 deletions.
diff --git a/cmake/ExternalTools.cmake b/cmake/ExternalTools.cmake
@@ -1,6 +1,24 @@
 include(ExternalProject)
 include(FetchContent)
 
+#############
+# PCG RANDOM
+#############
+
+option(ENABLE_PCG "Enable PCG random number generator" off)
+if (ENABLE_PCG)
+    FetchContent_Declare(
+        pcg-cpp
+        URL https://github.com/imneme/pcg-cpp/archive/ffd522e7188bef30a00c74dc7eb9de5faff90092.tar.gz
+        URL_HASH MD5=051b969bbaf924f35f2159813f93e341)
+    FetchContent_GetProperties(pcg-cpp)
+    if(NOT pcg-cpp_POPULATED)
+        FetchContent_Populate(pcg-cpp)
+    endif()
+    include_directories(SYSTEM ${pcg-cpp_SOURCE_DIR}/include)
+    add_definitions("-DENABLE_PCG")
+endif()
+
 #########
 # EXPRTK 
 #########

diff --git a/docs/_docs/install.md b/docs/_docs/install.md
@@ -74,6 +74,7 @@ CMake Option                         | Description
 `-DENABLE_PYTHON=OFF`                | Build python bindings (experimental)
 `-DENABLE_FREESASA=ON`               | Enable SASA routines (external download)
 `-DENABLE_TBB=OFF`                   | Build with Intel Threading Building Blocks (experimental)
+`-DENABLE_PCG=OFF`                   | Use PCG random number generator instead of C++'s Mersenne Twister
 `-DBUILD_STATIC=OFF`                 | Build statically linked binaries
 `-DCMAKE_BUILD_TYPE=Release`         | Alternatives: `Debug` or `RelWithDebInfo`
 `-DCMAKE_CXX_FLAGS_RELEASE="..."`    | Compiler options for Release mode

diff --git a/examples/cluster-ideal/cluster-ideal.yml b/examples/cluster-ideal/cluster-ideal.yml
@@ -27,7 +27,7 @@ energy:
 moves:
     - moltransrot: {molecule: water, dp: 20, dprot: 1, repeat: 1}
     - moltransrot: {molecule: other, dp: 20, dprot: 1, repeat: 1}
-    - cluster: {molecules: [water, other], dp: 20, dprot: 3, threshold: 7, satellites: [other]}
+    - cluster: {molecules: [water, other], dp: 19.99999, dprot: 3, threshold: 7, satellites: [other]}
 
 analysis:
     - savestate: {file: confout.pqr}

diff --git a/src/core.cpp b/src/core.cpp
@@ -212,7 +212,7 @@ Point ranunit(Random &rand, const Point &dir) {
 
 TEST_CASE("[Faunus] ranunit") {
     Random r;
-    int n = 2e5;
+    int n = 4e5;
     Point rtp(0, 0, 0);
     for (int i = 0; i < n; i++)
         rtp += xyz2rtp(ranunit(r));

diff --git a/src/random.cpp b/src/random.cpp
@@ -31,9 +31,14 @@ void to_json(nlohmann::json &j, const Random &random) {
     std::ostringstream stream;
     stream << random.engine; // dump engine state to stream
     j["seed"] = stream.str();
+    if constexpr (std::is_same<RandomNumberEngine, std::mt19937>::value) {
+        j["engine"] = "Mersenne Twister (std::mt19937)";
+    } else {
+        j["engine"] = "Permuted Congruential Generator (pcg32)";
+    }
 }
 
-void Random::seed() { engine = std::mt19937(std::random_device()()); }
+void Random::seed() { engine = RandomNumberEngine(std::random_device()()); }
 
 Random::Random() : dist01(0, 1) {}
 

diff --git a/src/random.h b/src/random.h
@@ -5,8 +5,18 @@
 #include <stdexcept>
 #include <nlohmann/json_fwd.hpp>
 
+#ifdef ENABLE_PCG
+#include <pcg_random.hpp>
+#endif
+
 namespace Faunus {
 
+#ifdef ENABLE_PCG
+using RandomNumberEngine = pcg32;
+#else
+using RandomNumberEngine = std::mt19937;
+#endif
+
 /**
  * @brief Random number generator
  *
@@ -22,7 +32,7 @@ class Random {
   private:
     std::uniform_real_distribution<double> dist01; //!< Uniform real distribution [0,1)
   public:
-    std::mt19937 engine; //!< Random number engine used for all operations
+    RandomNumberEngine engine; //!< Random number engine used for all operations
     Random();            //!< Constructor with deterministic seed
     void seed();         //!< Set a non-deterministic ("hardware") seed
     double operator()(); //!< Random double in uniform range [0,1)