Random: add seed parameter

configure.ac

@@ -1052,6 +1052,16 @@ fi
 AC_SUBST([TBBMALLOC_LIBS])
 AM_CONDITIONAL([HAVETBBMALLOC],[test -n "$TBBMALLOC_LIBS"])
 
+# check for a random seed
+AC_ARG_WITH([random-seed],
+    [AS_HELP_STRING([--with-random-seed=SHORT INT],
+       [Set the random seed.]
+    )],
+    [with_random_seed="$withval"],
+    [with_random_seed="0"]
+)
+AC_DEFINE_UNQUOTED([SWIFT_RANDOM_SEED_XOR], [$with_random_seed],[Value of the random seed.]),
+
 # Check for python.
 have_python="no"
 AC_ARG_WITH([python],

doc/RTD/source/random.rst

@@ -29,6 +29,11 @@ argument called the type of random number which is basically the nth random
 number for the specified seed, which is added to the particle ID, thus providing
 a distinct state per random number type.
 
+If the user wishes to run a simulation with a different set of random number,
+an option during the configuration (``--with-random-seed=INT``) is available.
+This option simply flip some bits in the initial number composed of the ID and the
+current simulation time through the binary operator XOR.
+
 Implementation
 ~~~~~~~~~~~~~~
 

src/common_io.c

@@ -455,6 +455,7 @@ void io_write_code_description(hid_t h_file) {
 #else
   io_write_attribute_s(h_grpcode, "MPI library", "Non-MPI version of SWIFT");
 #endif
+  io_write_attribute_i(h_grpcode, "RandomSeed", SWIFT_RANDOM_SEED_XOR);
   H5Gclose(h_grpcode);
 }
 

src/random.h

@@ -140,6 +140,7 @@ INLINE static double inl_erand48(uint16_t xsubi[3]) { return erand48(xsubi); }
 INLINE static double random_unit_interval(int64_t id,
                                           const integertime_t ti_current,
                                           const enum random_number_type type) {
+
   /* Start by packing the state into a sequence of 16-bit seeds for rand_r. */
   uint16_t buff[9];
   id += type;
@@ -151,6 +152,9 @@ INLINE static double random_unit_interval(int64_t id,
      value to get 18 bytes of state. */
   buff[8] = 6178;
 
+  /* Use the random seed to generate a new random number */
+  buff[0] = buff[0] ^ (uint16_t)SWIFT_RANDOM_SEED_XOR;
+
   /* Shuffle the buffer values, this will be our source of entropy for
      the erand48 generator. */
   uint32_t seed16 = 0;

tests/testRandom.c

@@ -100,6 +100,9 @@ int main(int argc, char* argv[]) {
   message("Seed = %d", seed);
   srand(seed);
 
+  /* Log the swift random seed */
+  message("SWIFT random seed = %d", SWIFT_RANDOM_SEED_XOR);
+
   /* Time-step size */
   const int time_bin = 30;
 

tests/testRandomSpacing.c

@@ -49,6 +49,9 @@ int main(int argc, char* argv[]) {
   message("Seed = %d", seed);
   srand(seed);
 
+  /* Log the swift random seed */
+  message("SWIFT random seed = %d", SWIFT_RANDOM_SEED_XOR);
+
   /* Time-step size */
   const int time_bin = 30;