Add some comments and move metis seed

Author: filikat

highs/ipm/hipo/factorhighs/Analyse.cpp

@@ -104,28 +104,32 @@ Int Analyse::getPermutation() {
     }
   }
 
-  // call Metis
-  Int metis_seed = 42;
-
   Int options[METIS_NOPTIONS];
   METIS_SetDefaultOptions(options);
-  // fix seed of rng inside Metis, to make it deterministic (?)
-  options[METIS_OPTION_SEED] = metis_seed;
+  options[METIS_OPTION_SEED] = kMetisSeed;
 
   // set logging of Metis depending on debug level
   options[METIS_OPTION_DBGLVL] = 0;
   if (log_->debug(2))
     options[METIS_OPTION_DBGLVL] = METIS_DBG_INFO | METIS_DBG_COARSEN;
 
   if (log_) log_->printDevInfo("Running Metis\n");
+
 #ifdef METIS_THREAD_SAFE
-  unsigned rng_state = metis_seed;
+  // Thread-safe version of Metis that stores state of random-number generator
+  // in local variable.
+  if (log_) log_->printDevInfo("Using thread-safe Metis\n");
+  unsigned rng_state = kMetisSeed;
   Int status = METIS_NodeND(&n_, temp_ptr.data(), temp_rows.data(), NULL,
                             options, perm_.data(), iperm_.data(), &rng_state);
 #else
+  // Default version of Metis. This gives non-deterministic results if multiple
+  // matrices are reordered concurrently.
+  if (log_) log_->printDevInfo("Using non-thread-safe Metis\n");
   Int status = METIS_NodeND(&n_, temp_ptr.data(), temp_rows.data(), NULL,
                             options, perm_.data(), iperm_.data());
 #endif
+
   if (log_) log_->printDevInfo("Metis done\n");
   if (status != METIS_OK) {
     if (log_) log_->printDevInfo("Error with Metis\n");

highs/ipm/hipo/factorhighs/FactorHiGHSSettings.h

@@ -51,6 +51,9 @@ const double kDynamicDiagCoeff = 1e-24;
 const Int kMaxRefinementIter = 3;
 const double kRefinementTolerance = 1e-12;
 
+// metis
+const Int kMetisSeed = 42;
+
 struct Regul {
   double primal{};
   double dual{};