Merge pull request #4691 from markdewing/fix_4690

Guard batch size computation against zero

src/QMCDrivers/WFOpt/QMCCostFunctionBatched.cpp

@@ -191,16 +191,26 @@ void QMCCostFunctionBatched::getConfigurations(const std::string& aroot)
   }
 }
 
-// Input - sample_size  - number of samples to process
-//       - batch_size  -  process samples in batch_size at a time
-// Output - num_batches - number of batches to use
-//        - final_batch_size - the last batch size.  May be smaller than batch_size
-//                             if the number of samples is not a multiple of the batch size
+  /** Compute number of batches and final batch size given the number of samples
+   *   and a batch size.
+   * \param[in] sample_size number of samples to process.
+   * \param[in] batch_size process samples in batch_size at a time (typically the number of walkers in a crowd).
+   * \param[out] num_batches number of batches to use.
+   * \param[out] final_batch_size the last batch size.  May be smaller than batch_size
+   *             if the number of samples is not a multiple of the batch size.
+   *
+   * There may be cases where the batch size is zero. One cause is when the number of walkers per
+   *  rank is less than the number of crowds.
+   */
 void compute_batch_parameters(int sample_size, int batch_size, int& num_batches, int& final_batch_size)
 {
-  num_batches      = sample_size / batch_size;
+  if (batch_size == 0)
+    num_batches = 0;
+  else
+    num_batches = sample_size / batch_size;
+
   final_batch_size = batch_size;
-  if (sample_size % batch_size != 0)
+  if (batch_size != 0 && sample_size % batch_size != 0)
   {
     num_batches += 1;
     final_batch_size = sample_size % batch_size;

src/QMCDrivers/tests/test_QMCCostFunctionBatched.cpp

@@ -40,6 +40,11 @@ TEST_CASE("compute_batch_parameters", "[drivers]")
   compute_batch_parameters(sample_size, batch_size, num_batches, final_batch_size);
   CHECK(num_batches == 3);
   CHECK(final_batch_size == 3);
+
+  batch_size = 0;
+  compute_batch_parameters(sample_size, batch_size, num_batches, final_batch_size);
+  CHECK(num_batches == 0);
+  CHECK(final_batch_size == 0);
 }
 
 namespace testing