Student Scheduling: Concurrent Modification

- improved handing of concurrent modification errors during backtracking (caused by a model change during the search) - retry computation of the neighbour selection up to five times rather than skipping to the next student/request
UniTime · Nov 15, 2020 · 12657a9 · 12657a9
1 parent 55c3eab
commit 12657a9
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Showing 3 changed files with 35 additions and 18 deletions.
diff --git a/src/org/cpsolver/studentsct/heuristics/selection/BacktrackSelection.java b/src/org/cpsolver/studentsct/heuristics/selection/BacktrackSelection.java
@@ -4,6 +4,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.ConcurrentModificationException;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -114,15 +115,20 @@ public Neighbour<Request, Enrollment> selectNeighbour(Solution<Request, Enrollme
             Enrollment e = request.getAssignment(solution.getAssignment());
             if (e != null && request instanceof FreeTimeRequest) continue;
             if (e != null && e.getPriority() == 0 && ((CourseRequest)request).getSelectedChoices().isEmpty()) continue;
-            Neighbour<Request, Enrollment> n = iRBtNSel.selectNeighbour(solution, request);
-            if (iRBtNSel.getContext() != null) {
-                iNbrIterations ++;
-                iTotalTime += iRBtNSel.getContext().getTime();
-                if (iRBtNSel.getContext().isTimeoutReached()) iNbrTimeoutReached ++;
-                if (n == null) iNbrNoSolution ++;
+            for (int i = 0; i < 5; i++) {
+                try {
+                    Neighbour<Request, Enrollment> n = iRBtNSel.selectNeighbour(solution, request);
+                    if (iRBtNSel.getContext() != null) {
+                        iNbrIterations ++;
+                        iTotalTime += iRBtNSel.getContext().getTime();
+                        if (iRBtNSel.getContext().isTimeoutReached()) iNbrTimeoutReached ++;
+                        if (n == null) iNbrNoSolution ++;
+                    }
+                    if (n != null && n.value(solution.getAssignment()) <= 0.0)
+                        return n;
+                    break;
+                } catch (ConcurrentModificationException ex) {}
             }
-            if (n != null && n.value(solution.getAssignment()) <= 0.0)
-                return n;
         }
         return null;
     }

diff --git a/src/org/cpsolver/studentsct/heuristics/selection/BranchBoundSelection.java b/src/org/cpsolver/studentsct/heuristics/selection/BranchBoundSelection.java
@@ -5,6 +5,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
+import java.util.ConcurrentModificationException;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -192,9 +193,13 @@ public Neighbour<Request, Enrollment> selectNeighbour(Solution<Request, Enrollme
         Student student = null;
         while ((student = nextStudent()) != null) {
             Progress.getInstance(solution.getModel()).incProgress();
-            Neighbour<Request, Enrollment> neighbour = getSelection(solution.getAssignment(), student).select();
-            if (neighbour != null)
-                return neighbour;
+            for (int i = 0; i < 5; i++) {
+                try {
+                    Neighbour<Request, Enrollment> neighbour = getSelection(solution.getAssignment(), student).select();
+                    if (neighbour != null) return neighbour;
+                    break;
+                } catch (ConcurrentModificationException e) {}
+            }
         }
         return null;
     }

diff --git a/src/org/cpsolver/studentsct/heuristics/selection/SwapStudentSelection.java b/src/org/cpsolver/studentsct/heuristics/selection/SwapStudentSelection.java
@@ -4,6 +4,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.ConcurrentModificationException;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedList;
@@ -162,13 +163,18 @@ public Neighbour<Request, Enrollment> selectNeighbour(Solution<Request, Enrollme
             Progress.getInstance(solution.getModel()).incProgress();
             if (student.isComplete(solution.getAssignment()) || student.nrAssignedRequests(solution.getAssignment()) == 0)
                 continue;
-            Selection selection = getSelection(solution.getAssignment(), student);
-            Neighbour<Request, Enrollment> neighbour = selection.select();
-            if (neighbour != null) {
-                addStudent(student);
-                return neighbour;
-            } else
-                iProblemStudents.addAll(selection.getProblemStudents());
+            for (int i = 0; i < 5; i++) {
+                try {
+                    Selection selection = getSelection(solution.getAssignment(), student);
+                    Neighbour<Request, Enrollment> neighbour = selection.select();
+                    if (neighbour != null) {
+                        addStudent(student);
+                        return neighbour;
+                    } else
+                        iProblemStudents.addAll(selection.getProblemStudents());
+                    break;
+                } catch (ConcurrentModificationException e) {}
+            }
         }
         return null;
     }