Update constraint_engine.py

constraint_engine.py

@@ -315,3 +315,35 @@ class ConstraintEngine:
                     for lit in at_slot: self.model.AddImplication(is_active.Not(), lit.Not())
                     literals.append(is_active)
                 self.model.Add(sum(literals) <= self.max_continuous_stretch)
+
+    def _apply_no_first_hour_free(self):
+        """
+        HARD CONSTRAINT:
+        Ensures that the first hour (slot 0) of every day is never free for any section.
+        Every section must have at least one class starting at slot 0 every day.
+        """
+        from collections import defaultdict
+        import constants as const
+        
+        tasks_by_sec = defaultdict(list)
+        for task in self.tasks:
+            tasks_by_sec[task.section.section_id].append(task)
+
+        for sec_id, sec_tasks in tasks_by_sec.items():
+            if not sec_tasks: continue
+
+            for day in range(const.NUM_WORKING_DAYS):
+                first_slot = day * const.NUM_TEACHING_SLOTS_PER_DAY
+                
+                starts_at_first = []
+                for task in sec_tasks:
+                    start_var = self.task_vars[task.task_id][0]
+                    # We create a boolean variable that is True if start_var == first_slot
+                    at_first = self.model.NewBoolVar(f"hard_at1st_{task.task_id}_d{day}")
+                    self.model.Add(start_var == first_slot).OnlyEnforceIf(at_first)
+                    self.model.Add(start_var != first_slot).OnlyEnforceIf(at_first.Not())
+                    starts_at_first.append(at_first)
+
+                if starts_at_first:
+                    # At least one task MUST start at the first slot
+                    self.model.AddBoolOr(starts_at_first)