add more API on cp_model.py; gate_scheduling sat problem from or-tools.discuss

examples/python/gate_scheduling_sat.py

@@ -69,15 +69,20 @@ def main():
     demands.append(jobs[i][1])
 
     performed_on_m0 = model.NewBoolVar('perform_%i_on_m0' % i)
+
     performed.append(performed_on_m0)
-    start0 = model.NewIntVar(0, horizon, 'start_%i_on_m0' % i)
-    end0 = model.NewIntVar(0, horizon, 'end_%i_on_m0' % i)
+    start0 = model.NewOptionalIntVar(
+        0, horizon, performed_on_m0, 'start_%i_on_m0' % i)
+    end0 = model.NewOptionalIntVar(
+        0, horizon, performed_on_m0, 'end_%i_on_m0' % i)
     interval0 = model.NewOptionalIntervalVar(
         start0, duration, end0, performed_on_m0, 'interval_%i_on_m0' % i)
     intervals0.append(interval0)
 
-    start1 = model.NewIntVar(0, horizon, 'start_%i_on_m1' % i)
-    end1 = model.NewIntVar(0, horizon, 'end_%i_on_m1' % i)
+    start1 = model.NewOptionalIntVar(
+        0, horizon, performed_on_m0.Not(), 'start_%i_on_m1' % i)
+    end1 = model.NewOptionalIntVar(
+        0, horizon, performed_on_m0.Not(), 'end_%i_on_m1' % i)
     interval1 = model.NewOptionalIntervalVar(
         start1, duration, end1, performed_on_m0.Not(), 'interval_%i_on_m1' % i)
     intervals1.append(interval1)

ortools/sat/cp_model_presolve.cc

@@ -338,12 +338,25 @@ MUST_USE_RESULT bool MarkConstraintAsFalse(ConstraintProto* ct,
 
 bool PresolveEnforcementLiteral(ConstraintProto* ct, PresolveContext* context) {
   if (!HasEnforcementLiteral(*ct)) return false;
+
   const int literal = ct->enforcement_literal(0);
   if (context->LiteralIsTrue(literal)) {
     context->UpdateRuleStats("true enforcement literal");
     ct->clear_enforcement_literal();
     return true;
   }
+
+  // TODO(user): because the cumulative and disjunctive constraint refer to this
+  // interval, we cannot simply remove the constraint even if we know that this
+  // optional interval will not be present. We could fix that by removing this
+  // interval from these constraints, but it is difficult to do that in a
+  // general code, so we will need the presolve for these constraint to take
+  // care of that, and then we would be able to remove this interval if it is
+  // not longer used.
+  if (ct->constraint_case() == ConstraintProto::ConstraintCase::kInterval) {
+    return false;
+  }
+
   if (context->LiteralIsFalse(literal)) {
     context->UpdateRuleStats("false enforcement literal");
     return RemoveConstraint(ct, context);

ortools/sat/python/cp_model.py

@@ -296,13 +296,16 @@ class SumArray(IntegerExpression):
 class IntVar(IntegerExpression):
   """Represents a IntegerExpression containing only a single variable."""
 
-  def __init__(self, model, lb, ub, name):
+  def __init__(self, model, lb, ub, is_present_index, name):
     self.__model = model
     self.__index = len(model.variables)
     self.__var = model.variables.add()
     self.__var.domain.extend([lb, ub])
     self.__var.name = name
     self.__negation = None
+    if is_present_index is not None:
+      self.__var.enforcement_literal.append(is_present_index)
+
 
   def Index(self):
     return self.__index
@@ -449,10 +452,14 @@ class CpModel(object):
   # Integer variable.
 
   def NewIntVar(self, lb, ub, name):
-    return IntVar(self.__model, lb, ub, name)
+    return IntVar(self.__model, lb, ub, None, name)
+
+  def NewOptionalIntVar(self, lb, ub, is_present, name):
+    is_present_index = self.GetOrMakeBooleanIndex(is_present)
+    return IntVar(self.__model, lb, ub, is_present_index, name)
 
   def NewBoolVar(self, name):
-    return IntVar(self.__model, 0, 1, name)
+    return IntVar(self.__model, 0, 1, None, name)
 
   # Integer constraints.