From bd51fa48c4c65dbd0054bb34e940fd8703150981 Mon Sep 17 00:00:00 2001
From: Laurent Perron <lperron@google.com>
Date: Sun, 12 Dec 2021 08:11:11 +0100
Subject: [PATCH] implement #2699

---
 examples/cpp/jobshop_sat.cc | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/examples/cpp/jobshop_sat.cc b/examples/cpp/jobshop_sat.cc
index 567e0aaf4f..64e1716a39 100644
--- a/examples/cpp/jobshop_sat.cc
+++ b/examples/cpp/jobshop_sat.cc
@@ -383,6 +383,12 @@ void CreateMachines(
     // Create circuit constraint on a machine. Node 0 is both the source and
     // sink, i.e. the first and last job.
     CircuitConstraint circuit = cp_model.AddCircuitConstraint();
+
+    // If all intervals are optional, a solution without any performed
+    // interval in this resource requires an empty circuit.
+    BoolVar empty_circuit = cp_model.NewBoolVar();
+    circuit.AddArc(0, 0, empty_circuit);
+
     for (int i = 0; i < num_intervals; ++i) {
       const int job_i = machine_to_tasks[m][i].job;
       const MachineTaskData& tail = machine_to_tasks[m][i];
@@ -395,6 +401,10 @@ void CreateMachines(
       // Node to sink.
       circuit.AddArc(i + 1, 0, cp_model.NewBoolVar());
 
+      // If the circuit is empty, the interval cannot be performed.
+      cp_model.AddImplication(empty_circuit,
+                              Not(tail.interval.PresenceBoolVar()));
+
       // Used to constrain the size of the tail interval.
       std::vector<BoolVar> literals;
       std::vector<int64_t> transitions;