Added a method to hot-start MIPModel.

src/solvers/MIPModel.java

@@ -8,6 +8,7 @@ import ilog.cplex.IloCplex;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashSet;
+import java.util.OptionalInt;
 import xmtrclusters.Problem;
 
 /**
@@ -229,4 +230,45 @@ public final class MIPModel {
     }
     return clusters;
   }
+
+  /**
+   * Adds a hot start to the MIP.
+   * @param start an initial solution (collection of clusters)
+   * @throws IloException if CPLEX cannot process the starting solution
+   */
+  public void hotStart(final Collection<Collection<Integer>> start)
+              throws IloException {
+    // Set initial values for the x variables (only).
+    int nT = problem.getNTrans();
+    double[][] xx = new double[nT][nT];
+    for (Collection<Integer> c : start) {
+      for (int t : c) {
+        for (int t1 : c) {
+          if (t < t1) {
+            xx[t][t1] = 1.0;
+            xx[t1][t] = 1.0;
+          }
+        }
+      }
+      // Make the smallest element of c the anchor.
+      OptionalInt m = c.stream().mapToInt(i -> i).min();
+      if (m.isPresent()) {
+        int mm = m.getAsInt();
+        xx[mm][mm] = 1.0;
+      }
+    }
+    // Flatten x and xx into vectors.
+    IloNumVar[] vars = new IloNumVar[nT * nT];
+    double[] vals = new double[nT * nT];
+    int index = 0;
+    for (int t = 0; t < nT; t++) {
+      for (int t1 = 0; t1 < nT; t1++) {
+        vars[index] = x[t][t1];
+        vals[index] = xx[t][t1];
+        index += 1;
+      }
+    }
+    // Add the MIP start.
+    mip.addMIPStart(vars, vals);
+  }
 }