// (c) 2023-2025 Fair Isaac Corporation
import static com.dashoptimization.objects.Utils.sum;
import java.util.Arrays;
import com.dashoptimization.ColumnType;
import com.dashoptimization.XPRSconstants;
import com.dashoptimization.XPRSenumerations;
import com.dashoptimization.objects.Variable;
import com.dashoptimization.objects.XpressProblem;
/**
* Load balancing of train wagons. econd version, using heuristic solution as
* start solution for MIP.
*/
public class Wagon {
/* Box weights */
private static final int[] WEIGHT = { 34, 6, 8, 17, 16, 5, 13, 21, 25, 31, 14, 13, 33, 9, 25, 25 };
private static final int NBOXES = WEIGHT.length; /* Number of boxes */
private static final int NWAGONS = 3; /* Number of wagons */
private static final int WMAX = 100; /* Weight limit of the wagons */
private static final int[] HeurSol = new int[NBOXES]; /* Heuristic solution: for each box */
/* Variables */
private static Variable[][] load;
private static Variable maxweight;
private static void model(XpressProblem p) {
// Create load[box,wagon] (binary)
load = p.addVariables(NBOXES, NWAGONS).withType(ColumnType.Binary)
.withName((b, w) -> String.format("load_%d_%d", b + 1, w + 1)).toArray();
// Create maxweight (continuous with lb=ceil((sum(b in BOXES) WEIGHT(b))/NBOXES)
maxweight = p.addVariable(Math.ceil(Arrays.stream(WEIGHT).sum() / (double) NBOXES), XPRSconstants.PLUSINFINITY,
ColumnType.Continuous, "maxweight");
// Every box into one wagon: forall(b in BOXES) sum(w in WAGONS) load(b,w) = 1
p.addConstraints(NBOXES, b -> sum(load[b]).eq(1.0));
// Limit the weight loaded into every wagon: forall(w in WAGONS) sum(b in BOXES)
// WEIGHT(b)*load(b,w) <= maxweight
p.addConstraints(NWAGONS,
w -> sum(NBOXES, b -> load[b][w].mul(WEIGHT[b])).leq(maxweight).setName(String.format("wmilit_%d", w)));
// Minimize maxweight
p.setObjective(maxweight, XPRSenumerations.ObjSense.MINIMIZE);
p.writeProb("Wagon.lp", "l");
}
/**
* LPT (Longest processing time) heuristic: One at a time, place the heaviest
* unassigned box onto the wagon with the least load
*
* @return A dense vector with the heuristic solution.
*/
private static double heuristic() {
Integer[] ORDERW = new Integer[NBOXES]; // Box indices sorted in decreasing weight order
int[] CurNum = new int[NWAGONS]; // For each wagon w, this is the number of boxes currently loaded
int[] CurWeight = new int[NWAGONS]; // For each wagon w, this is the current weight, i.e. the sum of weights of
// loaded boxes
int[][] Load = new int[NWAGONS][NBOXES]; // Load[w][i] (for i=0..CurNum[w]-1) contains the box index of the i-th
// box loaded on wagon w
// Copy the box indices into array ORDERW and sort them in decreasing
// order of box weights (the sorted indices are returned in array ORDERW)
for (int b = 0; b < NBOXES; b++)
ORDERW[b] = b;
Arrays.sort(ORDERW, (i1, i2) -> ((Integer) WEIGHT[i2]).compareTo(WEIGHT[i1]));
// Distribute the loads to the wagons using the LPT heuristic
for (int b = 0; b < NBOXES; b++) {
int v = 0; /* Find wagon v with the smallest load */
for (int w = 0; w < NWAGONS; w++)
if (CurWeight[w] <= CurWeight[v])
v = w;
Load[v][CurNum[v]] = ORDERW[b]; /* Add current box to wagon v */
CurNum[v]++; /* Increase the counter of boxes on v */
CurWeight[v] += WEIGHT[ORDERW[b]]; /* Update current weight of the wagon */
}
// Calculate the solution value
double heurobj = 0; /* heuristic solution objective value (max wagon weight) */
for (int w = 0; w < NWAGONS; w++)
if (CurWeight[w] > heurobj)
heurobj = CurWeight[w];
// Solution printing
System.out.printf("Heuristic solution:%n Max weight: %.0f\n", heurobj);
for (int w = 0; w < NWAGONS; w++) {
System.out.printf(" %d:", w + 1);
for (int i = 0; i < CurNum[w]; i++)
System.out.print(" " + (Load[w][i] + 1));
System.out.printf(" (total weight: %d)\n", CurWeight[w]);
}
// Save the heuristic solution into the HeurSol array
for (int w = 0; w < NWAGONS; w++)
for (int i = 0; i < CurNum[w]; i++)
HeurSol[Load[w][i]] = w;
return heurobj;
}
private static void optimization(XpressProblem p) {
// Get the solution from the heuristic solution we have found
// Send the solution to the optimizer
Variable[] heurmipvar = new Variable[NBOXES];
double[] heurmipsol = new double[NBOXES];
for (int b = 0; b < NBOXES; b++) {
heurmipvar[b] = load[b][HeurSol[b]];
heurmipsol[b] = 1.0;
}
p.addMipSol(heurmipsol, heurmipvar, "heuristic");
p.optimize();
if (p.attributes().getSolStatus() != XPRSenumerations.SolStatus.OPTIMAL
&& p.attributes().getSolStatus() != XPRSenumerations.SolStatus.FEASIBLE)
throw new RuntimeException("failed to optimize with status " + p.attributes().getSolStatus());
System.out.println(String.format(
"Problem status:\n\tSolve status: %s\n\tLP status: %s\n\tMIP status: %s\n\tSol status: %s",
p.attributes().getSolveStatus(), p.attributes().getLPStatus(), p.attributes().getMIPStatus(),
p.attributes().getSolStatus()));
// An integer solution has been found
if (p.attributes().getMIPStatus() == XPRSenumerations.MIPStatus.OPTIMAL
|| p.attributes().getMIPStatus() == XPRSenumerations.MIPStatus.SOLUTION) {
double[] sol = p.getSolution();
System.out.printf("Optimal solution:\n Max weight: %.0f%n", p.attributes().getObjVal());
for (int w = 0; w < NWAGONS; w++) {
double tot_weight = 0.;
System.out.printf(" %d:", w + 1);
for (int b = 0; b < NBOXES; b++)
if (load[b][w].getValue(sol) * WEIGHT[b] > .5) {
System.out.printf(" %.0f", load[b][w].getValue(sol) * WEIGHT[b]);
tot_weight += load[b][w].getValue(sol) * WEIGHT[b];
}
System.out.printf(" (total weight: %.0f)%n", tot_weight);
}
}
}
public static void main(String[] args) {
if (heuristic() <= WMAX) {
System.out.println("Heuristic solution fits capacity limits");
} else {
try (XpressProblem prob = new XpressProblem()) {
/* Suppress outputs */
model(prob);
optimization(prob);
}
}
}
}
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