FlowExample.java

// Copyright 2010-2024 Google LLC
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.google.ortools.java;

import com.google.ortools.Loader;
import com.google.ortools.graph.MaxFlow;
import com.google.ortools.graph.MinCostFlow;

/**
 * Sample showing how to model using the flow solver.
 *
 */
public class FlowExample {
  private static void solveMinCostFlow() {
    System.out.println("Min Cost Flow Problem - Simple interface");
    final int numSources = 4;
    final int numTargets = 4;
    final int[][] costs = {
        {90, 75, 75, 80}, {35, 85, 55, 65}, {125, 95, 90, 105}, {45, 110, 95, 115}};
    final int expectedCost = 275;
    MinCostFlow minCostFlow = new MinCostFlow();
    for (int source = 0; source < numSources; ++source) {
      for (int target = 0; target < numTargets; ++target) {
        minCostFlow.addArcWithCapacityAndUnitCost(
            source, numSources + target, 1, costs[source][target]);
      }
    }
    for (int node = 0; node < numSources; ++node) {
      minCostFlow.setNodeSupply(node, 1);
      minCostFlow.setNodeSupply(numSources + node, -1);
    }
    if (minCostFlow.solve() == MinCostFlow.Status.OPTIMAL) {
      final long totalFlowCost = minCostFlow.getOptimalCost();
      System.out.println("total flow = " + totalFlowCost + "/" + expectedCost);
      for (int i = 0; i < minCostFlow.getNumArcs(); ++i) {
        if (minCostFlow.getFlow(i) > 0) {
          System.out.println("From source " + minCostFlow.getTail(i) + " to target "
              + minCostFlow.getHead(i) + ": cost " + minCostFlow.getUnitCost(i));
        }
      }
    } else {
      System.out.println("No solution found");
    }
  }

  private static void solveMaxFlow() {
    System.out.println("Max Flow Problem - Simple interface");
    final int[] tails = {0, 0, 0, 0, 1, 2, 3, 3, 4};
    final int[] heads = {1, 2, 3, 4, 3, 4, 4, 5, 5};
    final int[] capacities = {5, 8, 5, 3, 4, 5, 6, 6, 4};
    final int expectedTotalFlow = 10;
    MaxFlow maxFlow = new MaxFlow();
    for (int i = 0; i < tails.length; ++i) {
      maxFlow.addArcWithCapacity(tails[i], heads[i], capacities[i]);
    }
    if (maxFlow.solve(0, 5) == MaxFlow.Status.OPTIMAL) {
      System.out.println("Total flow " + maxFlow.getOptimalFlow() + "/" + expectedTotalFlow);
      for (int i = 0; i < maxFlow.getNumArcs(); ++i) {
        System.out.println("From source " + maxFlow.getTail(i) + " to target " + maxFlow.getHead(i)
            + ": " + maxFlow.getFlow(i) + " / " + maxFlow.getCapacity(i));
      }
      // TODO(user): Our SWIG configuration does not currently handle these
      // functions correctly in Java:
      // maxFlow.getSourceSideMinCut(...);
      // maxFlow.getSinkSideMinCut(...);
    } else {
      System.out.println("There was an issue with the input.");
    }
  }

  public static void main(String[] args) throws Exception {
    Loader.loadNativeLibraries();
    FlowExample.solveMinCostFlow();
    FlowExample.solveMaxFlow();
  }
}