pyflow_example.py

#!/usr/bin/env python3
# Copyright 2010-2022 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
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"""MaxFlow and MinCostFlow examples."""

from absl import app
from ortools.graph.python import max_flow
from ortools.graph.python import min_cost_flow


def MaxFlow():
    """MaxFlow simple interface example."""
    print('MaxFlow on a simple network.')
    tails = [0, 0, 0, 0, 1, 2, 3, 3, 4]
    heads = [1, 2, 3, 4, 3, 4, 4, 5, 5]
    capacities = [5, 8, 5, 3, 4, 5, 6, 6, 4]
    expected_total_flow = 10
    smf = max_flow.SimpleMaxFlow()
    for i in range(0, len(tails)):
        smf.add_arc_with_capacity(tails[i], heads[i], capacities[i])
    if smf.solve(0, 5) == smf.OPTIMAL:
        print('Total flow', smf.optimal_flow(), '/', expected_total_flow)
        for i in range(smf.num_arcs()):
            print('From source %d to target %d: %d / %d' %
                  (smf.tail(i), smf.head(i), smf.flow(i), smf.capacity(i)))
        print('Source side min-cut:', smf.get_source_side_min_cut())
        print('Sink side min-cut:', smf.get_sink_side_min_cut())
    else:
        print('There was an issue with the max flow input.')


def MinCostFlow():
    """MinCostFlow simple interface example.

  Note that this example is actually a linear sum assignment example and will
  be more efficiently solved with the pywrapgraph.LinearSumAssignment class.
  """
    print('MinCostFlow on 4x4 matrix.')
    num_sources = 4
    num_targets = 4
    costs = [[90, 75, 75, 80], [35, 85, 55, 65], [125, 95, 90, 105],
             [45, 110, 95, 115]]
    expected_cost = 275
    smcf = min_cost_flow.SimpleMinCostFlow()
    for source in range(0, num_sources):
        for target in range(0, num_targets):
            smcf.add_arc_with_capacity_and_unit_cost(source,
                                                     num_sources + target, 1,
                                                     costs[source][target])
    for node in range(0, num_sources):
        smcf.set_node_supply(node, 1)
        smcf.set_node_supply(num_sources + node, -1)
    status = smcf.solve()
    if status == smcf.OPTIMAL:
        print('Total flow', smcf.optimal_cost(), '/', expected_cost)
        for i in range(0, smcf.num_arcs()):
            if smcf.flow(i) > 0:
                print('From source %d to target %d: cost %d' %
                      (smcf.tail(i), smcf.head(i) - num_sources,
                       smcf.unit_cost(i)))
    else:
        print('There was an issue with the min cost flow input.')


def main(_=None):
    MaxFlow()
    MinCostFlow()


if __name__ == '__main__':
    app.run(main)