bubble_example.py

from __future__ import print_function
from ortools.graph import pywrapgraph

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

  # Define four parallel arrays: start_nodes, end_nodes, capacities, and unit costs
  # between each pair. For instance, the arc from node 0 to node 1 has a
  # capacity of 15 and a unit cost of 4.

  start_nodes = [ 0, 0,  1, 1,  1,  2, 2,  3, 4]
  end_nodes   = [ 1, 2,  2, 3,  4,  3, 4,  4, 2]
  capacities  = [15, 8, 20, 4, 10, 15, 4, 20, 5]
  unit_costs  = [ 4, 4,  2, 2,  6,  1, 3,  2, 3]

  # Define an array of supplies at each node.

  supplies = [20, 0, 0, -5, -15]


  # Instantiate a SimpleMinCostFlow solver.
  min_cost_flow = pywrapgraph.SimpleMinCostFlow()

  # Add each arc.
  for i in range(0, len(start_nodes)):
    min_cost_flow.AddArcWithCapacityAndUnitCost(start_nodes[i], end_nodes[i],
                                                capacities[i], unit_costs[i])

  # Add node supplies.

  for i in range(0, len(supplies)):
    min_cost_flow.SetNodeSupply(i, supplies[i])


  # Find the minimum cost flow between node 0 and node 4.
  if min_cost_flow.Solve() == min_cost_flow.OPTIMAL:
    print('Minimum cost:', min_cost_flow.OptimalCost())
    print('')
    print('  Arc    Flow / Capacity  Cost')
    for i in range(min_cost_flow.NumArcs()):
      cost = min_cost_flow.Flow(i) * min_cost_flow.UnitCost(i)
      print('%1s -> %1s   %3s  / %3s       %3s' % (
          min_cost_flow.Tail(i),
          min_cost_flow.Head(i),
          min_cost_flow.Flow(i),
          min_cost_flow.Capacity(i),
          cost))
  else:
    print('There was an issue with the min cost flow input.')

if __name__ == '__main__':
  main()