93 lines
2.6 KiB
Python
93 lines
2.6 KiB
Python
# Copyright 2010 Hakan Kjellerstrand hakank@gmail.com
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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"""
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Set covering in Google CP Solver.
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Placing of firestations, from Winston 'Operations Research', page 486.
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Compare with the following models:
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* MiniZinc: http://www.hakank.org/minizinc/set_covering.mzn
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* ECLiPSe : http://www.hakank.org/eclipse/set_covering.ecl
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* Comet : http://www.hakank.org/comet/set_covering.co
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* Gecode : http://www.hakank.org/gecode/set_covering.cpp
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* SICStus : http://www.hakank.org/sicstus/set_covering.pl
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This model was created by Hakan Kjellerstrand (hakank@gmail.com)
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Also see my other Google CP Solver models:
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http://www.hakank.org/google_or_tools/
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"""
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from __future__ import print_function
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from ortools.constraint_solver import pywrapcp
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def main(unused_argv):
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# Create the solver.
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solver = pywrapcp.Solver("Set covering")
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#
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# data
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#
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min_distance = 15
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num_cities = 6
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distance = [[0, 10, 20, 30, 30, 20], [10, 0, 25, 35, 20, 10],
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[20, 25, 0, 15, 30, 20], [30, 35, 15, 0, 15, 25],
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[30, 20, 30, 15, 0, 14], [20, 10, 20, 25, 14, 0]]
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#
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# declare variables
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#
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x = [solver.IntVar(0, 1, "x[%i]" % i) for i in range(num_cities)]
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#
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# constraints
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#
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# objective to minimize
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z = solver.Sum(x)
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# ensure that all cities are covered
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for i in range(num_cities):
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b = [x[j] for j in range(num_cities) if distance[i][j] <= min_distance]
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solver.Add(solver.SumGreaterOrEqual(b, 1))
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objective = solver.Minimize(z, 1)
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#
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# solution and search
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#
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solution = solver.Assignment()
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solution.Add(x)
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solution.AddObjective(z)
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collector = solver.LastSolutionCollector(solution)
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solver.Solve(
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solver.Phase(x + [z], solver.INT_VAR_DEFAULT, solver.INT_VALUE_DEFAULT),
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[collector, objective])
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print("z:", collector.ObjectiveValue(0))
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print("x:", [collector.Value(0, x[i]) for i in range(num_cities)])
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print("failures:", solver.Failures())
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print("branches:", solver.Branches())
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print("WallTime:", solver.WallTime())
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if __name__ == "__main__":
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main("cp sample")
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