77 lines
3.4 KiB
C++
77 lines
3.4 KiB
C++
// Copyright 2010-2025 Google LLC
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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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#ifndef OR_TOOLS_SAT_PRIMARY_VARIABLES_H_
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#define OR_TOOLS_SAT_PRIMARY_VARIABLES_H_
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#include <cstdint>
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#include <utility>
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#include <vector>
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#include "ortools/sat/cp_model.pb.h"
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namespace operations_research {
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namespace sat {
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// This structs defines a way of splitting the variables of the model in two
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// groups: primary variables and secondary variables. Those are specified so
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// that the value of secondary_variables[i] is uniquely fixed by applying the
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// constraint dependency_resolution_constraint_index[i] to the values of the
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// primary variables and the values of the variables in the set
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// {secondary_variables[0], ..., secondary_variables[i-1]}.
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//
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// The set of primary variables is implicitly defined by the set of variables
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// that are not in secondary_variables.
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//
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// A useful property of this structure is that given an assignment of primary
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// variables that corresponds to a feasible solution, we can deduce all the
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// values of the secondary variables. Note that if the values of the primary
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// variables are unfeasible, then it might not be possible to deduce the values
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// of the secondary variables.
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struct VariableRelationships {
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std::vector<int> secondary_variables;
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std::vector<ConstraintProto> dependency_resolution_constraint;
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// A pair of(x, y) means that one needs to compute the value of y before
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// computing the value of x. This defines an implicit dependency DAG for
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// computing the secondary variables from the primary.
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std::vector<std::pair<int, int>> variable_dependencies;
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// The list of model constraints that are redundant (ie., satisfied by
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// construction) when the secondary variables are computed from the primary
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// ones. In other words, a model has a solution for a set of primary
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// variables {x_i} if and only if all the variable bounds and non-redundant
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// constraints are satisfied after the secondary variables have been computed
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// from the primary ones.
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std::vector<int> redundant_constraint_indices;
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};
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// Compute the variable relationships for a given model. Note that there are
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// multiple possible ways variables can be split in primary and secondary, so
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// this function will use an heuristic to try to find as many secondary
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// variables as possible. This runs in linear time in the model size (ie., the
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// sum of the number of variables over the constraints).
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VariableRelationships ComputeVariableRelationships(const CpModelProto& model);
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// Given a vector with a partial solution where only the primary variables have
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// a correct value, this function will overwrite the values of the secondary
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// variables so that the solution is complete and valid.
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bool ComputeAllVariablesFromPrimaryVariables(
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const CpModelProto& model, const VariableRelationships& relationships,
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std::vector<int64_t>* solution);
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} // namespace sat
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} // namespace operations_research
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#endif // OR_TOOLS_SAT_PRIMARY_VARIABLES_H_
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