CP-SAT

This directory contains a next-gen Constraint Programming (CP) solver with clause learning. It is built on top of an efficient SAT/max-SAT solver whose code is also in this directory.

The technology started in 2009. A complete presentation is available here.

CP-SAT was described during the CPAIOR 2020 masterclass. The recording is available here.

To begin, skim cp_model.proto to understand how optimization problems can be modeled using the solver. You can then solve a model with the functions in cp_model_solver.h.

Parameters

• sat_parameters.proto: The SAT solver parameters. Also contains max-SAT and CP parameters.

Model

The optimization model description and related utilities:

• cp_model.proto: Proto describing a general Constraint Programming model.
• cp_model_utils.h: Utilities to manipulate and create a cp_model.proto.
• cp_model_checker.h: Tools to validate cp_model.proto and test a solution feasibility.
• cp_model_solver.h: The solver API.
• cp_model_presolve.h: Presolve a cp_model.proto by applying simple rules to simplify a subsequent solve.

SAT solver

Standalone SAT solver and related files. Note that this is more than a basic SAT solver as it already includes non-clause constraints. However, these do not work on the general integer problems that the CP solver handles.

Pure SAT solver:

• sat_base.h: SAT core classes.
• clause.h: SAT clause propagator with the two-watcher mechanism. Also contains propagators for binary clauses.
• sat_solver.h: The SatSolver code.
• simplification.h: SAT postsolver and presolver.
• symmetry.h: Dynamic symmetry breaking constraint in SAT. Not used by default.

Extension:

• pb_constraint.h: Implementation of a Pseudo-Boolean constraint propagator for SAT. Pseudo-Boolean constraints are simply another name used in the SAT community for linear constraints on Booleans.
• no_cycle.h: Implementation of a no-cycle constraint on a graph whose arc presences are controlled by Boolean. This is a SAT propagator, not used in CP.
• encoding.h: Basic algorithm to encode integer variables into a binary representation. This is not used by the CP solver, just by the max-SAT core based algorithm in optimization.h.

Input/output:

• drat_writer.h: Write UNSAT proof in the DRAT format. This allows to check the correctness of an UNSAT proof with the third party program DRAT-trim.
• opb_reader.h: Parser for the .opb format for Pseudo-Boolean optimization problems.
• sat_cnf_reader.h: Parser for the classic SAT .cnf format. Also parses max-SAT files.
• boolean_problem.proto: Deprecated by cp_model.proto.

CP solver

CP solver built on top of the SAT solver:

• integer.h: The entry point, which defines the core of the solver.

Basic constraints:

• all_different.h: Propagation algorithms for the AllDifferent constraint.
• integer_expr.h: Propagation algorithms for integer expression (linear, max, min, div, mod, ...).
• table.h: Propagation algorithms for the table and automaton constraints.
• precedences.h: Propagation algorithms for integer inequalities (integer difference logic theory).
• cp_constraints.h: Propagation algorithms for other classic CP constraints (XOR, circuit, non-overlapping rectangles, ...)
• linear_programming_constraint.h: Constraint that solves an LP relaxation of a set of constraints and uses the dual-ray to explain an eventual infeasibility. Also implements reduced-cost fixing.
• flow_costs.h: Deprecated. Network flow constraint. We use the generic linear_programming_constraint instead.

Scheduling constraints:

• intervals.h: Definition and utility for manipulating "interval" variables (a.k.a. task or activities). This is the basic CP variable type used in scheduling problems.
• disjunctive.h: Propagation algorithms for the disjunctive scheduling constraint.
• cumulative.h, timetable.h, timetable_edgefinding.h: Propagation algorithms for the cumulative scheduling constraint.
• cumulative_energy.h: Propagation algorithms for a more general cumulative constraint.

Packing constraints

• diffn.h: 2D no_overlap constraints (aka non overlapping rectangles). Note these use 2 intervals, one for each dimension to represent a rectangle.
• diffn_util.h: rectangle utilities.

Linear relaxation

• linear_constraint.h: Data structure to represent a linear constraint: lb <= sum (ai * xi) <= ub.
• linear_constraint_manager.h: Utility to manage a set of linear constraints (addition, removal, sorting...)
• linear_programming_constraint.h: Sat propagator that interact with the simplex. It propagates variables bounds from the SAT solver to the simplex, and extract dual information from the simplex to be used by the sat solver.
• linear_model.h: Represents a relaxed view of the model as a set of linear constraints.
• linear_relaxation.h: Builds the static (linear model) and the dynamic (cut generators) linear view of a cp_model.
• cuts.h: Data Data structures and cut generators for the integer constraints.
• routing_cuts.h: Data structures and cut generators for the routing constraints (circuit and routes).
• scheduling_cuts.h: Data structures and cut generators for the scheduling constraints (no_overlap and cumulative).
• diffn_cuts.h: Data structures and cut generators for the packing constraints (no_overlap_2d).

Other

• model.h: Generic class that implements a basic dependency injection framework for singleton objects and manages the memory ownership of all the solver classes.
• optimization.h: Algorithms to solve an optimization problem using a satisfiability solver as an opaque-box.
• lp_utils.h: Utility to scale and convert a MIP model into CP.

Recipes

You can find a set a code recipes in the documentation directory.