CPMpy Hexaly interface (cpmpy.solvers.hexaly)

Interface to Hexaly’s API

Hexaly is a global optimization solver that supports nonlinear and a few global constraints.

Always use cp.SolverLookup.get("hexaly") to instantiate the solver object.

Installation

Requires that the ‘hexaly’ python package is installed:

$ pip install hexaly -i https://pip.hexaly.com

It also requires to install the Hexaly Optimizer with a Hexaly license (for example a free academic license) You can read more about available licences at https://www.hexaly.com/

See detailed installation instructions at: https://www.hexaly.com/docs/last/installation/pythonsetup.html

The rest of this documentation is for advanced users.

List of classes

CPM_hexaly

Interface to Hexaly's API

class cpmpy.solvers.hexaly.CPM_hexaly(cpm_model=None, subsolver=None)

Interface to Hexaly’s API

Creates the following attributes (see parent constructor for more):

• hex_model: object, Hexaly’s model object

• hex_solver: object, Hexaly’s solver object (to solve hex_model)

Documentation of the solver’s own Python API: https://www.hexaly.com/docs/last/pythonapi/index.html

add(cpm_expr_orig)

Eagerly add a constraint to the underlying solver.

Any CPMpy expression given is immediately transformed (through transform()) and then posted to the solver in this function.

This can raise ‘NotImplementedError’ for any constraint not supported after transformation

The variables used in expressions given to add are stored as ‘user variables’. Those are the only ones the user knows and cares about (and will be populated with a value after solve). All other variables are auxiliary variables created by transformations.

Parameters:

cpm_expr (Expression or list of Expression) – CPMpy expression, or list thereof

Returns:

self

get_core()

For use with s.solve(assumptions=[...]). Only meaningful if the solver returned UNSAT.

Typically implemented in SAT-based solvers

Returns a small subset of assumption literals that are unsat together. (a literal is either a _BoolVarImpl or a NegBoolView in case of its negation, e.g. x or ~x) Setting these literals to True makes the model UNSAT, setting any to False makes it SAT

has_objective()

Returns whether the solver has an objective function or not.

static installed()

static license_ok()

maximize(expr)

Post the given expression to the solver as objective to maximize

maximize() can be called multiple times, only the last one is stored

minimize(expr)

Post the given expression to the solver as objective to minimize

minimize() can be called multiple times, only the last one is stored

property native_model

Returns the solver’s underlying native model (for direct solver access).

objective(expr, minimize=True)

Post the given expression to the solver as objective to minimize/maximize

‘objective()’ can be called multiple times, only the last one is stored

(technical side note: any constraints created during conversion of the objective

are permanently posted to the solver)

objective_value()

Returns the value of the objective function of the latest solver run on this model

Returns:

an integer or ‘None’ if it is not run, or a satisfaction problem

solution_hint(cpm_vars: List[_NumVarImpl], vals: List[int | bool])

For warmstarting the solver with a variable assignment

Typically implemented in SAT-based solvers

Parameters:

• cpm_vars – list of CPMpy variables

• vals – list of (corresponding) values for the variables

solve(time_limit: float | None = None, solution_callback=None, **kwargs)

Call the Hexaly solver

Parameters:

• time_limit – maximum solve time in seconds (float, optional)

• kwargs – any keyword argument, sets parameters of solver object

Arguments that correspond to solver parameters:

• nb_threads: number of threads used to parallelize the search.

• iteration_limit: max number of iterations

• verbosity: verbosity level

full list of parameters availble at: https://www.hexaly.com/docs/last/pythonapi/optimizer/hxparam.html

solveAll(display: Expression | List[Expression] | Callable | None = None, time_limit: float | None = None, solution_limit: int | None = None, call_from_model=False, **kwargs)

A shorthand to (efficiently) compute all solutions, map them to CPMpy and optionally display the solutions.

Parameters:

• display – either a list of CPMpy expressions, OR a callback function, called with the variables after value-mapping default/None: nothing displayed

• solution_limit – stop after this many solutions (default: None)

• time_limit (float) – maximum solve time in seconds

Returns:

number of solutions found

Note

Hexaly does not support exhaustive search to find all solutions. Set time_limit to do a limited search.

solver_var(cpm_var)

Creates solver variable for cpmpy variable or returns from cache if previously created

solver_vars(cpm_vars)

Like solver_var() but for arbitrary shaped lists/tensors

status()

static supported()

Check for support in current system setup. Return True if the system has package installed or supports solver, else returns False.

Returns:

Solver support by current system setup.

Return type:

[bool]

supported_global_constraints: frozenset[str] = frozenset({'abs', 'div', 'element', 'max', 'min', 'mod', 'pow'})

supported_reified_global_constraints: frozenset[str] = frozenset({})

transform(cpm_expr)

Transform arbitrary CPMpy expressions to constraints the solver supports

Implemented through chaining multiple solver-independent transformation functions from the cpmpy/transformations/ directory.

See the Adding a new solver docs on readthedocs for more information.

Parameters:

cpm_expr (Expression or list of Expression) – CPMpy expression, or list thereof

Returns:

list of Expression

classmethod version() → str | None

Returns the installed version of the solver’s Python API.

class cpmpy.solvers.hexaly.HexSolutionPrinter(solver, display=None, solution_limit=None, verbose=False)

Native Hexaly callback for solution printing.

Use with CPM_hexaly as follows:

cb = HexSolutionPrinter(s, display=vars)
s.solve(solution_callback=cb)

For multiple variables (single or NDVarArray), use:

For a custom print function, use for example:

def myprint():
    print(f"x0={x[0].value()}, x1={x[1].value()}")

cb = HexSolutionPrinter(s, display=myprint)

Optionally retrieve the solution count with cb.solution_count().

Parameters:

• verbose (bool, default = False) – whether to print info on every solution found

• display – either a list of CPMpy expressions, OR a callback function, called with the variables after value-mapping default/None: nothing displayed

• solution_limit (default = None) – stop after this many solutions

on_solution_callback(optimizer, cb_type)

Called on each new solution.

solution_count()