linpy/polyhedra.py

   1 # Copyright 2014 MINES ParisTech
   2 #
   3 # This file is part of LinPy.
   4 #
   5 # LinPy is free software: you can redistribute it and/or modify
   6 # it under the terms of the GNU General Public License as published by
   7 # the Free Software Foundation, either version 3 of the License, or
   8 # (at your option) any later version.
   9 #
  10 # LinPy is distributed in the hope that it will be useful,
  11 # but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13 # GNU General Public License for more details.
  14 #
  15 # You should have received a copy of the GNU General Public License
  16 # along with LinPy.  If not, see <http://www.gnu.org/licenses/>.
  17
  18 import functools
  19 import math
  20 import numbers
  21
  22 from . import islhelper
  23
  24 from .islhelper import mainctx, libisl
  25 from .geometry import GeometricObject, Point
  26 from .linexprs import LinExpr, Rational
  27 from .domains import Domain
  28
  29
  30 __all__ = [
  31     'Polyhedron',
  32     'Lt', 'Le', 'Eq', 'Ne', 'Ge', 'Gt',
  33     'Empty', 'Universe',
  34 ]
  35
  36
  37 class Polyhedron(Domain):
  38     """
  39     A convex polyhedron (or simply "polyhedron") is the space defined by a
  40     system of linear equalities and inequalities. This space can be
  41     unbounded.
  42     """
  43
  44     __slots__ = (
  45         '_equalities',
  46         '_inequalities',
  47         '_symbols',
  48         '_dimension',
  49     )
  50
  51     def __new__(cls, equalities=None, inequalities=None):
  52         """
  53         Return a polyhedron from two sequences of linear expressions: equalities
  54         is a list of expressions equal to 0, and inequalities is a list of
  55         expressions greater or equal to 0. For example, the polyhedron
  56         0 <= x <= 2, 0 <= y <= 2 can be constructed with:
  57
  58         >>> x, y = symbols('x y')
  59         >>> square = Polyhedron([], [x, 2 - x, y, 2 - y])
  60
  61         It may be easier to use comparison operators LinExpr.__lt__(),
  62         LinExpr.__le__(), LinExpr.__ge__(), LinExpr.__gt__(), or functions Lt(),
  63         Le(), Eq(), Ge() and Gt(), using one of the following instructions:
  64
  65         >>> x, y = symbols('x y')
  66         >>> square = (0 <= x) & (x <= 2) & (0 <= y) & (y <= 2)
  67         >>> square = Le(0, x, 2) & Le(0, y, 2)
  68
  69         It is also possible to build a polyhedron from a string.
  70
  71         >>> square = Polyhedron('0 <= x <= 2, 0 <= y <= 2')
  72
  73         Finally, a polyhedron can be constructed from a GeometricObject
  74         instance, calling the GeometricObject.aspolyedron() method. This way, it
  75         is possible to compute the polyhedral hull of a Domain instance, i.e.,
  76         the convex hull of two polyhedra:
  77
  78         >>> square = Polyhedron('0 <= x <= 2, 0 <= y <= 2')
  79         >>> square2 = Polyhedron('2 <= x <= 4, 2 <= y <= 4')
  80         >>> Polyhedron(square | square2)
  81         """
  82         if isinstance(equalities, str):
  83             if inequalities is not None:
  84                 raise TypeError('too many arguments')
  85             return cls.fromstring(equalities)
  86         elif isinstance(equalities, GeometricObject):
  87             if inequalities is not None:
  88                 raise TypeError('too many arguments')
  89             return equalities.aspolyhedron()
  90         sc_equalities = []
  91         if equalities is not None:
  92             for equality in equalities:
  93                 if not isinstance(equality, LinExpr):
  94                     raise TypeError('equalities must be linear expressions')
  95                 sc_equalities.append(equality.scaleint())
  96         sc_inequalities = []
  97         if inequalities is not None:
  98             for inequality in inequalities:
  99                 if not isinstance(inequality, LinExpr):
 100                     raise TypeError('inequalities must be linear expressions')
 101                 sc_inequalities.append(inequality.scaleint())
 102         symbols = cls._xsymbols(sc_equalities + sc_inequalities)
 103         islbset = cls._toislbasicset(sc_equalities, sc_inequalities, symbols)
 104         return cls._fromislbasicset(islbset, symbols)
 105
 106     @property
 107     def equalities(self):
 108         """
 109         The tuple of equalities. This is a list of LinExpr instances that are
 110         equal to 0 in the polyhedron.
 111         """
 112         return self._equalities
 113
 114     @property
 115     def inequalities(self):
 116         """
 117         The tuple of inequalities. This is a list of LinExpr instances that are
 118         greater or equal to 0 in the polyhedron.
 119         """
 120         return self._inequalities
 121
 122     @property
 123     def constraints(self):
 124         """
 125         The tuple of constraints, i.e., equalities and inequalities. This is
 126         semantically equivalent to: equalities + inequalities.
 127         """
 128         return self._equalities + self._inequalities
 129
 130     @property
 131     def polyhedra(self):
 132         return self,
 133
 134     def make_disjoint(self):
 135         return self
 136
 137     def isuniverse(self):
 138         islbset = self._toislbasicset(self.equalities, self.inequalities,
 139             self.symbols)
 140         universe = bool(libisl.isl_basic_set_is_universe(islbset))
 141         libisl.isl_basic_set_free(islbset)
 142         return universe
 143
 144     def aspolyhedron(self):
 145         return self
 146
 147     def __contains__(self, point):
 148         if not isinstance(point, Point):
 149             raise TypeError('point must be a Point instance')
 150         if self.symbols != point.symbols:
 151             raise ValueError('arguments must belong to the same space')
 152         for equality in self.equalities:
 153             if equality.subs(point.coordinates()) != 0:
 154                 return False
 155         for inequality in self.inequalities:
 156             if inequality.subs(point.coordinates()) < 0:
 157                 return False
 158         return True
 159
 160     def subs(self, symbol, expression=None):
 161         equalities = [equality.subs(symbol, expression)
 162             for equality in self.equalities]
 163         inequalities = [inequality.subs(symbol, expression)
 164             for inequality in self.inequalities]
 165         return Polyhedron(equalities, inequalities)
 166
 167     def _asinequalities(self):
 168         inequalities = list(self.equalities)
 169         inequalities.extend([-expression for expression in self.equalities])
 170         inequalities.extend(self.inequalities)
 171         return inequalities
 172
 173     def widen(self, other):
 174         """
 175         Compute the standard widening of two polyhedra, à la Halbwachs.
 176
 177         In its current implementation, this method is slow and should not be
 178         used on large polyhedra.
 179         """
 180         if not isinstance(other, Polyhedron):
 181             raise ValueError('argument must be a Polyhedron instance')
 182         inequalities1 = self._asinequalities()
 183         inequalities2 = other._asinequalities()
 184         inequalities = []
 185         for inequality1 in inequalities1:
 186             if other <= Polyhedron(inequalities=[inequality1]):
 187                 inequalities.append(inequality1)
 188         for inequality2 in inequalities2:
 189             for i in range(len(inequalities1)):
 190                 inequalities3 = inequalities1[:i] + inequalities[i + 1:]
 191                 inequalities3.append(inequality2)
 192                 polyhedron3 = Polyhedron(inequalities=inequalities3)
 193                 if self == polyhedron3:
 194                     inequalities.append(inequality2)
 195                     break
 196         return Polyhedron(inequalities=inequalities)
 197
 198     @classmethod
 199     def _fromislbasicset(cls, islbset, symbols):
 200         islconstraints = islhelper.isl_basic_set_constraints(islbset)
 201         equalities = []
 202         inequalities = []
 203         for islconstraint in islconstraints:
 204             constant = libisl.isl_constraint_get_constant_val(islconstraint)
 205             constant = islhelper.isl_val_to_int(constant)
 206             coefficients = {}
 207             for index, symbol in enumerate(symbols):
 208                 coefficient = libisl.isl_constraint_get_coefficient_val(islconstraint,
 209                     libisl.isl_dim_set, index)
 210                 coefficient = islhelper.isl_val_to_int(coefficient)
 211                 if coefficient != 0:
 212                     coefficients[symbol] = coefficient
 213             expression = LinExpr(coefficients, constant)
 214             if libisl.isl_constraint_is_equality(islconstraint):
 215                 equalities.append(expression)
 216             else:
 217                 inequalities.append(expression)
 218         libisl.isl_basic_set_free(islbset)
 219         self = object().__new__(Polyhedron)
 220         self._equalities = tuple(equalities)
 221         self._inequalities = tuple(inequalities)
 222         self._symbols = cls._xsymbols(self.constraints)
 223         self._dimension = len(self._symbols)
 224         return self
 225
 226     @classmethod
 227     def _toislbasicset(cls, equalities, inequalities, symbols):
 228         dimension = len(symbols)
 229         indices = {symbol: index for index, symbol in enumerate(symbols)}
 230         islsp = libisl.isl_space_set_alloc(mainctx, 0, dimension)
 231         islbset = libisl.isl_basic_set_universe(libisl.isl_space_copy(islsp))
 232         islls = libisl.isl_local_space_from_space(islsp)
 233         for equality in equalities:
 234             isleq = libisl.isl_equality_alloc(libisl.isl_local_space_copy(islls))
 235             for symbol, coefficient in equality.coefficients():
 236                 islval = str(coefficient).encode()
 237                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 238                 index = indices[symbol]
 239                 isleq = libisl.isl_constraint_set_coefficient_val(isleq,
 240                     libisl.isl_dim_set, index, islval)
 241             if equality.constant != 0:
 242                 islval = str(equality.constant).encode()
 243                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 244                 isleq = libisl.isl_constraint_set_constant_val(isleq, islval)
 245             islbset = libisl.isl_basic_set_add_constraint(islbset, isleq)
 246         for inequality in inequalities:
 247             islin = libisl.isl_inequality_alloc(libisl.isl_local_space_copy(islls))
 248             for symbol, coefficient in inequality.coefficients():
 249                 islval = str(coefficient).encode()
 250                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 251                 index = indices[symbol]
 252                 islin = libisl.isl_constraint_set_coefficient_val(islin,
 253                     libisl.isl_dim_set, index, islval)
 254             if inequality.constant != 0:
 255                 islval = str(inequality.constant).encode()
 256                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 257                 islin = libisl.isl_constraint_set_constant_val(islin, islval)
 258             islbset = libisl.isl_basic_set_add_constraint(islbset, islin)
 259         return islbset
 260
 261     @classmethod
 262     def fromstring(cls, string):
 263         domain = Domain.fromstring(string)
 264         if not isinstance(domain, Polyhedron):
 265             raise ValueError('non-polyhedral expression: {!r}'.format(string))
 266         return domain
 267
 268     def __repr__(self):
 269         strings = []
 270         for equality in self.equalities:
 271             strings.append('Eq({}, 0)'.format(equality))
 272         for inequality in self.inequalities:
 273             strings.append('Ge({}, 0)'.format(inequality))
 274         if len(strings) == 1:
 275             return strings[0]
 276         else:
 277             return 'And({})'.format(', '.join(strings))
 278
 279     def _repr_latex_(self):
 280         strings = []
 281         for equality in self.equalities:
 282             strings.append('{} = 0'.format(equality._repr_latex_().strip('$')))
 283         for inequality in self.inequalities:
 284             strings.append('{} \\ge 0'.format(inequality._repr_latex_().strip('$')))
 285         return '$${}$$'.format(' \\wedge '.join(strings))
 286
 287     @classmethod
 288     def fromsympy(cls, expr):
 289         domain = Domain.fromsympy(expr)
 290         if not isinstance(domain, Polyhedron):
 291             raise ValueError('non-polyhedral expression: {!r}'.format(expr))
 292         return domain
 293
 294     def tosympy(self):
 295         import sympy
 296         constraints = []
 297         for equality in self.equalities:
 298             constraints.append(sympy.Eq(equality.tosympy(), 0))
 299         for inequality in self.inequalities:
 300             constraints.append(sympy.Ge(inequality.tosympy(), 0))
 301         return sympy.And(*constraints)
 302
 303
 304 class EmptyType(Polyhedron):
 305     """
 306     The empty polyhedron, whose set of constraints is not satisfiable.
 307     """
 308
 309     __slots__ = Polyhedron.__slots__
 310
 311     def __new__(cls):
 312         self = object().__new__(cls)
 313         self._equalities = (Rational(1),)
 314         self._inequalities = ()
 315         self._symbols = ()
 316         self._dimension = 0
 317         return self
 318
 319     def widen(self, other):
 320         if not isinstance(other, Polyhedron):
 321             raise ValueError('argument must be a Polyhedron instance')
 322         return other
 323
 324     def __repr__(self):
 325         return 'Empty'
 326
 327     def _repr_latex_(self):
 328         return '$$\\emptyset$$'
 329
 330 Empty = EmptyType()
 331
 332
 333 class UniverseType(Polyhedron):
 334     """
 335     The universe polyhedron, whose set of constraints is always satisfiable,
 336     i.e. is empty.
 337     """
 338
 339     __slots__ = Polyhedron.__slots__
 340
 341     def __new__(cls):
 342         self = object().__new__(cls)
 343         self._equalities = ()
 344         self._inequalities = ()
 345         self._symbols = ()
 346         self._dimension = ()
 347         return self
 348
 349     def __repr__(self):
 350         return 'Universe'
 351
 352     def _repr_latex_(self):
 353         return '$$\\Omega$$'
 354
 355 Universe = UniverseType()
 356
 357
 358 def _polymorphic(func):
 359     @functools.wraps(func)
 360     def wrapper(left, right):
 361         if not isinstance(left, LinExpr):
 362             if isinstance(left, numbers.Rational):
 363                 left = Rational(left)
 364             else:
 365                 raise TypeError('left must be a a rational number '
 366                     'or a linear expression')
 367         if not isinstance(right, LinExpr):
 368             if isinstance(right, numbers.Rational):
 369                 right = Rational(right)
 370             else:
 371                 raise TypeError('right must be a a rational number '
 372                     'or a linear expression')
 373         return func(left, right)
 374     return wrapper
 375
 376 @_polymorphic
 377 def Lt(left, right):
 378     """
 379     Create the polyhedron with constraints expr1 < expr2 < expr3 ...
 380     """
 381     return Polyhedron([], [right - left - 1])
 382
 383 @_polymorphic
 384 def Le(left, right):
 385     """
 386     Create the polyhedron with constraints expr1 <= expr2 <= expr3 ...
 387     """
 388     return Polyhedron([], [right - left])
 389
 390 @_polymorphic
 391 def Eq(left, right):
 392     """
 393     Create the polyhedron with constraints expr1 == expr2 == expr3 ...
 394     """
 395     return Polyhedron([left - right], [])
 396
 397 @_polymorphic
 398 def Ne(left, right):
 399     """
 400     Create the domain such that expr1 != expr2 != expr3 ... The result is a
 401     Domain, not a Polyhedron.
 402     """
 403     return ~Eq(left, right)
 404
 405 @_polymorphic
 406 def Gt(left, right):
 407     """
 408     Create the polyhedron with constraints expr1 > expr2 > expr3 ...
 409     """
 410     return Polyhedron([], [left - right - 1])
 411
 412 @_polymorphic
 413 def Ge(left, right):
 414     """
 415     Create the polyhedron with constraints expr1 >= expr2 >= expr3 ...
 416     """
 417     return Polyhedron([], [left - right])