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         if not isinstance(other, Polyhedron):
 178             raise ValueError('argument must be a Polyhedron instance')
 179         inequalities1 = self._asinequalities()
 180         inequalities2 = other._asinequalities()
 181         inequalities = []
 182         for inequality1 in inequalities1:
 183             if other <= Polyhedron(inequalities=[inequality1]):
 184                 inequalities.append(inequality1)
 185         for inequality2 in inequalities2:
 186             for i in range(len(inequalities1)):
 187                 inequalities3 = inequalities1[:i] + inequalities[i + 1:]
 188                 inequalities3.append(inequality2)
 189                 polyhedron3 = Polyhedron(inequalities=inequalities3)
 190                 if self == polyhedron3:
 191                     inequalities.append(inequality2)
 192                     break
 193         return Polyhedron(inequalities=inequalities)
 194
 195     @classmethod
 196     def _fromislbasicset(cls, islbset, symbols):
 197         islconstraints = islhelper.isl_basic_set_constraints(islbset)
 198         equalities = []
 199         inequalities = []
 200         for islconstraint in islconstraints:
 201             constant = libisl.isl_constraint_get_constant_val(islconstraint)
 202             constant = islhelper.isl_val_to_int(constant)
 203             coefficients = {}
 204             for index, symbol in enumerate(symbols):
 205                 coefficient = libisl.isl_constraint_get_coefficient_val(islconstraint,
 206                     libisl.isl_dim_set, index)
 207                 coefficient = islhelper.isl_val_to_int(coefficient)
 208                 if coefficient != 0:
 209                     coefficients[symbol] = coefficient
 210             expression = LinExpr(coefficients, constant)
 211             if libisl.isl_constraint_is_equality(islconstraint):
 212                 equalities.append(expression)
 213             else:
 214                 inequalities.append(expression)
 215         libisl.isl_basic_set_free(islbset)
 216         self = object().__new__(Polyhedron)
 217         self._equalities = tuple(equalities)
 218         self._inequalities = tuple(inequalities)
 219         self._symbols = cls._xsymbols(self.constraints)
 220         self._dimension = len(self._symbols)
 221         return self
 222
 223     @classmethod
 224     def _toislbasicset(cls, equalities, inequalities, symbols):
 225         dimension = len(symbols)
 226         indices = {symbol: index for index, symbol in enumerate(symbols)}
 227         islsp = libisl.isl_space_set_alloc(mainctx, 0, dimension)
 228         islbset = libisl.isl_basic_set_universe(libisl.isl_space_copy(islsp))
 229         islls = libisl.isl_local_space_from_space(islsp)
 230         for equality in equalities:
 231             isleq = libisl.isl_equality_alloc(libisl.isl_local_space_copy(islls))
 232             for symbol, coefficient in equality.coefficients():
 233                 islval = str(coefficient).encode()
 234                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 235                 index = indices[symbol]
 236                 isleq = libisl.isl_constraint_set_coefficient_val(isleq,
 237                     libisl.isl_dim_set, index, islval)
 238             if equality.constant != 0:
 239                 islval = str(equality.constant).encode()
 240                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 241                 isleq = libisl.isl_constraint_set_constant_val(isleq, islval)
 242             islbset = libisl.isl_basic_set_add_constraint(islbset, isleq)
 243         for inequality in inequalities:
 244             islin = libisl.isl_inequality_alloc(libisl.isl_local_space_copy(islls))
 245             for symbol, coefficient in inequality.coefficients():
 246                 islval = str(coefficient).encode()
 247                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 248                 index = indices[symbol]
 249                 islin = libisl.isl_constraint_set_coefficient_val(islin,
 250                     libisl.isl_dim_set, index, islval)
 251             if inequality.constant != 0:
 252                 islval = str(inequality.constant).encode()
 253                 islval = libisl.isl_val_read_from_str(mainctx, islval)
 254                 islin = libisl.isl_constraint_set_constant_val(islin, islval)
 255             islbset = libisl.isl_basic_set_add_constraint(islbset, islin)
 256         return islbset
 257
 258     @classmethod
 259     def fromstring(cls, string):
 260         domain = Domain.fromstring(string)
 261         if not isinstance(domain, Polyhedron):
 262             raise ValueError('non-polyhedral expression: {!r}'.format(string))
 263         return domain
 264
 265     def __repr__(self):
 266         strings = []
 267         for equality in self.equalities:
 268             strings.append('Eq({}, 0)'.format(equality))
 269         for inequality in self.inequalities:
 270             strings.append('Ge({}, 0)'.format(inequality))
 271         if len(strings) == 1:
 272             return strings[0]
 273         else:
 274             return 'And({})'.format(', '.join(strings))
 275
 276     def _repr_latex_(self):
 277         strings = []
 278         for equality in self.equalities:
 279             strings.append('{} = 0'.format(equality._repr_latex_().strip('$')))
 280         for inequality in self.inequalities:
 281             strings.append('{} \\ge 0'.format(inequality._repr_latex_().strip('$')))
 282         return '$${}$$'.format(' \\wedge '.join(strings))
 283
 284     @classmethod
 285     def fromsympy(cls, expr):
 286         domain = Domain.fromsympy(expr)
 287         if not isinstance(domain, Polyhedron):
 288             raise ValueError('non-polyhedral expression: {!r}'.format(expr))
 289         return domain
 290
 291     def tosympy(self):
 292         import sympy
 293         constraints = []
 294         for equality in self.equalities:
 295             constraints.append(sympy.Eq(equality.tosympy(), 0))
 296         for inequality in self.inequalities:
 297             constraints.append(sympy.Ge(inequality.tosympy(), 0))
 298         return sympy.And(*constraints)
 299
 300
 301 class EmptyType(Polyhedron):
 302     """
 303     The empty polyhedron, whose set of constraints is not satisfiable.
 304     """
 305
 306     __slots__ = Polyhedron.__slots__
 307
 308     def __new__(cls):
 309         self = object().__new__(cls)
 310         self._equalities = (Rational(1),)
 311         self._inequalities = ()
 312         self._symbols = ()
 313         self._dimension = 0
 314         return self
 315
 316     def widen(self, other):
 317         if not isinstance(other, Polyhedron):
 318             raise ValueError('argument must be a Polyhedron instance')
 319         return other
 320
 321     def __repr__(self):
 322         return 'Empty'
 323
 324     def _repr_latex_(self):
 325         return '$$\\emptyset$$'
 326
 327 Empty = EmptyType()
 328
 329
 330 class UniverseType(Polyhedron):
 331     """
 332     The universe polyhedron, whose set of constraints is always satisfiable,
 333     i.e. is empty.
 334     """
 335
 336     __slots__ = Polyhedron.__slots__
 337
 338     def __new__(cls):
 339         self = object().__new__(cls)
 340         self._equalities = ()
 341         self._inequalities = ()
 342         self._symbols = ()
 343         self._dimension = ()
 344         return self
 345
 346     def __repr__(self):
 347         return 'Universe'
 348
 349     def _repr_latex_(self):
 350         return '$$\\Omega$$'
 351
 352 Universe = UniverseType()
 353
 354
 355 def _polymorphic(func):
 356     @functools.wraps(func)
 357     def wrapper(left, right):
 358         if not isinstance(left, LinExpr):
 359             if isinstance(left, numbers.Rational):
 360                 left = Rational(left)
 361             else:
 362                 raise TypeError('left must be a a rational number '
 363                     'or a linear expression')
 364         if not isinstance(right, LinExpr):
 365             if isinstance(right, numbers.Rational):
 366                 right = Rational(right)
 367             else:
 368                 raise TypeError('right must be a a rational number '
 369                     'or a linear expression')
 370         return func(left, right)
 371     return wrapper
 372
 373 @_polymorphic
 374 def Lt(left, right):
 375     """
 376     Create the polyhedron with constraints expr1 < expr2 < expr3 ...
 377     """
 378     return Polyhedron([], [right - left - 1])
 379
 380 @_polymorphic
 381 def Le(left, right):
 382     """
 383     Create the polyhedron with constraints expr1 <= expr2 <= expr3 ...
 384     """
 385     return Polyhedron([], [right - left])
 386
 387 @_polymorphic
 388 def Eq(left, right):
 389     """
 390     Create the polyhedron with constraints expr1 == expr2 == expr3 ...
 391     """
 392     return Polyhedron([left - right], [])
 393
 394 @_polymorphic
 395 def Ne(left, right):
 396     """
 397     Create the domain such that expr1 != expr2 != expr3 ... The result is a
 398     Domain, not a Polyhedron.
 399     """
 400     return ~Eq(left, right)
 401
 402 @_polymorphic
 403 def Gt(left, right):
 404     """
 405     Create the polyhedron with constraints expr1 > expr2 > expr3 ...
 406     """
 407     return Polyhedron([], [left - right - 1])
 408
 409 @_polymorphic
 410 def Ge(left, right):
 411     """
 412     Create the polyhedron with constraints expr1 >= expr2 >= expr3 ...
 413     """
 414     return Polyhedron([], [left - right])