pypol/linear.py

   1 import ast
   2 import functools
   3 import numbers
   4 import re
   5
   6 from fractions import Fraction, gcd
   7
   8 from . import isl
   9 from .isl import libisl
  10
  11
  12 __all__ = [
  13     'Expression', 'Constant', 'Symbol', 'symbols',
  14     'eq', 'le', 'lt', 'ge', 'gt',
  15     'Polyhedron',
  16     'Empty', 'Universe'
  17 ]
  18
  19
  20 def _polymorphic_method(func):
  21     @functools.wraps(func)
  22     def wrapper(a, b):
  23         if isinstance(b, Expression):
  24             return func(a, b)
  25         if isinstance(b, numbers.Rational):
  26             b = Constant(b)
  27             return func(a, b)
  28         return NotImplemented
  29     return wrapper
  30
  31 def _polymorphic_operator(func):
  32     # A polymorphic operator should call a polymorphic method, hence we just
  33     # have to test the left operand.
  34     @functools.wraps(func)
  35     def wrapper(a, b):
  36         if isinstance(a, numbers.Rational):
  37             a = Constant(a)
  38             return func(a, b)
  39         elif isinstance(a, Expression):
  40             return func(a, b)
  41         raise TypeError('arguments must be linear expressions')
  42     return wrapper
  43
  44
  45 _main_ctx = isl.Context()
  46
  47
  48 class Expression:
  49     """
  50     This class implements linear expressions.
  51     """
  52
  53     __slots__ = (
  54         '_coefficients',
  55         '_constant',
  56         '_symbols',
  57         '_dimension',
  58     )
  59
  60     def __new__(cls, coefficients=None, constant=0):
  61         if isinstance(coefficients, str):
  62             if constant:
  63                 raise TypeError('too many arguments')
  64             return cls.fromstring(coefficients)
  65         if isinstance(coefficients, dict):
  66             coefficients = coefficients.items()
  67         if coefficients is None:
  68             return Constant(constant)
  69         coefficients = [(symbol, coefficient)
  70                 for symbol, coefficient in coefficients if coefficient != 0]
  71         if len(coefficients) == 0:
  72             return Constant(constant)
  73         elif len(coefficients) == 1 and constant == 0:
  74             symbol, coefficient = coefficients[0]
  75             if coefficient == 1:
  76                 return Symbol(symbol)
  77         self = object().__new__(cls)
  78         self._coefficients = {}
  79         for symbol, coefficient in coefficients:
  80             if isinstance(symbol, Symbol):
  81                 symbol = symbol.name
  82             elif not isinstance(symbol, str):
  83                 raise TypeError('symbols must be strings or Symbol instances')
  84             if isinstance(coefficient, Constant):
  85                 coefficient = coefficient.constant
  86             if not isinstance(coefficient, numbers.Rational):
  87                 raise TypeError('coefficients must be rational numbers or Constant instances')
  88             self._coefficients[symbol] = coefficient
  89         if isinstance(constant, Constant):
  90             constant = constant.constant
  91         if not isinstance(constant, numbers.Rational):
  92             raise TypeError('constant must be a rational number or a Constant instance')
  93         self._constant = constant
  94         self._symbols = tuple(sorted(self._coefficients))
  95         self._dimension = len(self._symbols)
  96         return self
  97
  98     @classmethod
  99     def _fromast(cls, node):
 100         if isinstance(node, ast.Module) and len(node.body) == 1:
 101             return cls._fromast(node.body[0])
 102         elif isinstance(node, ast.Expr):
 103             return cls._fromast(node.value)
 104         elif isinstance(node, ast.Name):
 105             return Symbol(node.id)
 106         elif isinstance(node, ast.Num):
 107             return Constant(node.n)
 108         elif isinstance(node, ast.UnaryOp) and isinstance(node.op, ast.USub):
 109             return -cls._fromast(node.operand)
 110         elif isinstance(node, ast.BinOp):
 111             left = cls._fromast(node.left)
 112             right = cls._fromast(node.right)
 113             if isinstance(node.op, ast.Add):
 114                 return left + right
 115             elif isinstance(node.op, ast.Sub):
 116                 return left - right
 117             elif isinstance(node.op, ast.Mult):
 118                 return left * right
 119             elif isinstance(node.op, ast.Div):
 120                 return left / right
 121         raise SyntaxError('invalid syntax')
 122
 123     @classmethod
 124     def fromstring(cls, string):
 125         string = re.sub(r'(\d+|\))\s*([^\W\d_]\w*|\()', r'\1*\2', string)
 126         tree = ast.parse(string, 'eval')
 127         return cls._fromast(tree)
 128
 129     @property
 130     def symbols(self):
 131         return self._symbols
 132
 133     @property
 134     def dimension(self):
 135         return self._dimension
 136
 137     def coefficient(self, symbol):
 138         if isinstance(symbol, Symbol):
 139             symbol = str(symbol)
 140         elif not isinstance(symbol, str):
 141             raise TypeError('symbol must be a string or a Symbol instance')
 142         try:
 143             return self._coefficients[symbol]
 144         except KeyError:
 145             return 0
 146
 147     __getitem__ = coefficient
 148
 149     def coefficients(self):
 150         for symbol in self.symbols:
 151             yield symbol, self.coefficient(symbol)
 152
 153     @property
 154     def constant(self):
 155         return self._constant
 156
 157     def isconstant(self):
 158         return False
 159
 160     def values(self):
 161         for symbol in self.symbols:
 162             yield self.coefficient(symbol)
 163         yield self.constant
 164
 165     def issymbol(self):
 166         return False
 167
 168     def __bool__(self):
 169         return True
 170
 171     def __pos__(self):
 172         return self
 173
 174     def __neg__(self):
 175         return self * -1
 176
 177     @_polymorphic_method
 178     def __add__(self, other):
 179         coefficients = dict(self.coefficients())
 180         for symbol, coefficient in other.coefficients():
 181             if symbol in coefficients:
 182                 coefficients[symbol] += coefficient
 183             else:
 184                 coefficients[symbol] = coefficient
 185         constant = self.constant + other.constant
 186         return Expression(coefficients, constant)
 187
 188     __radd__ = __add__
 189
 190     @_polymorphic_method
 191     def __sub__(self, other):
 192         coefficients = dict(self.coefficients())
 193         for symbol, coefficient in other.coefficients():
 194             if symbol in coefficients:
 195                 coefficients[symbol] -= coefficient
 196             else:
 197                 coefficients[symbol] = -coefficient
 198         constant = self.constant - other.constant
 199         return Expression(coefficients, constant)
 200
 201     def __rsub__(self, other):
 202         return -(self - other)
 203
 204     @_polymorphic_method
 205     def __mul__(self, other):
 206         if other.isconstant():
 207             coefficients = dict(self.coefficients())
 208             for symbol in coefficients:
 209                 coefficients[symbol] *= other.constant
 210             constant = self.constant * other.constant
 211             return Expression(coefficients, constant)
 212         if isinstance(other, Expression) and not self.isconstant():
 213             raise ValueError('non-linear expression: '
 214                     '{} * {}'.format(self._parenstr(), other._parenstr()))
 215         return NotImplemented
 216
 217     __rmul__ = __mul__
 218
 219     @_polymorphic_method
 220     def __truediv__(self, other):
 221         if other.isconstant():
 222             coefficients = dict(self.coefficients())
 223             for symbol in coefficients:
 224                 coefficients[symbol] = \
 225                         Fraction(coefficients[symbol], other.constant)
 226             constant = Fraction(self.constant, other.constant)
 227             return Expression(coefficients, constant)
 228         if isinstance(other, Expression):
 229             raise ValueError('non-linear expression: '
 230                 '{} / {}'.format(self._parenstr(), other._parenstr()))
 231         return NotImplemented
 232
 233     def __rtruediv__(self, other):
 234         if isinstance(other, self):
 235             if self.isconstant():
 236                 constant = Fraction(other, self.constant)
 237                 return Expression(constant=constant)
 238             else:
 239                 raise ValueError('non-linear expression: '
 240                         '{} / {}'.format(other._parenstr(), self._parenstr()))
 241         return NotImplemented
 242
 243     def __str__(self):
 244         string = ''
 245         i = 0
 246         for symbol in self.symbols:
 247             coefficient = self.coefficient(symbol)
 248             if coefficient == 1:
 249                 if i == 0:
 250                     string += symbol
 251                 else:
 252                     string += ' + {}'.format(symbol)
 253             elif coefficient == -1:
 254                 if i == 0:
 255                     string += '-{}'.format(symbol)
 256                 else:
 257                     string += ' - {}'.format(symbol)
 258             else:
 259                 if i == 0:
 260                     string += '{}*{}'.format(coefficient, symbol)
 261                 elif coefficient > 0:
 262                     string += ' + {}*{}'.format(coefficient, symbol)
 263                 else:
 264                     assert coefficient < 0
 265                     coefficient *= -1
 266                     string += ' - {}*{}'.format(coefficient, symbol)
 267             i += 1
 268         constant = self.constant
 269         if constant != 0 and i == 0:
 270             string += '{}'.format(constant)
 271         elif constant > 0:
 272             string += ' + {}'.format(constant)
 273         elif constant < 0:
 274             constant *= -1
 275             string += ' - {}'.format(constant)
 276         if string == '':
 277             string = '0'
 278         return string
 279
 280     def _parenstr(self, always=False):
 281         string = str(self)
 282         if not always and (self.isconstant() or self.issymbol()):
 283             return string
 284         else:
 285             return '({})'.format(string)
 286
 287     def __repr__(self):
 288         string = '{}({{'.format(self.__class__.__name__)
 289         for i, (symbol, coefficient) in enumerate(self.coefficients()):
 290             if i != 0:
 291                 string += ', '
 292             string += '{!r}: {!r}'.format(symbol, coefficient)
 293         string += '}}, {!r})'.format(self.constant)
 294         return string
 295
 296     @_polymorphic_method
 297     def __eq__(self, other):
 298         # "normal" equality
 299         # see http://docs.sympy.org/dev/tutorial/gotchas.html#equals-signs
 300         return isinstance(other, Expression) and \
 301                 self._coefficients == other._coefficients and \
 302                 self.constant == other.constant
 303
 304     def __hash__(self):
 305         return hash((tuple(sorted(self._coefficients.items())), self._constant))
 306
 307     def _toint(self):
 308         lcm = functools.reduce(lambda a, b: a*b // gcd(a, b),
 309                 [value.denominator for value in self.values()])
 310         return self * lcm
 311
 312     @_polymorphic_method
 313     def _eq(self, other):
 314         return Polyhedron(equalities=[(self - other)._toint()])
 315
 316     @_polymorphic_method
 317     def __le__(self, other):
 318         return Polyhedron(inequalities=[(other - self)._toint()])
 319
 320     @_polymorphic_method
 321     def __lt__(self, other):
 322         return Polyhedron(inequalities=[(other - self)._toint() - 1])
 323
 324     @_polymorphic_method
 325     def __ge__(self, other):
 326         return Polyhedron(inequalities=[(self - other)._toint()])
 327
 328     @_polymorphic_method
 329     def __gt__(self, other):
 330         return Polyhedron(inequalities=[(self - other)._toint() - 1])
 331
 332     @classmethod
 333     def fromsympy(cls, expr):
 334         import sympy
 335         coefficients = {}
 336         constant = 0
 337         for symbol, coefficient in expr.as_coefficients_dict().items():
 338             coefficient = Fraction(coefficient.p, coefficient.q)
 339             if symbol == sympy.S.One:
 340                 constant = coefficient
 341             elif isinstance(symbol, sympy.Symbol):
 342                 symbol = symbol.name
 343                 coefficients[symbol] = coefficient
 344             else:
 345                 raise ValueError('non-linear expression: {!r}'.format(expr))
 346         return cls(coefficients, constant)
 347
 348     def tosympy(self):
 349         import sympy
 350         expr = 0
 351         for symbol, coefficient in self.coefficients():
 352             term = coefficient * sympy.Symbol(symbol)
 353             expr += term
 354         expr += self.constant
 355         return expr
 356
 357
 358 class Constant(Expression):
 359
 360     def __new__(cls, numerator=0, denominator=None):
 361         self = object().__new__(cls)
 362         if denominator is None:
 363             if isinstance(numerator, numbers.Rational):
 364                 self._constant = numerator
 365             elif isinstance(numerator, Constant):
 366                 self._constant = numerator.constant
 367             else:
 368                 raise TypeError('constant must be a rational number or a Constant instance')
 369         else:
 370             self._constant = Fraction(numerator, denominator)
 371         self._coefficients = {}
 372         self._symbols = ()
 373         self._dimension = 0
 374         return self
 375
 376     def isconstant(self):
 377         return True
 378
 379     def __bool__(self):
 380         return bool(self.constant)
 381
 382     def __repr__(self):
 383         if self.constant.denominator == 1:
 384             return '{}({!r})'.format(self.__class__.__name__, self.constant)
 385         else:
 386             return '{}({!r}, {!r})'.format(self.__class__.__name__,
 387                 self.constant.numerator, self.constant.denominator)
 388
 389     @classmethod
 390     def fromsympy(cls, expr):
 391         import sympy
 392         if isinstance(expr, sympy.Rational):
 393             return cls(expr.p, expr.q)
 394         elif isinstance(expr, numbers.Rational):
 395             return cls(expr)
 396         else:
 397             raise TypeError('expr must be a sympy.Rational instance')
 398
 399
 400 class Symbol(Expression):
 401
 402     __slots__ = Expression.__slots__ + (
 403         '_name',
 404     )
 405
 406     def __new__(cls, name):
 407         if isinstance(name, Symbol):
 408             name = name.name
 409         elif not isinstance(name, str):
 410             raise TypeError('name must be a string or a Symbol instance')
 411         self = object().__new__(cls)
 412         self._coefficients = {name: 1}
 413         self._constant = 0
 414         self._symbols = tuple(name)
 415         self._name = name
 416         self._dimension = 1
 417         return self
 418
 419     @property
 420     def name(self):
 421         return self._name
 422
 423     def issymbol(self):
 424         return True
 425
 426     def __repr__(self):
 427         return '{}({!r})'.format(self.__class__.__name__, self._name)
 428
 429     @classmethod
 430     def fromsympy(cls, expr):
 431         import sympy
 432         if isinstance(expr, sympy.Symbol):
 433             return cls(expr.name)
 434         else:
 435             raise TypeError('expr must be a sympy.Symbol instance')
 436
 437
 438 def symbols(names):
 439     if isinstance(names, str):
 440         names = names.replace(',', ' ').split()
 441     return (Symbol(name) for name in names)
 442
 443
 444 @_polymorphic_operator
 445 def eq(a, b):
 446     return a.__eq__(b)
 447
 448 @_polymorphic_operator
 449 def le(a, b):
 450     return a.__le__(b)
 451
 452 @_polymorphic_operator
 453 def lt(a, b):
 454     return a.__lt__(b)
 455
 456 @_polymorphic_operator
 457 def ge(a, b):
 458     return a.__ge__(b)
 459
 460 @_polymorphic_operator
 461 def gt(a, b):
 462     return a.__gt__(b)
 463
 464
 465 class Polyhedron:
 466     """
 467     This class implements polyhedrons.
 468     """
 469
 470     __slots__ = (
 471         '_equalities',
 472         '_inequalities',
 473         '_constraints',
 474         '_symbols',
 475     )
 476
 477     def __new__(cls, equalities=None, inequalities=None):
 478         if isinstance(equalities, str):
 479             if inequalities is not None:
 480                 raise TypeError('too many arguments')
 481             return cls.fromstring(equalities)
 482         self = super().__new__(cls)
 483         self._equalities = []
 484         if equalities is not None:
 485             for constraint in equalities:
 486                 for value in constraint.values():
 487                     if value.denominator != 1:
 488                         raise TypeError('non-integer constraint: '
 489                                 '{} == 0'.format(constraint))
 490                 self._equalities.append(constraint)
 491         self._equalities = tuple(self._equalities)
 492         self._inequalities = []
 493         if inequalities is not None:
 494             for constraint in inequalities:
 495                 for value in constraint.values():
 496                     if value.denominator != 1:
 497                         raise TypeError('non-integer constraint: '
 498                                 '{} <= 0'.format(constraint))
 499                 self._inequalities.append(constraint)
 500         self._inequalities = tuple(self._inequalities)
 501         self._constraints = self._equalities + self._inequalities
 502         self._symbols = set()
 503         for constraint in self._constraints:
 504             self.symbols.update(constraint.symbols)
 505         self._symbols = tuple(sorted(self._symbols))
 506         return self
 507
 508     @classmethod
 509     def _fromast(cls, node):
 510         if isinstance(node, ast.Module) and len(node.body) == 1:
 511             return cls._fromast(node.body[0])
 512         elif isinstance(node, ast.Expr):
 513             return cls._fromast(node.value)
 514         elif isinstance(node, ast.BinOp) and isinstance(node.op, ast.BitAnd):
 515             equalities1, inequalities1 = cls._fromast(node.left)
 516             equalities2, inequalities2 = cls._fromast(node.right)
 517             equalities = equalities1 + equalities2
 518             inequalities = inequalities1 + inequalities2
 519             return equalities, inequalities
 520         elif isinstance(node, ast.Compare):
 521             equalities = []
 522             inequalities = []
 523             left = Expression._fromast(node.left)
 524             for i in range(len(node.ops)):
 525                 op = node.ops[i]
 526                 right = Expression._fromast(node.comparators[i])
 527                 if isinstance(op, ast.Lt):
 528                     inequalities.append(right - left - 1)
 529                 elif isinstance(op, ast.LtE):
 530                     inequalities.append(right - left)
 531                 elif isinstance(op, ast.Eq):
 532                     equalities.append(left - right)
 533                 elif isinstance(op, ast.GtE):
 534                     inequalities.append(left - right)
 535                 elif isinstance(op, ast.Gt):
 536                     inequalities.append(left - right - 1)
 537                 else:
 538                     break
 539                 left = right
 540             else:
 541                 return equalities, inequalities
 542         raise SyntaxError('invalid syntax')
 543
 544     @classmethod
 545     def fromstring(cls, string):
 546         string = string.strip()
 547         string = re.sub(r'^\{\s*|\s*\}$', '', string)
 548         string = re.sub(r'([^<=>])=([^<=>])', r'\1==\2', string)
 549         string = re.sub(r'(\d+|\))\s*([^\W\d_]\w*|\()', r'\1*\2', string)
 550         tokens = re.split(r',|;|and|&&|/\\|∧', string, flags=re.I)
 551         tokens = ['({})'.format(token) for token in tokens]
 552         string = ' & '.join(tokens)
 553         tree = ast.parse(string, 'eval')
 554         equalities, inequalities = cls._fromast(tree)
 555         return cls(equalities, inequalities)
 556
 557     @property
 558     def equalities(self):
 559         return self._equalities
 560
 561     @property
 562     def inequalities(self):
 563         return self._inequalities
 564
 565     @property
 566     def constraints(self):
 567         return self._constraints
 568
 569     @property
 570     def symbols(self):
 571         return self._symbols
 572
 573     @property
 574     def dimension(self):
 575         return len(self.symbols)
 576
 577     def __bool__(self):
 578         return not self.is_empty()
 579
 580     def __contains__(self, value):
 581         # is the value in the polyhedron?
 582         raise NotImplementedError
 583
 584     def __eq__(self, other):
 585         # works correctly when symbols is not passed
 586         # should be equal if values are the same even if symbols are different
 587         bset = self._toisl()
 588         other = other._toisl()
 589         return bool(libisl.isl_basic_set_plain_is_equal(bset, other))
 590
 591     def isempty(self):
 592         bset = self._toisl()
 593         return bool(libisl.isl_basic_set_is_empty(bset))
 594
 595     def isuniverse(self):
 596         bset = self._toisl()
 597         return bool(libisl.isl_basic_set_is_universe(bset))
 598
 599     def isdisjoint(self, other):
 600         # return true if the polyhedron has no elements in common with other
 601         #symbols = self._symbolunion(other)
 602         bset = self._toisl()
 603         other = other._toisl()
 604         return bool(libisl.isl_set_is_disjoint(bset, other))
 605
 606     def issubset(self, other):
 607         # check if self(bset) is a subset of other
 608         symbols = self._symbolunion(other)
 609         bset = self._toisl(symbols)
 610         other = other._toisl(symbols)
 611         return bool(libisl.isl_set_is_strict_subset(other, bset))
 612
 613     def __le__(self, other):
 614         return self.issubset(other)
 615
 616     def __lt__(self, other):
 617         symbols = self._symbolunion(other)
 618         bset = self._toisl(symbols)
 619         other = other._toisl(symbols)
 620         return bool(libisl.isl_set_is_strict_subset(other, bset))
 621
 622     def issuperset(self, other):
 623         # test whether every element in other is in the polyhedron
 624         raise NotImplementedError
 625
 626     def __ge__(self, other):
 627         return self.issuperset(other)
 628
 629     def __gt__(self, other):
 630         symbols = self._symbolunion(other)
 631         bset = self._toisl(symbols)
 632         other = other._toisl(symbols)
 633         bool(libisl.isl_set_is_strict_subset(other, bset))
 634         raise NotImplementedError
 635
 636     def union(self, *others):
 637         # return a new polyhedron with elements from the polyhedron and all
 638         # others (convex union)
 639         raise NotImplementedError
 640
 641     def __or__(self, other):
 642         return self.union(other)
 643
 644     def intersection(self, *others):
 645         # return a new polyhedron with elements common to the polyhedron and all
 646         # others
 647         # a poor man's implementation could be:
 648         # equalities = list(self.equalities)
 649         # inequalities = list(self.inequalities)
 650         # for other in others:
 651         #     equalities.extend(other.equalities)
 652         #     inequalities.extend(other.inequalities)
 653         # return self.__class__(equalities, inequalities)
 654         raise NotImplementedError
 655
 656     def __and__(self, other):
 657         return self.intersection(other)
 658
 659     def difference(self, other):
 660         # return a new polyhedron with elements in the polyhedron that are not in the other
 661         symbols = self._symbolunion(other)
 662         bset = self._toisl(symbols)
 663         other = other._toisl(symbols)
 664         difference = libisl.isl_set_subtract(bset, other)
 665         return difference
 666
 667     def __sub__(self, other):
 668         return self.difference(other)
 669
 670     def __str__(self):
 671         constraints = []
 672         for constraint in self.equalities:
 673             constraints.append('{} == 0'.format(constraint))
 674         for constraint in self.inequalities:
 675             constraints.append('{} >= 0'.format(constraint))
 676         return '{{{}}}'.format(', '.join(constraints))
 677
 678     def __repr__(self):
 679         if self.isempty():
 680             return 'Empty'
 681         elif self.isuniverse():
 682             return 'Universe'
 683         else:
 684             equalities = list(self.equalities)
 685             inequalities = list(self.inequalities)
 686             return '{}(equalities={!r}, inequalities={!r})' \
 687                     ''.format(self.__class__.__name__, equalities, inequalities)
 688
 689     def _symbolunion(self, *others):
 690         symbols = set(self.symbols)
 691         for other in others:
 692             symbols.update(other.symbols)
 693         return sorted(symbols)
 694
 695     def _toisl(self, symbols=None):
 696         if symbols is None:
 697             symbols = self.symbols
 698         dimension = len(symbols)
 699         space = libisl.isl_space_set_alloc(_main_ctx, 0, dimension)
 700         bset = libisl.isl_basic_set_universe(libisl.isl_space_copy(space))
 701         ls = libisl.isl_local_space_from_space(space)
 702         for equality in self.equalities:
 703             ceq = libisl.isl_equality_alloc(libisl.isl_local_space_copy(ls))
 704             for symbol, coefficient in equality.coefficients():
 705                 val = str(coefficient).encode()
 706                 val = libisl.isl_val_read_from_str(_main_ctx, val)
 707                 dim = symbols.index(symbol)
 708                 ceq = libisl.isl_constraint_set_coefficient_val(ceq, libisl.isl_dim_set, dim, val)
 709             if equality.constant != 0:
 710                 val = str(equality.constant).encode()
 711                 val = libisl.isl_val_read_from_str(_main_ctx, val)
 712                 ceq = libisl.isl_constraint_set_constant_val(ceq, val)
 713             bset = libisl.isl_basic_set_add_constraint(bset, ceq)
 714         for inequality in self.inequalities:
 715             cin = libisl.isl_inequality_alloc(libisl.isl_local_space_copy(ls))
 716             for symbol, coefficient in inequality.coefficients():
 717                 val = str(coefficient).encode()
 718                 val = libisl.isl_val_read_from_str(_main_ctx, val)
 719                 dim = symbols.index(symbol)
 720                 cin = libisl.isl_constraint_set_coefficient_val(cin, libisl.isl_dim_set, dim, val)
 721             if inequality.constant != 0:
 722                 val = str(inequality.constant).encode()
 723                 val = libisl.isl_val_read_from_str(_main_ctx, val)
 724                 cin = libisl.isl_constraint_set_constant_val(cin, val)
 725             bset = libisl.isl_basic_set_add_constraint(bset, cin)
 726         bset = isl.BasicSet(bset)
 727         return bset
 728
 729     @classmethod
 730     def _fromisl(cls, bset, symbols):
 731         raise NotImplementedError
 732         equalities = ...
 733         inequalities = ...
 734         return cls(equalities, inequalities)
 735         '''takes basic set  in isl form and puts back into python version of polyhedron
 736         isl example code gives isl form as:
 737             "{[i] : exists (a : i = 2a and i >= 10 and i <= 42)}")
 738             our printer is giving form as:
 739             { [i0, i1] : 2i1 >= -2 - i0 } '''
 740
 741 Empty = eq(0,1)
 742
 743 Universe = Polyhedron()
 744
 745
 746 if __name__ == '__main__':
 747     #p = Polyhedron('2a + 2b + 1 == 0') # empty
 748     p = Polyhedron('3x + 2y + 3 == 0, y == 0') # not empty
 749     ip = p._toisl()
 750     print(ip)
 751     print(ip.constraints())