Improve hash functions in linexprs

[linpy.git] / pypol / linexprs.py

diff --git a/pypol/linexprs.py b/pypol/linexprs.py
index 9ab5c86..3aef337 100644 (file)
--- a/pypol/linexprs.py
+++ b/pypol/linexprs.py
@@ -3,6 +3,7 @@ import functools
 import numbers
 import re
 
+from collections import OrderedDict
 from fractions import Fraction, gcd
 
 
@@ -35,6 +36,7 @@ class Expression:
         '_constant',
         '_symbols',
         '_dimension',
+        '_hash',
     )
 
     def __new__(cls, coefficients=None, constant=0):
@@ -67,14 +69,16 @@ class Expression:
                 raise TypeError('coefficients must be rational numbers '
                     'or Constant instances')
             self._coefficients[symbol] = coefficient
+        self._coefficients = OrderedDict(sorted(self._coefficients.items()))
         if isinstance(constant, Constant):
             constant = constant.constant
         if not isinstance(constant, numbers.Rational):
             raise TypeError('constant must be a rational number '
                 'or a Constant instance')
         self._constant = constant
-        self._symbols = tuple(sorted(self._coefficients))
+        self._symbols = tuple(self._coefficients)
         self._dimension = len(self._symbols)
+        self._hash = hash((tuple(self._coefficients.items()), self._constant))
         return self
 
     def coefficient(self, symbol):
@@ -90,8 +94,7 @@ class Expression:
     __getitem__ = coefficient
 
     def coefficients(self):
-        for symbol in self.symbols:
-            yield symbol, self.coefficient(symbol)
+        yield from self._coefficients.items()
 
     @property
     def constant(self):
@@ -105,6 +108,9 @@ class Expression:
     def dimension(self):
         return self._dimension
 
+    def __hash__(self):
+        return self._hash
+
     def isconstant(self):
         return False
 
@@ -219,9 +225,6 @@ class Expression:
         from .polyhedra import Gt
         return Gt(self, other)
 
-    def __hash__(self):
-        return hash((tuple(sorted(self._coefficients.items())), self._constant))
-
     def _toint(self):
         lcm = functools.reduce(lambda a, b: a*b // gcd(a, b),
             [value.denominator for value in self.values()])
@@ -347,11 +350,12 @@ class Symbol(Expression):
             raise TypeError('name must be a string or a Symbol instance')
         name = name.strip()
         self = object().__new__(cls)
-        self._coefficients = {name: 1}
+        self._coefficients = OrderedDict([(name, 1)])
         self._constant = 0
         self._symbols = tuple(name)
         self._name = name
         self._dimension = 1
+        self._hash = hash(self._name)
         return self
 
     @property
@@ -397,9 +401,10 @@ class Constant(Expression):
             self._constant = numerator.constant
         else:
             self._constant = Fraction(numerator, denominator)
-        self._coefficients = {}
+        self._coefficients = OrderedDict()
         self._symbols = ()
         self._dimension = 0
+        self._hash = hash(self._constant)
         return self
 
     def isconstant(self):