Introduce default assignment simplifications

There are many simplifications in pystencils that are usually not applied. They can be applied now by using a simple argument in the configuration.

Fixes #39 (closed)

pystencils/simp/simplifications.py

@@ -3,13 +3,22 @@ from typing import Callable, List, Sequence, Union
 from collections import defaultdict
 
 import sympy as sp
+from sympy.codegen.rewriting import optims_c99, optimize
+from sympy.codegen.rewriting import ReplaceOptim
 
 from pystencils.assignment import Assignment
-from pystencils.astnodes import Node
+from pystencils.astnodes import Node, SympyAssignment
 from pystencils.field import AbstractField, Field
 from pystencils.sympyextensions import subs_additive, is_constant, recursive_collect
 
 
+# Evaluates all constant terms
+evaluate_constant_terms = ReplaceOptim(lambda e: hasattr(e, 'is_constant') and e.is_constant and not e.is_integer,
+                                       lambda p: p.evalf())
+
+sympy_optimisations = [evaluate_constant_terms] + list(optims_c99)
+
+
 def sort_assignments_topologically(assignments: Sequence[Union[Assignment, Node]]) -> List[Union[Assignment, Node]]:
     """Sorts assignments in topological order, such that symbols used on rhs occur first on a lhs"""
     edges = []
@@ -223,3 +232,16 @@ def apply_on_all_subexpressions(operation: Callable[[sp.Expr], sp.Expr]):
 
     f.__name__ = operation.__name__
     return f
+
+
+def apply_sympy_optimisations(assignments):
+    """Applies default sympy optimisations. See sympy.codegen.rewriting"""
+
+    assignments = [Assignment(a.lhs, optimize(a.rhs, sympy_optimisations))
+                   if hasattr(a, 'lhs')
+                   else a for a in assignments]
+    assignments_nodes = [a.atoms(SympyAssignment) for a in assignments]
+    for a in chain.from_iterable(assignments_nodes):
+        a.optimize(sympy_optimisations)
+
+    return assignments