Source code for sympy.simplify.cse_main
""" Tools for doing common subexpression elimination.
"""
from __future__ import print_function, division
import difflib
from sympy.core import Basic, Mul, Add, Pow, sympify, Tuple
from sympy.core.singleton import S
from sympy.core.basic import preorder_traversal
from sympy.core.function import _coeff_isneg
from sympy.core.exprtools import factor_terms
from sympy.core.compatibility import iterable, xrange
from sympy.utilities.iterables import numbered_symbols, \
sift, topological_sort, ordered
from . import cse_opts
# (preprocessor, postprocessor) pairs which are commonly useful. They should
# each take a sympy expression and return a possibly transformed expression.
# When used in the function ``cse()``, the target expressions will be transformed
# by each of the preprocessor functions in order. After the common
# subexpressions are eliminated, each resulting expression will have the
# postprocessor functions transform them in *reverse* order in order to undo the
# transformation if necessary. This allows the algorithm to operate on
# a representation of the expressions that allows for more optimization
# opportunities.
# ``None`` can be used to specify no transformation for either the preprocessor or
# postprocessor.
basic_optimizations = [(cse_opts.sub_pre, cse_opts.sub_post),
(factor_terms, None)]
# sometimes we want the output in a different format; non-trivial
# transformations can be put here for users
# ===============================================================
def reps_toposort(r):
"""Sort replacements `r` so (k1, v1) appears before (k2, v2)
if k2 is in v1's free symbols. This orders items in the
way that cse returns its results (hence, in order to use the
replacements in a substitution option it would make sense
to reverse the order).
Examples
========
>>> from sympy.simplify.cse_main import reps_toposort
>>> from sympy.abc import x, y
>>> from sympy import Eq
>>> for l, r in reps_toposort([(x, y + 1), (y, 2)]):
... print(Eq(l, r))
...
y == 2
x == y + 1
"""
r = sympify(r)
E = []
for c1, (k1, v1) in enumerate(r):
for c2, (k2, v2) in enumerate(r):
if k1 in v2.free_symbols:
E.append((c1, c2))
return [r[i] for i in topological_sort((range(len(r)), E))]
def cse_separate(r, e):
"""Move expressions that are in the form (symbol, expr) out of the
expressions and sort them into the replacements using the reps_toposort.
Examples
========
>>> from sympy.simplify.cse_main import cse_separate
>>> from sympy.abc import x, y, z
>>> from sympy import cos, exp, cse, Eq, symbols
>>> x0, x1 = symbols('x:2')
>>> eq = (x + 1 + exp((x + 1)/(y + 1)) + cos(y + 1))
>>> cse([eq, Eq(x, z + 1), z - 2], postprocess=cse_separate) in [
... [[(x0, y + 1), (x, z + 1), (x1, x + 1)],
... [x1 + exp(x1/x0) + cos(x0), z - 2]],
... [[(x1, y + 1), (x, z + 1), (x0, x + 1)],
... [x0 + exp(x0/x1) + cos(x1), z - 2]]]
...
True
"""
d = sift(e, lambda w: w.is_Equality and w.lhs.is_Symbol)
r = r + [w.args for w in d[True]]
e = d[False]
return [reps_toposort(r), e]
# ====end of cse postprocess idioms===========================
def preprocess_for_cse(expr, optimizations):
""" Preprocess an expression to optimize for common subexpression
elimination.
Parameters
----------
expr : sympy expression
The target expression to optimize.
optimizations : list of (callable, callable) pairs
The (preprocessor, postprocessor) pairs.
Returns
-------
expr : sympy expression
The transformed expression.
"""
for pre, post in optimizations:
if pre is not None:
expr = pre(expr)
return expr
def postprocess_for_cse(expr, optimizations):
""" Postprocess an expression after common subexpression elimination to
return the expression to canonical sympy form.
Parameters
----------
expr : sympy expression
The target expression to transform.
optimizations : list of (callable, callable) pairs, optional
The (preprocessor, postprocessor) pairs. The postprocessors will be
applied in reversed order to undo the effects of the preprocessors
correctly.
Returns
-------
expr : sympy expression
The transformed expression.
"""
if optimizations is None:
optimizations = cse_optimizations
for pre, post in reversed(optimizations):
if post is not None:
expr = post(expr)
return expr
[docs]def opt_cse(exprs, order='canonical'):
"""Find optimization opportunities in Adds, Muls, Pows and negative
coefficient Muls
Parameters
----------
exprs : list of sympy expressions
The expressions to optimize.
order : string, 'none' or 'canonical'
The order by which Mul and Add arguments are processed. For large
expressions where speed is a concern, use the setting order='none'.
Returns
-------
opt_subs : dictionary of expression substitutions
The expression substitutions which can be useful to optimize CSE.
Examples
--------
>>> from sympy.simplify.cse_main import opt_cse
>>> from sympy.abc import x
>>> opt_subs = opt_cse([x**-2])
>>> print(opt_subs)
{x**(-2): 1/(x**2)}
"""
from sympy.matrices import Matrix
opt_subs = dict()
adds = set()
muls = set()
seen_subexp = set()
def _find_opts(expr):
if expr.is_Atom:
return
if iterable(expr):
list(map(_find_opts, expr))
return
if expr in seen_subexp:
return expr
seen_subexp.add(expr)
list(map(_find_opts, expr.args))
if _coeff_isneg(expr):
neg_expr = -expr
if not neg_expr.is_Atom:
opt_subs[expr] = Mul(S.NegativeOne, neg_expr, evaluate=False)
seen_subexp.add(neg_expr)
expr = neg_expr
if expr.is_Mul:
muls.add(expr)
elif expr.is_Add:
adds.add(expr)
elif expr.is_Pow:
if _coeff_isneg(expr.exp):
opt_subs[expr] = Pow(Pow(expr.base, -expr.exp), S.NegativeOne,
evaluate=False)
for e in exprs:
if isinstance(e, Basic):
_find_opts(e)
## Process Adds and commutative Muls
def _match_common_args(Func, funcs):
if order != 'none':
funcs = list(ordered(funcs))
else:
funcs = sorted(funcs, key=lambda x: len(x.args))
func_args = [set(e.args) for e in funcs]
for i in xrange(len(func_args)):
for j in xrange(i + 1, len(func_args)):
com_args = func_args[i].intersection(func_args[j])
if len(com_args) > 1:
com_func = Func(*com_args)
# for all sets, replace the common symbols by the function
# over them, to allow recursive matches
diff_i = func_args[i].difference(com_args)
func_args[i] = diff_i | set([com_func])
if diff_i:
opt_subs[funcs[i]] = Func(Func(*diff_i), com_func,
evaluate=False)
diff_j = func_args[j].difference(com_args)
func_args[j] = diff_j | set([com_func])
opt_subs[funcs[j]] = Func(Func(*diff_j), com_func,
evaluate=False)
for k in xrange(j + 1, len(func_args)):
if not com_args.difference(func_args[k]):
diff_k = func_args[k].difference(com_args)
func_args[k] = diff_k | set([com_func])
opt_subs[funcs[k]] = Func(Func(*diff_k), com_func,
evaluate=False)
# split muls into commutative
comutative_muls = set()
for m in muls:
c, nc = m.args_cnc(cset=True)
if c:
c_mul = Mul(*c)
if nc:
opt_subs[m] = Mul(c_mul, Mul(*nc), evaluate=False)
if len(c) > 1:
comutative_muls.add(c_mul)
_match_common_args(Add, adds)
_match_common_args(Mul, comutative_muls)
return opt_subs
[docs]def tree_cse(exprs, symbols, opt_subs=None, order='canonical'):
"""Perform raw CSE on expression tree, taking opt_subs into account.
Parameters
==========
exprs : list of sympy expressions
The expressions to reduce.
symbols : infinite iterator yielding unique Symbols
The symbols used to label the common subexpressions which are pulled
out.
opt_subs : dictionary of expression substitutions
The expressions to be substituted before any CSE action is performed.
order : string, 'none' or 'canonical'
The order by which Mul and Add arguments are processed. For large
expressions where speed is a concern, use the setting order='none'.
"""
from sympy.matrices import Matrix
if opt_subs is None:
opt_subs = dict()
## Find repeated sub-expressions
to_eliminate = set()
seen_subexp = set()
def _find_repeated(expr):
if expr.is_Atom:
return
if iterable(expr):
args = expr
else:
if expr in seen_subexp:
to_eliminate.add(expr)
return
seen_subexp.add(expr)
if expr in opt_subs:
expr = opt_subs[expr]
args = expr.args
list(map(_find_repeated, args))
for e in exprs:
if isinstance(e, Basic):
_find_repeated(e)
## Rebuild tree
replacements = []
subs = dict()
def _rebuild(expr):
if expr.is_Atom:
return expr
if iterable(expr):
new_args = [_rebuild(arg) for arg in expr]
return expr.func(*new_args)
if expr in subs:
return subs[expr]
orig_expr = expr
if expr in opt_subs:
expr = opt_subs[expr]
# If enabled, parse Muls and Adds arguments by order to ensure
# replacement order independent from hashes
if order != 'none':
if expr.is_Mul:
c, nc = expr.args_cnc()
args = list(ordered(c)) + nc
elif expr.is_Add:
args = list(ordered(expr.args))
else:
args = expr.args
else:
args = expr.args
new_args = list(map(_rebuild, args))
if new_args != args:
new_expr = expr.func(*new_args)
else:
new_expr = expr
if orig_expr in to_eliminate:
sym = next(symbols)
subs[orig_expr] = sym
replacements.append((sym, new_expr))
return sym
else:
return new_expr
reduced_exprs = []
for e in exprs:
if isinstance(e, Basic):
reduced_e = _rebuild(e)
else:
reduced_e = e
reduced_exprs.append(reduced_e)
return replacements, reduced_exprs
[docs]def cse(exprs, symbols=None, optimizations=None, postprocess=None,
order='canonical'):
""" Perform common subexpression elimination on an expression.
Parameters
==========
exprs : list of sympy expressions, or a single sympy expression
The expressions to reduce.
symbols : infinite iterator yielding unique Symbols
The symbols used to label the common subexpressions which are pulled
out. The ``numbered_symbols`` generator is useful. The default is a
stream of symbols of the form "x0", "x1", etc. This must be an
infinite iterator.
optimizations : list of (callable, callable) pairs
The (preprocessor, postprocessor) pairs of external optimization
functions. Optionally 'basic' can be passed for a set of predefined
basic optimizations. Such 'basic' optimizations were used by default
in old implementation, however they can be really slow on larger
expressions. Now, no pre or post optimizations are made by default.
postprocess : a function which accepts the two return values of cse and
returns the desired form of output from cse, e.g. if you want the
replacements reversed the function might be the following lambda:
lambda r, e: return reversed(r), e
order : string, 'none' or 'canonical'
The order by which Mul and Add arguments are processed. If set to
'canonical', arguments will be canonically ordered. If set to 'none',
ordering will be faster but dependent on expressions hashes, thus
machine dependent and variable. For large expressions where speed is a
concern, use the setting order='none'.
Returns
=======
replacements : list of (Symbol, expression) pairs
All of the common subexpressions that were replaced. Subexpressions
earlier in this list might show up in subexpressions later in this
list.
reduced_exprs : list of sympy expressions
The reduced expressions with all of the replacements above.
"""
from sympy.matrices import Matrix
if symbols is None:
symbols = numbered_symbols()
else:
# In case we get passed an iterable with an __iter__ method instead of
# an actual iterator.
symbols = iter(symbols)
if optimizations is None:
optimizations = list()
elif optimizations == 'basic':
optimizations = basic_optimizations
# Handle the case if just one expression was passed.
if isinstance(exprs, Basic):
exprs = [exprs]
# Preprocess the expressions to give us better optimization opportunities.
reduced_exprs = [preprocess_for_cse(e, optimizations) for e in exprs]
# Find other optimization opportunities.
opt_subs = opt_cse(reduced_exprs, order)
# Main CSE algorithm.
replacements, reduced_exprs = tree_cse(reduced_exprs, symbols, opt_subs,
order)
# Postprocess the expressions to return the expressions to canonical form.
for i, (sym, subtree) in enumerate(replacements):
subtree = postprocess_for_cse(subtree, optimizations)
replacements[i] = (sym, subtree)
reduced_exprs = [postprocess_for_cse(e, optimizations)
for e in reduced_exprs]
if isinstance(exprs, Matrix):
reduced_exprs = [Matrix(exprs.rows, exprs.cols, reduced_exprs)]
if postprocess is None:
return replacements, reduced_exprs
return postprocess(replacements, reduced_exprs)
