simplifications.py 7.7 KB
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from itertools import chain
from typing import Callable, List, Sequence, Union
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import sympy as sp

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from pystencils.assignment import Assignment
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from pystencils.astnodes import Node
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from pystencils.field import AbstractField, Field
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from pystencils.sympyextensions import subs_additive
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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 = []
    for c1, e1 in enumerate(assignments):
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        if hasattr(e1, 'lhs') and hasattr(e1, 'rhs'):
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            symbols = [e1.lhs]
        elif isinstance(e1, Node):
            symbols = e1.symbols_defined
        else:
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            raise NotImplementedError("Cannot sort topologically. Object of type " + type(e1) + " cannot be handled.")

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        for lhs in symbols:
            for c2, e2 in enumerate(assignments):
                if isinstance(e2, Assignment) and lhs in e2.rhs.free_symbols:
                    edges.append((c1, c2))
                elif isinstance(e2, Node) and lhs in e2.undefined_symbols:
                    edges.append((c1, c2))
    return [assignments[i] for i in sp.topological_sort((range(len(assignments)), edges))]
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def sympy_cse(ac):
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    """Searches for common subexpressions inside the equation collection.

    Searches is done in both the existing subexpressions as well as the assignments themselves.
    It uses the sympy subexpression detection to do this. Return a new equation collection
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    with the additional subexpressions found
    """
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    symbol_gen = ac.subexpression_symbol_generator
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    all_assignments = [e for e in chain(ac.subexpressions, ac.main_assignments) if isinstance(e, Assignment)]
    other_objects = [e for e in chain(ac.subexpressions, ac.main_assignments) if not isinstance(e, Assignment)]
    replacements, new_eq = sp.cse(all_assignments, symbols=symbol_gen)

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    replacement_eqs = [Assignment(*r) for r in replacements]
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    modified_subexpressions = new_eq[:len(ac.subexpressions)]
    modified_update_equations = new_eq[len(ac.subexpressions):]
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    new_subexpressions = sort_assignments_topologically(other_objects + replacement_eqs + modified_subexpressions)
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    return ac.copy(modified_update_equations, new_subexpressions)
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def sympy_cse_on_assignment_list(assignments: List[Assignment]) -> List[Assignment]:
    """Extracts common subexpressions from a list of assignments."""
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    from pystencils.simp.assignment_collection import AssignmentCollection
    ec = AssignmentCollection([], assignments)
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    return sympy_cse(ec).all_assignments


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def subexpression_substitution_in_existing_subexpressions(ac):
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    """Goes through the subexpressions list and replaces the term in the following subexpressions."""
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    result = []
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    for outer_ctr, s in enumerate(ac.subexpressions):
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        new_rhs = s.rhs
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        for inner_ctr in range(outer_ctr):
            sub_expr = ac.subexpressions[inner_ctr]
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            new_rhs = subs_additive(new_rhs, sub_expr.lhs, sub_expr.rhs, required_match_replacement=1.0)
            new_rhs = new_rhs.subs(sub_expr.rhs, sub_expr.lhs)
        result.append(Assignment(s.lhs, new_rhs))
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    return ac.copy(ac.main_assignments, result)
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def subexpression_substitution_in_main_assignments(ac):
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    """Replaces already existing subexpressions in the equations of the assignment_collection."""
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    result = []
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    for s in ac.main_assignments:
        new_rhs = s.rhs
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        for sub_expr in ac.subexpressions:
            new_rhs = subs_additive(new_rhs, sub_expr.lhs, sub_expr.rhs, required_match_replacement=1.0)
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        result.append(Assignment(s.lhs, new_rhs))
    return ac.copy(result)
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def add_subexpressions_for_divisions(ac):
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    r"""Introduces subexpressions for all divisions which have no constant in the denominator.
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    For example :math:`\frac{1}{x}` is replaced while :math:`\frac{1}{3}` is not replaced.
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    """
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    divisors = set()

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    def search_divisors(term):
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        if term.func == sp.Pow:
            if term.exp.is_integer and term.exp.is_number and term.exp < 0:
                divisors.add(term)
        else:
            for a in term.args:
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                search_divisors(a)
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    for eq in ac.all_assignments:
        search_divisors(eq.rhs)
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    divisors = sorted(list(divisors), key=lambda x: str(x))
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    new_symbol_gen = ac.subexpression_symbol_generator
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    substitutions = {divisor: new_symbol for new_symbol, divisor in zip(new_symbol_gen, divisors)}
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    return ac.new_with_substitutions(substitutions, True)
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def add_subexpressions_for_sums(ac):
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    r"""Introduces subexpressions for all sums - i.e. splits addends into subexpressions."""
    addends = []

    def contains_sum(term):
        if term.func == sp.add.Add:
            return True
        if term.is_Atom:
            return False
        return any([contains_sum(a) for a in term.args])

    def search_addends(term):
        if term.func == sp.add.Add:
            if all([not contains_sum(a) for a in term.args]):
                addends.extend(term.args)
        for a in term.args:
            search_addends(a)

    for eq in ac.all_assignments:
        search_addends(eq.rhs)

    addends = [a for a in addends if not isinstance(a, sp.Symbol) or isinstance(a, AbstractField.AbstractAccess)]
    new_symbol_gen = ac.subexpression_symbol_generator
    substitutions = {addend: new_symbol for new_symbol, addend in zip(new_symbol_gen, addends)}
    return ac.new_with_substitutions(substitutions, True, substitute_on_lhs=False)


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def add_subexpressions_for_field_reads(ac, subexpressions=True, main_assignments=True):
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    r"""Substitutes field accesses on rhs of assignments with subexpressions

    Can change semantics of the update rule (which is the goal of this transformation)
    This is useful if a field should be update in place - all values are loaded before into subexpression variables,
    then the new values are computed and written to the same field in-place.
    """
    field_reads = set()
    if subexpressions:
        for assignment in ac.subexpressions:
            field_reads.update(assignment.rhs.atoms(Field.Access))
    if main_assignments:
        for assignment in ac.main_assignments:
            field_reads.update(assignment.rhs.atoms(Field.Access))
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    substitutions = {fa: next(ac.subexpression_symbol_generator) for fa in field_reads}
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    return ac.new_with_substitutions(substitutions, add_substitutions_as_subexpressions=True, substitute_on_lhs=False)


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def transform_rhs(assignment_list, transformation, *args, **kwargs):
    """Applies a transformation function on the rhs of each element of the passed assignment list
    If the list also contains other object, like AST nodes, these are ignored.
    Additional parameters are passed to the transformation function"""
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    return [Assignment(a.lhs, transformation(a.rhs, *args, **kwargs)) if hasattr(a, 'lhs') and hasattr(a, 'rhs') else a
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            for a in assignment_list]


def transform_lhs_and_rhs(assignment_list, transformation, *args, **kwargs):
    return [Assignment(transformation(a.lhs, *args, **kwargs),
                       transformation(a.rhs, *args, **kwargs))
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            if hasattr(a, 'lhs') and hasattr(a, 'rhs') else a
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            for a in assignment_list]


def apply_to_all_assignments(operation: Callable[[sp.Expr], sp.Expr]):
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    """Applies sympy expand operation to all equations in collection."""
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    def f(ac):
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        return ac.copy(transform_rhs(ac.main_assignments, operation))
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    f.__name__ = operation.__name__
    return f


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def apply_on_all_subexpressions(operation: Callable[[sp.Expr], sp.Expr]):
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    """Applies the given operation on all subexpressions of the AC."""
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    def f(ac):
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        return ac.copy(ac.main_assignments, transform_rhs(ac.subexpressions, operation))
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    f.__name__ = operation.__name__
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    return f