expr_utils.py

"""
Copyright 2008-2011 Free Software Foundation, Inc.
This file is part of GNU Radio

GNU Radio Companion is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

GNU Radio Companion is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
"""

import string
VAR_CHARS = string.letters + string.digits + '_'

class graph(object):
    """
    Simple graph structure held in a dictionary.
    """

    def __init__(self): self._graph = dict()

    def __str__(self): return str(self._graph)

    def add_node(self, node_key):
        if self._graph.has_key(node_key): return
        self._graph[node_key] = set()

    def remove_node(self, node_key):
        if not self._graph.has_key(node_key): return
        for edges in self._graph.values():
            if node_key in edges: edges.remove(node_key)
        self._graph.pop(node_key)

    def add_edge(self, src_node_key, dest_node_key):
        self._graph[src_node_key].add(dest_node_key)

    def remove_edge(self, src_node_key, dest_node_key):
        self._graph[src_node_key].remove(dest_node_key)

    def get_nodes(self): return self._graph.keys()

    def get_edges(self, node_key): return self._graph[node_key]

def expr_split(expr):
    """
    Split up an expression by non alphanumeric characters, including underscore.
    Leave strings in-tact.
    #TODO ignore escaped quotes, use raw strings.
    
    Args:
        expr: an expression string
    
    Returns:
        a list of string tokens that form expr
    """
    toks = list()
    tok = ''
    quote = ''
    for char in expr:
        if quote or char in VAR_CHARS:
            if char == quote: quote = ''
            tok += char
        elif char in ("'", '"'):
            toks.append(tok)
            tok = char
            quote = char
        else:
            toks.append(tok)
            toks.append(char)
            tok = ''
    toks.append(tok)
    return filter(lambda t: t, toks)

def expr_replace(expr, replace_dict):
    """
    Search for vars in the expression and add the prepend.
    
    Args:
        expr: an expression string
        replace_dict: a dict of find:replace
    
    Returns:
        a new expression with the prepend
    """
    expr_splits = expr_split(expr)
    for i, es in enumerate(expr_splits):
        if es in replace_dict.keys():
            expr_splits[i] = replace_dict[es]
    return ''.join(expr_splits)

def get_variable_dependencies(expr, vars):
    """
    Return a set of variables used in this expression.
    
    Args:
        expr: an expression string
        vars: a list of variable names
    
    Returns:
        a subset of vars used in the expression
    """
    expr_toks = expr_split(expr)
    return set(filter(lambda v: v in expr_toks, vars))

def get_graph(exprs):
    """
    Get a graph representing the variable dependencies
    
    Args:
        exprs: a mapping of variable name to expression
    
    Returns:
        a graph of variable deps
    """
    vars = exprs.keys()
    #get dependencies for each expression, load into graph
    var_graph = graph()
    for var in vars: var_graph.add_node(var)
    for var, expr in exprs.iteritems():
        for dep in get_variable_dependencies(expr, vars):
            if dep != var: var_graph.add_edge(dep, var)
    return var_graph

def sort_variables(exprs):
    """
    Get a list of variables in order of dependencies.
    
    Args:
        exprs: a mapping of variable name to expression
    
    Returns:
        a list of variable names
    @throws Exception circular dependencies
    """
    var_graph = get_graph(exprs)
    sorted_vars = list()
    #determine dependency order
    while var_graph.get_nodes():
        #get a list of nodes with no edges
        indep_vars = filter(lambda var: not var_graph.get_edges(var), var_graph.get_nodes())
        if not indep_vars: raise Exception('circular dependency caught in sort_variables')
        #add the indep vars to the end of the list
        sorted_vars.extend(sorted(indep_vars))
        #remove each edge-less node from the graph
        for var in indep_vars: var_graph.remove_node(var)
    return reversed(sorted_vars)

def sort_objects(objects, get_id, get_expr):
    """
    Sort a list of objects according to their expressions.
    
    Args:
        objects: the list of objects to sort
        get_id: the function to extract an id from the object
        get_expr: the function to extract an expression from the object
    
    Returns:
        a list of sorted objects
    """
    id2obj = dict([(get_id(obj), obj) for obj in objects])
    #map obj id to expression code
    id2expr = dict([(get_id(obj), get_expr(obj)) for obj in objects])
    #sort according to dependency
    sorted_ids = sort_variables(id2expr)
    #return list of sorted objects
    return [id2obj[id] for id in sorted_ids]

if __name__ == '__main__':
    for i in sort_variables({'x':'1', 'y':'x+1', 'a':'x+y', 'b':'y+1', 'c':'a+b+x+y'}): print i