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cssautomaton.py
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cssautomaton.py
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"""Representation of a CSS automaton and functions to build from a selector"""
import re
import copy
from enum import Enum
from collections import defaultdict
import cssselect_parser
from cssselect_parser import Element
from lxml import etree
# yeah -- using two cssselector libraries, but one has the AST the other does
# matching...
from lxml.cssselect import CSSSelector
import cssfile
# * selector for convenience
_isany = cssselect_parser.parse("*").pop().parsed_tree
# regular expression for parsing transitions
_tranRE = re.compile("(\w*) -- ([csn0]), (.*) --> (\w*)")
_q1grp = 1
_arrgrp = 2
_nsgrp = 3
_q2grp = 4
class CSSAutConstructionException(Exception):
pass
def fromstring(cssstring, namespaces = {}):
"""Build a CSSAutomaton from a CSS string
:param cssstring:
The CSS string to build the automaton from.
:param namespaces:
A dict of 'namespace' : 'uri' pairs to use for selector evaluation
:returns:
A CSSAutomaton representing the given CSS
"""
css = cssselect_parser.parse(cssstring).pop().parsed_tree
return fromselector(css, namespaces)
def fromselector(css, namespaces = {}):
"""Build a CSSAutomaton from a cssselect selector (parsed tree)
:param css:
The cssselect selector to build the automaton from.
:param namespaces:
A dict of 'namespace' : 'uri' pairs to use for selector evaluation
:returns:
A CSSAutomaton representing the given CSS
"""
aut = CSSAutomaton(namespaces)
aut.qinit = aut._new_state()
aut.qfinal = aut._new_state()
aut.add_tran(Tran(aut.qinit, Arrow.child, _isany, aut.qinit))
aut.add_tran(Tran(aut.qinit, Arrow.sibling, _isany, aut.qinit))
__build_aut_from_selector(aut, css, aut.qinit, aut.qfinal, Combinator.noop)
return aut
def fromlist(slist):
"""Takes a list where the first two items are strings giving the name of
qinit and qfinal respectively, and the remaining items are string
representations of transitions
q1 -- {c, n, s, 0}, <selector> --> q2
where c is child, n neighbour, s sibling, and 0 noop.
Adds the transitions to the automaton.
:param trans:
list of strings, where the first two are the init and final states
and the rest represent transitions
"""
aut = CSSAutomaton()
aut.qinit = State(slist[0])
aut.qfinal = State(slist[1])
for t in slist[2:]:
m = _tranRE.match(t)
if m is None:
raise CSSAutConstructionException("Unparsable transition" + t)
css = cssselect_parser.parse(m.group(_nsgrp)).pop().parsed_tree
arr = {
"c" : Arrow.child,
"n" : Arrow.neighbour,
"s" : Arrow.sibling,
"0" : Arrow.noop
}[m.group(_arrgrp)]
aut.add_tran(Tran(State(m.group(_q1grp)),
arr,
css,
State(m.group(_q2grp))))
return aut
def intersect(aut1, aut2):
"""Forms the intersection of two CSSAutomaton objects.
Throws exception if namespaces of both automata are not the same.
:param css1:
A CSSAutomaton, the first operand of the intersection
:param css2:
A CSSAutomaton, the second operand of the intersection
:returns:
A CSSAutomaton that is the intersection of the two
"""
if (aut1.namespaces != aut2.namespaces):
raise CSSAutConstructionException("Can only intersect automata with matching namespaces.")
aut = CSSAutomaton(aut1.namespaces)
aut.qinit = _product_states(aut1.qinit, aut2.qinit)
aut.qfinal = _product_states(aut1.qfinal, aut2.qfinal)
for t1 in aut1:
for t2 in aut2:
t = _product_trans(t1, t2)
if t is not None:
aut.add_tran(t)
return aut
class State:
"""A state of a CSS automaton"""
def __init__(self, name):
""":param name:
the name of the state as a string
"""
self.name = name
def __eq__(self, other):
if isinstance(other, self.__class__):
return self.name == other.name
else:
return False
def __ne__(self, other):
return not self.__eq__(other)
def __str__(self):
return self.name
def __repr__(self):
return str(self) + "<" + repr(id(self)) + ">"
def __hash__(self):
return hash(self.name)
class Arrow(Enum):
"""The kinds of directional arrows that can label transitions"""
child = 1
neighbour = 2
sibling = 3
noop = 4
def __str__(self):
if (self == Arrow.child):
return ">"
elif (self == Arrow.neighbour):
return "+"
elif (self == Arrow.sibling):
return "~"
elif (self == Arrow.noop):
return "."
class Combinator(Enum):
child = 1
descendant = 2
neighbour = 3
sibling = 4
noop = 5
def __str__(self):
if (self == Combinator.child):
return ">"
if (self == Combinator.descendant):
return ">>"
elif (self == Combinator.neighbour):
return "+"
elif (self == Combinator.sibling):
return "~"
elif (self == Combinator.noop):
return "."
class Tran:
"""A generic CSS automaton transition"""
def __init__(self, q1, arrow, node_selector, q2):
""":param q1:
The source state (as State)
:param arrow:
The type of arrow labelling the transition (as Arrow)
:param node_selector:
A cssselect selector with no sibling or descendant combinators
:param q2:
The target state (as State)
"""
self.q1 = q1
self.arrow = arrow
self.node_selector = node_selector
self.q2 = q2
def __eq__(self, other):
if isinstance(other, self.__class__):
# the use of str is a bit of a hack, but if the cssselect library
# didn't implement __eq__...
return (self.q1 == other.q1 and
self.arrow == other.arrow and
str(self.node_selector) == str(other.node_selector) and
self.q2 == other.q2)
else:
return False
def __ne__(self, other):
return not self.__eq__(other)
def __hash__(self):
return (hash(self.q1) +
hash(self.arrow) +
# all our transitions will be unique by q1, arrow q2 anyhow
#hash(self.node_selector) +
hash(self.q2))
def __str__(self):
return (str(self.q1) +
"-- " +
str(self.arrow) +
", " +
str(self.node_selector) +
" --> " +
str(self.q2))
class CSSComponents:
"""A class to hold information about a CSSAutomaton:
num_trans -- the number of transitions in the automaton
states -- the set of states as State
namespaces -- the set of namespaces as Strings appearing in the
selectors
elements -- the set of elements as Strings appearing in the
selectors
ids -- the set of ids as strings appearing in the selectors
"""
def __init__(self):
self.states = set([])
self.namespaces = set([])
self.elements = set([])
self.ids = set([])
class CSSAutomaton:
_nextstate = 0
def __init__(self, namespaces = {}):
"""An empty CSS automaton
:param namespaces:
A dict of 'namespace' : 'uri' pairs to use in selector evaluation
"""
# map from q to trans to q
self.trans = defaultdict(set)
# map from q to trans from q
self.trans_fwd = defaultdict(set)
self.namespaces = namespaces
self.qinit = None
self.qfinal = None
def add_tran(self, tran):
"""Add a new transition to the automaton
:param tran:
The new transition as a Tran
"""
self.trans[tran.q2].add(tran)
self.trans_fwd[tran.q1].add(tran)
def trans_from(self, q):
"""Returns the set of transitions from a given state.
:param q:
The State to get the transitions from.
:returns:
The set of transitions (as Tran) from q
"""
return self.trans_fwd[q]
def accepts(self, node):
"""Detects if the node of an XML document is accepted by the automaton.
The CSS matching in this method is quite inefficient, so probably only
really good for testing purposes (but that's all we need it for anyway).
:param node:
The node to test, as a Element from etree in lxml
:returns:
True iff the node is accepted
"""
# worklist of (node, state) pairs, repeat until empty or we find
# (root, init)
worklist = set([(node, self.qfinal)])
donelist = set([])
while len(worklist) > 0:
(n, q) = worklist.pop()
donelist.add((n, q))
for t in self.trans[q]:
next_n = None
next_q = t.q1
if t.arrow == Arrow.child:
next_n = n.getparent()
elif t.arrow == Arrow.neighbour:
next_n = n.getprevious()
elif t.arrow == Arrow.sibling:
next_n = n.getprevious()
next_q = t.q2
elif t.arrow == Arrow.noop:
next_n = n
# this seems quite inefficient... (esp getroottree...)
matcher = CSSSelector(cssfile.selector_str(t.node_selector),
namespaces=self.namespaces)
if next_n is not None and next_n in matcher(next_n.getroottree()):
if (next_n.getparent() is None and
next_q == self.qinit):
return True
elif not (next_n, next_q) in donelist:
worklist.add((next_n, next_q))
return False
def components(self):
"""This method computes the sets of components on each call.
:returns:
A completed CSSAutomatonComponents object for the aut
"""
comps = CSSComponents()
comps.num_trans = sum([len(ts) for ts in self.trans.values()])
comps.states.add(self.qinit)
comps.states.add(self.qfinal)
for t in self:
comps.states.add(t.q1)
comps.states.add(t.q2)
sel = t.node_selector
while True:
s = (sel if type(sel).__name__ != "Negation"
else sel.subselector)
stype = type(s).__name__
if stype == "Element":
if s.namespace is not None:
comps.namespaces.add(s.namespace)
if s.element is not None:
comps.elements.add(s.element)
elif stype == "Hash":
comps.ids.add(s.id)
# move on or finish
if type(sel).__name__ == "Element":
break
sel = sel.selector
return comps
# only weakly protected so test.py can use it
def _new_state(self):
"""Returns a fresh state, for use in automaton construction"""
state = State("q" + str(self._nextstate))
self._nextstate += 1
return state
def __eq__(self, other):
if isinstance(other, self.__class__):
return (self.qinit == other.qinit and
self.qfinal == other.qfinal and
self.namespaces == other.namespaces and
self.trans == other.trans)
else:
return False
def __ne__(self, other):
return not self.__eq__(other)
def __str__(self):
lines = ["Init: " + str(self.qinit),
"Final: " + str(self.qfinal),
"Namespaces: " + str(self.namespaces),
""]
for t in self:
lines.append(" " + str(t))
return '\n'.join(lines)
def __repr__(self):
return "Id: " + repr(id(self)) + "\n" + str(self)
def __iter__(self):
"""iterates over Tran-s of automaton"""
for _, ts in self.trans.items():
for t in ts:
yield t
def __build_aut_from_selector(aut, selector, qin, qout, combinator):
"""Fills in the aut object with transitions to represent selector, recursive.
Does not add transitions to navigate from root to any node in tree.
:param aut:
A CSSAutomaton to add the transitions to
:param selector:
A Selector object from cssselect
:param qin:
A State from which any new transitions will be added
:param qout:
A State to exit to after the automaton for selector is done
:param combinator:
A Combinator that says how to get from the given selector to the one
immediately after (next selector not given as an argument) (i.e. how
to reach qout)
"""
stype = type(selector).__name__
if stype == "CombinedSelector":
qmid = aut._new_state()
new_combinator = {
">" : Combinator.child,
" " : Combinator.descendant,
"+" : Combinator.neighbour,
"~" : Combinator.sibling
}[selector.combinator]
__build_aut_from_selector(aut,
selector.selector,
qin,
qmid,
new_combinator)
qstart = qmid
node_selector = selector.subselector
else:
qstart = qin
node_selector = selector
if combinator == Combinator.child:
aut.add_tran(Tran(qstart, Arrow.child, node_selector, qout))
qloop = aut._new_state()
aut.add_tran(Tran(qstart, Arrow.child, node_selector, qloop))
aut.add_tran(Tran(qloop, Arrow.sibling, _isany, qloop))
aut.add_tran(Tran(qloop, Arrow.neighbour, _isany, qout))
elif combinator == Combinator.descendant:
aut.add_tran(Tran(qstart, Arrow.child, node_selector, qout))
qloop = aut._new_state()
aut.add_tran(Tran(qstart, Arrow.child, node_selector, qloop))
aut.add_tran(Tran(qloop, Arrow.sibling, _isany, qloop))
aut.add_tran(Tran(qloop, Arrow.child, _isany, qloop))
aut.add_tran(Tran(qloop, Arrow.neighbour, _isany, qout))
aut.add_tran(Tran(qloop, Arrow.child, _isany, qout))
elif combinator == Combinator.neighbour:
aut.add_tran(Tran(qstart, Arrow.neighbour, node_selector, qout))
elif combinator == Combinator.sibling:
aut.add_tran(Tran(qstart, Arrow.neighbour, node_selector, qout))
qloop = aut._new_state()
aut.add_tran(Tran(qstart, Arrow.neighbour, node_selector, qloop))
aut.add_tran(Tran(qloop, Arrow.sibling, _isany, qloop))
aut.add_tran(Tran(qloop, Arrow.neighbour, _isany, qout))
elif combinator == Combinator.noop:
aut.add_tran(Tran(qstart, Arrow.noop, node_selector, qout))
else:
raise CSSAutConstructionException("Unrecognised combinator '" +
combinator +
"'")
def _product_states(q1, q2):
"""Form the product of q1 and q2
:param q1:
The first State
:param q2:
The second State
:returns:
A State for q1xq2
"""
return State("(" + q1.name + "," + q2.name + ")")
def _product_trans(t1, t2):
"""Form the product of t1 and t2
:param t1:
The first Tran
:param t2:
The second Tran
:returns:
A new Tran that is the intersection of t1 and t2 or None if
the transitions aren't compatible
"""
q1 = _product_states(t1.q1, t2.q1)
q2 = _product_states(t1.q2, t2.q2)
node_selector = _product_selectors(t1.node_selector,
t2.node_selector)
if node_selector is None:
return None
if t1.arrow == t2.arrow:
return Tran(q1, t1.arrow, node_selector, q2)
elif t1.arrow == Arrow.neighbour and t2.arrow == Arrow.sibling:
return Tran(q1, Arrow.neighbour, node_selector, q2)
elif t1.arrow == Arrow.sibling and t2.arrow == Arrow.neighbour:
return Tran(q1, Arrow.neighbour, node_selector, q2)
else:
return None
def _product_selectors(sel1, sel2):
"""Form the product of two cssselect selectors for a single node (i.e. no >, ,+,~)
:param sel1:
The first selector, as a cssselect object (parsed tree)
:param sel2:
The second selector, as a cssselect object (parsed tree)
:returns:
A cssselect parsed tree that is the intersection of the two, or
None if incompatible
"""
stype1 = type(sel1).__name__
if stype1 == "Element":
stype2 = type(sel2).__name__
if stype2 == "Element":
return _product_element_selectors(sel1, sel2)
else:
childprod = _product_selectors(sel2.selector, sel1)
if childprod is None:
return None
else:
sel = copy.copy(sel2)
sel.selector = childprod
return sel
else:
childprod = _product_selectors(sel1.selector, sel2)
if childprod is None:
return None
else:
sel = copy.copy(sel1)
sel.selector = childprod
return sel
def _product_element_selectors(ele1, ele2):
"""Returns the product of two cssselector Element objects
:param ele1:
The first Element object
:param ele2:
The second Element object
:returns:
An Element that is the intersection of the two, or None if incompatible
"""
namespace = None
if (ele1.namespace == ele2.namespace):
namespace = ele1.namespace
elif (ele1.namespace is None and ele2.namespace is not None):
namespace = ele2.namespace
elif (ele1.namespace is not None and ele2.namespace is None):
namespace = ele1.namespace
else:
return None
element = None
if (ele1.element == ele2.element):
element = ele1.element
elif (ele1.element is None and ele2.element is not None):
element = ele2.element
elif (ele1.element is not None and ele2.element is None):
element = ele1.element
else:
return None
return Element(namespace, element)