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from __future__ import absolute_import, division, unicode_literals

import re

from . import base
from ..constants import rcdataElements, spaceCharacters
spaceCharacters = "".join(spaceCharacters)

SPACES_REGEX = re.compile("[%s]+" % spaceCharacters)

class Filter(base.Filter):

spacePreserveElements = frozenset(["pre", "textarea"] + list(rcdataElements))

def __iter__(self):
        preserve = 0
        for token in base.Filter.__iter__(self):
            type = token["type"]
            if type == "StartTag" \
                    and (preserve or token["name"] in self.spacePreserveElements):
                preserve += 1

elif type == "EndTag" and preserve:
                preserve -= 1

elif not preserve and type == "SpaceCharacters" and token["data"]:
                # Test on token["data"] above to not introduce spaces where there were not
                token["data"] = " "

elif not preserve and type == "Characters":
                token["data"] = collapse_spaces(token["data"])

yield token

def collapse_spaces(text):
    return SPACES_REGEX.sub(' ', text)

/home/../usr/../opt/../usr/lib/python3.6/site-packages/html5lib/./../html5lib/filters/whitespace.py