import io
import itertools
import re

import bs4
import bs4.element

class BeautifulSoup(bs4.BeautifulSoup):
    """A wrapper of the original BeautifulSoup class, with convenience methods added."""

    IMAGE_ATTRS = {
        'img': 'src',
        'video': 'poster',
    }
    NON_BODY_TEXT_TAGS = frozenset([
        'img',
        'video',
    ])
    SENTENCE_END = re.compile(r'[.?!]\s*\W*\s*$')

    def __init__(self, src, parser='html5lib'):
        # WARNING!  It seems like it would be ideal to use the 'lxml' parser
        # for speed, but that doesn't work in our web application.  On
        # Debian stretch, at least, using lxml causes the web server WSGI
        # application to go into an infinite loop.
        super().__init__(src, parser)

    def _body_text(self, root):
        # "Body text" is all the strings under the root element, in order,
        # except:
        # * strings inside NON_BODY_TEXT_TAGS
        # * strings inside containers of NON_BODY_TEXT_TAGS.  A container is
        #   an element that has a NON_BODY_TEXT_TAGS element as its first child.
        #   For example, in <div> <video …> … </div>, none of the div's strings
        #   are included in the body text, because it's considered to be a
        #   <video> container, and any strings are probably a caption, fallback
        #   text, or other non-body text.
        started = False
        for child in root.children:
            child_type = type(child)
            if issubclass(child_type, bs4.element.Tag):
                if child.name in self.NON_BODY_TEXT_TAGS:
                    if not started:
                        break
                else:
                    yield from self._body_text(child)
            # It's not worth it to use issubclass here, because elements that
            # don't have body text like Comments and CDATA are subclasses of
            # NavigableString.
            elif child_type is bs4.element.NavigableString:
                if started:
                    yield child
                elif child.isspace():
                    pass
                else:
                    yield child
                    started = True

    def body_text(self):
        """Return an iterator of strings comprising this document's body text."""
        return self._body_text(self)

    def some_body_text(self, char_target=300):
        """Return an iterator of strings with some of this document's body text.

        This is the same as body_text, except after it yields a string that
        looks like the end of a sentence, it checks whether it has yielded
        at least `char_target` characters.  If so, the iterator stops.
        """
        # This implementation is likely to overshoot `char_target` a lot,
        # because it doesn't look inside the strings it yields, just at the
        # end of them.  We can implement something smarter later if needed.
        char_count = 0
        for s in self.body_text():
            yield s
            char_count += len(s)
            if (char_count > char_target) and self.SENTENCE_END.search(s):
                break

    @staticmethod
    def is_video_source(elem):
        try:
            return elem.name == 'source' and elem.parent.name == 'video'
        except AttributeError:
            return False

    def iter_images(self):
        """Return an iterator of all image elements in this document.

        Images include <img> and <video> with a poster attribute.
        """
        for elem in self.find_all(list(self.IMAGE_ATTRS.keys())):
            try:
                elem[self.IMAGE_ATTRS[elem.name]]
            except KeyError:
                pass
            else:
                yield elem

    def iter_videos(self):
        """Return an iterator of all video source elements in this document."""
        return self.find_all(self.is_video_source, src=True)


class SoupModelMixin:
    """Mixin for models to parse HTML with BeautifulSoup.

    Classes that use this mixin must define `SOUP_ATTRS`, a list of strings
    that name attributes with HTML in them.  After that, all the public methods
    are usable.
    """

    OG_PREVIEW_ATTR = 'data-ogpreview'
    SOUP_ATTRS = []

    def _get_soup(self):
        try:
            return self._soup
        except AttributeError:
            html = io.StringIO()
            for attr_name in self.SOUP_ATTRS:
                html.write(getattr(self, attr_name))
            html.seek(0)
            self._soup = BeautifulSoup(html)
            return self._soup

    def _elem_key(self, attr_name=OG_PREVIEW_ATTR, getvalue=int, fallback=999999):
        def elem_sort_key(elem):
            try:
                sort_key = getvalue(elem[attr_name])
            except (KeyError, ValueError):
                sort_key = fallback
            elem[attr_name] = sort_key
            return sort_key
        return elem_sort_key

    def _elem_pred(self, attr_name=OG_PREVIEW_ATTR, test=lambda n: n > 0):
        def elem_pred(elem):
            return test(elem[attr_name])
        return elem_pred

    def _sort_and_slice_elems(self, elem_seq, elem_key, pred, *slice_args):
        seq = filter(pred, sorted(elem_seq, key=elem_key))
        if slice_args:
            return itertools.islice(seq, *slice_args)
        else:
            return seq

    def get_description(self):
        """Return a string with a brief excerpt of body text from the HTML."""
        return ''.join(self._get_soup().some_body_text())

    def get_image_urls(self, *slice_args):
        """Return an iterator of source URL strings of all images in the HTML.

        Images include <img> sources and <video> poster attributes.
        """
        for elem in self._sort_and_slice_elems(
                self._get_soup().iter_images(),
                self._elem_key(),
                self._elem_pred(),
                *slice_args
        ):
            yield elem[BeautifulSoup.IMAGE_ATTRS[elem.name]]

    def get_one_image_url(self):
        return self.get_image_urls(1)

    def get_video_urls(self, *slice_args):
        """Return an iterator of source URL strings of all videos in the HTML."""
        for elem in self._sort_and_slice_elems(
                self._get_soup().iter_videos(),
                self._elem_key(),
                self._elem_pred(),
                *slice_args
        ):
            yield elem['src']

    def get_one_video_url(self):
        return self.get_video_urls(1)