"""
Simple HTML -> Telegram entity parser.
"""
import struct
from collections import deque
from html import escape, unescape
from html.parser import HTMLParser

from ..tl.types import (
    MessageEntityBold, MessageEntityItalic, MessageEntityCode,
    MessageEntityPre, MessageEntityEmail, MessageEntityUrl,
    MessageEntityTextUrl, MessageEntityMentionName
    )


# Helpers from markdown.py
def _add_surrogate(text):
    return ''.join(
        ''.join(chr(y) for y in struct.unpack('<HH', x.encode('utf-16le')))
        if (0x10000 <= ord(x) <= 0x10FFFF) else x for x in text
    )


def _del_surrogate(text):
    return text.encode('utf-16', 'surrogatepass').decode('utf-16')


class HTMLToTelegramParser(HTMLParser):
    def __init__(self):
        super().__init__()
        self.text = ''
        self.entities = []
        self._building_entities = {}
        self._open_tags = deque()
        self._open_tags_meta = deque()

    def handle_starttag(self, tag, attrs):
        self._open_tags.appendleft(tag)
        self._open_tags_meta.appendleft(None)

        attrs = dict(attrs)
        EntityType = None
        args = {}
        if tag == 'strong' or tag == 'b':
            EntityType = MessageEntityBold
        elif tag == 'em' or tag == 'i':
            EntityType = MessageEntityItalic
        elif tag == 'code':
            try:
                # If we're in the middle of a <pre> tag, this <code> tag is
                # probably intended for syntax highlighting.
                #
                # Syntax highlighting is set with
                #     <code class='language-...'>codeblock</code>
                # inside <pre> tags
                pre = self._building_entities['pre']
                try:
                    pre.language = attrs['class'][len('language-'):]
                except KeyError:
                    pass
            except KeyError:
                EntityType = MessageEntityCode
        elif tag == 'pre':
            EntityType = MessageEntityPre
            args['language'] = ''
        elif tag == 'a':
            try:
                url = attrs['href']
            except KeyError:
                return
            if url.startswith('mailto:'):
                url = url[len('mailto:'):]
                EntityType = MessageEntityEmail
            else:
                if self.get_starttag_text() == url:
                    EntityType = MessageEntityUrl
                else:
                    EntityType = MessageEntityTextUrl
                    args['url'] = url
                    url = None
            self._open_tags_meta.popleft()
            self._open_tags_meta.appendleft(url)

        if EntityType and tag not in self._building_entities:
            self._building_entities[tag] = EntityType(
                offset=len(self.text),
                # The length will be determined when closing the tag.
                length=0,
                **args)

    def handle_data(self, text):
        text = unescape(text)

        previous_tag = self._open_tags[0] if len(self._open_tags) > 0 else ''
        if previous_tag == 'a':
            url = self._open_tags_meta[0]
            if url:
                text = url

        for tag, entity in self._building_entities.items():
            entity.length += len(text.strip('\n'))

        self.text += text

    def handle_endtag(self, tag):
        try:
            self._open_tags.popleft()
            self._open_tags_meta.popleft()
        except IndexError:
            pass
        entity = self._building_entities.pop(tag, None)
        if entity:
            self.entities.append(entity)


def parse(html):
    """
    Parses the given HTML message and returns its stripped representation
    plus a list of the MessageEntity's that were found.

    :param message: the message with HTML to be parsed.
    :return: a tuple consisting of (clean message, [message entities]).
    """
    if not html:
        return html, []

    parser = HTMLToTelegramParser()
    parser.feed(_add_surrogate(html))
    return _del_surrogate(parser.text), parser.entities


def unparse(text, entities):
    """
    Performs the reverse operation to .parse(), effectively returning HTML
    given a normal text and its MessageEntity's.

    :param text: the text to be reconverted into HTML.
    :param entities: the MessageEntity's applied to the text.
    :return: a HTML representation of the combination of both inputs.
    """
    if not text or not entities:
        return text

    text = _add_surrogate(text)
    html = []
    last_offset = 0
    for entity in entities:
        if entity.offset > last_offset:
            html.append(escape(text[last_offset:entity.offset]))
        elif entity.offset < last_offset:
            continue

        skip_entity = False
        entity_text = escape(text[entity.offset:entity.offset + entity.length])
        entity_type = type(entity)

        if entity_type == MessageEntityBold:
            html.append('<strong>{}</strong>'.format(entity_text))
        elif entity_type == MessageEntityItalic:
            html.append('<em>{}</em>'.format(entity_text))
        elif entity_type == MessageEntityCode:
            html.append('<code>{}</code>'.format(entity_text))
        elif entity_type == MessageEntityPre:
            if entity.language:
                html.append(
                    "<pre>\n"
                    "    <code class='language-{}'>\n"
                    "        {}\n"
                    "    </code>\n"
                    "</pre>".format(entity.language, entity_text))
            else:
                html.append('<pre><code>{}</code></pre>'
                            .format(entity_text))
        elif entity_type == MessageEntityEmail:
            html.append('<a href="mailto:{0}">{0}</a>'.format(entity_text))
        elif entity_type == MessageEntityUrl:
            html.append('<a href="{0}">{0}</a>'.format(entity_text))
        elif entity_type == MessageEntityTextUrl:
            html.append('<a href="{}">{}</a>'
                        .format(escape(entity.url), entity_text))
        elif entity_type == MessageEntityMentionName:
            html.append('<a href="tg://user?id={}">{}</a>'
                        .format(entity.user_id, entity_text))
        else:
            skip_entity = True
        last_offset = entity.offset + (0 if skip_entity else entity.length)
    html.append(text[last_offset:])
    return _del_surrogate(''.join(html))