From 5f2f04c6c2676201cbb19e894d7d5bb3f3335ff0 Mon Sep 17 00:00:00 2001
From: Tulir Asokan <tulir@maunium.net>
Date: Mon, 22 Jan 2018 11:06:11 +0200
Subject: [PATCH] Add HTML parse mode (#554)

---
 telethon/extensions/html.py | 167 ++++++++++++++++++++++++++++++++++++
 telethon/telegram_client.py |   4 +-
 2 files changed, 170 insertions(+), 1 deletion(-)
 create mode 100644 telethon/extensions/html.py

diff --git a/telethon/extensions/html.py b/telethon/extensions/html.py
new file mode 100644
index 00000000..8cd170cb
--- /dev/null
+++ b/telethon/extensions/html.py
@@ -0,0 +1,167 @@
+"""
+Simple HTML -> Telegram entity parser.
+"""
+from html import escape, unescape
+from html.parser import HTMLParser
+from collections import deque
+
+from ..tl.types import (
+    MessageEntityBold, MessageEntityItalic, MessageEntityCode,
+    MessageEntityPre, MessageEntityEmail, MessageEntityUrl,
+    MessageEntityTextUrl
+)
+
+
+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]).
+    """
+    parser = HTMLToTelegramParser()
+    parser.feed(html)
+    return 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 entities:
+        return 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))
+        else:
+            skip_entity = True
+        last_offset = entity.offset + (0 if skip_entity else entity.length)
+    html.append(text[last_offset:])
+    return ''.join(html)
diff --git a/telethon/telegram_client.py b/telethon/telegram_client.py
index 5fe186f3..67644a7e 100644
--- a/telethon/telegram_client.py
+++ b/telethon/telegram_client.py
@@ -70,7 +70,7 @@ from .tl.types import (
     InputDocument, InputMediaDocument
 )
 from .tl.types.messages import DialogsSlice
-from .extensions import markdown
+from .extensions import markdown, html
 
 __log__ = logging.getLogger(__name__)
 
@@ -580,6 +580,8 @@ class TelegramClient(TelegramBareClient):
             parse_mode = parse_mode.lower()
             if parse_mode in {'md', 'markdown'}:
                 message, msg_entities = markdown.parse(message)
+            elif parse_mode.startswith('htm'):
+                message, msg_entities = html.parse(message)
             else:
                 raise ValueError('Unknown parsing mode: {}'.format(parse_mode))
         else: