#!/usr/local/bin/python3.6

import re
import sys
import email
import shlex
import mimetypes
import subprocess
from copy import copy
from hashlib import md5
from email import charset
from email import encoders
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.mime.nonmultipart import MIMENonMultipart
from os.path import basename, splitext, expanduser


charset.add_charset('utf-8', charset.SHORTEST, '8bit')


def pandoc(from_format, to_format='markdown', plain='markdown', title=None):
    markdown = ('markdown'
                '-blank_before_blockquote')

    if from_format == 'plain':
        from_format = plain
    if from_format == 'markdown':
        from_format = markdown
    if to_format == 'markdown':
        to_format = markdown

    command = 'pandoc -f {} -t {} --standalone --highlight-style=tango'
    if to_format in ('html', 'html5'):
        if title is not None:
            command += ' --variable=pagetitle:{}'.format(shlex.quote(title))
        command += ' --webtex --template={}'.format(
                expanduser('~/.pandoc/templates/email.html'))
    return command.format(from_format, to_format)


def gmailfy(payload):
    return payload.replace('<blockquote>',
                           '<blockquote class="gmail_quote" style="'
                           'padding: 0 7px 0 7px;'
                           'border-left: 2px solid #cccccc;'
                           'font-style: italic;'
                           'margin: 0 0 7px 3px;'
                           '">')


def make_alternative(message, part):
    alternative = convert(part, 'html',
                          pandoc(part.get_content_subtype(),
                                 to_format='html',
                                 title=message.get('Subject')))
    alternative.set_payload(gmailfy(alternative.get_payload()))
    return alternative


def make_replacement(message, part):
    return convert(part, 'plain', pandoc(part.get_content_subtype()))


def convert(part, to_subtype, command):
    payload = part.get_payload()
    if isinstance(payload, str):
        payload = payload.encode('utf-8')
    else:
        payload = part.get_payload(None, True)
        if not isinstance(payload, bytes):
            payload = payload.encode('utf-8')
    process = subprocess.run(
        shlex.split(command),
        input=payload, stdout=subprocess.PIPE, check=True)
    return MIMEText(process.stdout, to_subtype, 'utf-8')


def with_alternative(parent, part, from_signed,
                     make_alternative=make_alternative,
                     make_replacement=None):
    try:
        alternative = make_alternative(parent or part, from_signed or part)
        replacement = (make_replacement(parent or part, part)
                       if from_signed is None and make_replacement is not None
                       else part)
    except:
        return parent or part
    envelope = MIMEMultipart('alternative')
    if parent is None:
        for k, v in part.items():
            if (k.lower() != 'mime-version'
                    and not k.lower().startswith('content-')):
                envelope.add_header(k, v)
                del part[k]
    envelope.attach(replacement)
    envelope.attach(alternative)
    if parent is None:
        return envelope
    payload = parent.get_payload()
    payload[payload.index(part)] = envelope
    return parent


def tag_attachments(message):
    if message.get_content_type() == 'multipart/mixed':
        for part in message.get_payload():
            if (part.get_content_maintype() in ['image']
                    and 'Content-ID' not in part):
                filename = part.get_param('filename',
                                          header='Content-Disposition')
                if isinstance(filename, tuple):
                    filename = str(filename[2], filename[0] or 'us-ascii')
                if filename:
                    filename = splitext(basename(filename))[0]
                    if filename:
                        part.add_header('Content-ID', '<{}>'.format(filename))
    return message


def attachment_from_file_path(attachment_path):
    try:
        mime, encoding = mimetypes.guess_type(attachment_path, strict=False)
        maintype, subtype = mime.split('/')
        with open(attachment_path, 'rb') as payload:
            attachment = MIMENonMultipart(maintype, subtype)
            attachment.set_payload(payload.read())
            encoders.encode_base64(attachment)
            if encoding:
                attachment.add_header('Content-Encoding', encoding)
            return attachment
    except:
        return None


attachment_path_pattern = re.compile(r'\]\s*\(\s*file://(/[^)]*\S)\s*\)|'
                                     r'\]\s*:\s*file://(/.*\S)\s*$',
                                     re.MULTILINE)


def link_attachments(payload):
    attached = []
    attachments = []

    def on_match(match):
        if match.group(1):
            attachment_path = match.group(1)
            cid_fmt = '](cid:{})'
        else:
            attachment_path = match.group(2)
            cid_fmt = ']: cid:{}'
        attachment_id = md5(attachment_path.encode()).hexdigest()
        if attachment_id in attached:
            return cid_fmt.format(attachment_id)
        attachment = attachment_from_file_path(attachment_path)
        if attachment:
            attachment.add_header('Content-ID', '<{}>'.format(attachment_id))
            attachments.append(attachment)
            attached.append(attachment_id)
            return cid_fmt.format(attachment_id)
        return match.group()

    return attachments, attachment_path_pattern.sub(on_match, payload)


def with_local_attachments(parent, part, from_signed,
                           link_attachments=link_attachments):
    if from_signed is None:
        attachments, payload = link_attachments(part.get_payload())
        part.set_payload(payload)
    else:
        attachments, payload = link_attachments(from_signed.get_payload())
        from_signed = copy(from_signed)
        from_signed.set_payload(payload)
    if not attachments:
        return parent, part, from_signed
    if parent is None:
        parent = MIMEMultipart('mixed')
        for k, v in part.items():
            if (k.lower() != 'mime-version'
                    and not k.lower().startswith('content-')):
                parent.add_header(k, v)
                del part[k]
        parent.attach(part)
    for attachment in attachments:
        parent.attach(attachment)
    return parent, part, from_signed


def is_target(part, target_subtypes):
    return (part.get('Content-Disposition', 'inline') == 'inline'
            and part.get_content_maintype() == 'text'
            and part.get_content_subtype() in target_subtypes)


def pick_from_signed(part, target_subtypes):
    for from_signed in part.get_payload():
        if is_target(from_signed, target_subtypes):
            return from_signed


def seek_target(message, target_subtypes=['plain', 'markdown']):
    if message.is_multipart():
        if message.get_content_type() == 'multipart/signed':
            part = pick_from_signed(message, target_subtypes)
            if part is not None:
                return None, message, part
        elif message.get_content_type() == 'multipart/mixed':
            for part in message.get_payload():
                if part.is_multipart():
                    if part.get_content_type() == 'multipart/signed':
                        from_signed = pick_from_signed(part, target_subtypes)
                        if from_signed is not None:
                            return message, part, from_signed
                elif is_target(part, target_subtypes):
                    return message, part, None
    else:
        if is_target(message, target_subtypes):
            return None, message, None
    return None, None, None


def main():
    try:
        message = email.message_from_file(sys.stdin)
        parent, part, from_signed = seek_target(message)
        if (parent, part, from_signed) == (None, None, None):
            print(message)
            return
        tag_attachments(message)
        print(with_alternative(
             *with_local_attachments(parent, part, from_signed)))
    except (BrokenPipeError, KeyboardInterrupt):
        pass


if __name__ == '__main__':
    main()