#!/usr/bin/env python
'''
Utilities to provide various misc functions.
'''

import urllib.request
import urllib.error
import gzip



import aiohttp
import asyncio
from bs4 import BeautifulSoup
from urllib.parse import urljoin, urlsplit
import urllib.robotparser


class AsyncCrawler(object):
    '''
    docstring
    '''

    def __init__(self, baseurl=None, robots=None, concurrency=None):
        self.baseurl = baseurl
        self.robots = robots
        self.uncrawled = set()
        self.crawled = set()
        # self.headers = {'Accept-Encoding': 'gzip, deflate',
                        # 'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:61.0) Gecko/20100101 Firefox/61.0'}
        self.headers = {'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:61.0) Gecko/20100101 Firefox/61.0'}
        self.client_session = None
        self.semaphore = asyncio.BoundedSemaphore(concurrency)


    async def crawl_url(self, url=None):
        '''
        docstring
        '''
        urls = set()
        source = await self.get_source(url)
        if source:
            # add the URL we've just crawled
            self.crawled.add(url)
            for new_url in self.find_all_urls(source):
                urls.add(new_url)

        return urls


    def validate_url(self, url=None):
        '''
        Ensures we have a valid URL to crawl and that the site's robots.txt
        allows it.
        '''
        # ensure the URL is in a sane format
        url = standardise_url(url=url, base_url=self.baseurl)

        if url and self.robots.check(url=url):
            return url
        else:
            return False


    async def get_source(self, url=None):
        '''
        Obtains the page's source.
        '''
        print('semaphore held for {0}'.format(url))
        async with self.semaphore:
            async with self.client_session.get(url, timeout=5) as resp:
                try:
                    source = await resp.read()
                    return source
                except Exception:
                    return None


    def find_all_urls(self, source=None):
        '''
        Find all URLs in a page's source.
        '''
        urls = set()

        html = BeautifulSoup(source, 'lxml')
        hrefs = html.find_all('a', href=True)

        # build a set of URLs which are valid and haven't been crawled yet
        for href in hrefs:
            url = self.validate_url(url=href['href'])
            if url and url not in self.crawled:
                urls.add(url)

        return urls


    async def run_loop(self):
        '''
        function which runs the crawler
        '''
        # print('Crawling: {}'.format(self.baseurl))
        self.client_session = aiohttp.ClientSession(headers=self.headers)
        # provide the starting URL to the crawler
        self.uncrawled.add(self.baseurl)

        while len(self.uncrawled) > 0:
            url = self.uncrawled.pop()
            new_urls = await self.crawl_url(url=url)
            for url in new_urls:
                self.uncrawled.add(url)

        await self.client_session.close()
        return self.crawled


class RobotsTxt(object):
    '''
    needs a docstring
    '''

    def __init__(self, base_url=None):
        '''
        Manually retrieve robots.txt to allow us to set the user-agent.
        '''
        self.base_url = base_url
        self.headers = {'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:61.0) Gecko/20100101 Firefox/61.0'}

        robots_url = urljoin(self.base_url, 'robots.txt')
        request = urllib.request.Request(robots_url, headers=self.headers)

        robots = urllib.robotparser.RobotFileParser()
        robots.set_url(robots_url)

        try:
            response = urllib.request.urlopen(request, timeout=5)
        except urllib.error.HTTPError:
            robots.allow_all = True
        else:
            data = response.read()
            decoded_data = data.decode("utf-8").splitlines()
            robots.parse(decoded_data)

        self.robots = robots


    def check(self, url):
        '''
        Test if robots allows us to crawl that URL.
        '''
        return self.robots.can_fetch("*", url)


def standardise_url(url=None, base_url=None):
    '''
    If `base_url` is None then we attempt to standarise the URL to ensure it can
    be prepended to relative URLs. If no scheme has been provided then we default
    to http as any sane https-only site should 301 redirect http > https.

    If `base_url` is set, we standardise URLs to strip queries and fragments (we
    don't want to scrape in-page anchors etc). Any relative URLs will be appended
    to the base url.

    Returns a standardised URL as a string.
    '''
    default_proto = 'http'
    delim = '://'

    split_url = urlsplit(url)

    if not base_url:
        # This will sanitise the initial url provided by the user.
        if split_url.scheme and split_url.scheme.startswith('http'):
            return "".join([split_url.scheme, delim, split_url.netloc])
        elif (split_url.path and not split_url.scheme and not split_url.netloc):
            return "".join([default_proto, delim, split_url.path])
    else:
        # Sanitise discovered URLs. We already expect them in the format
        # protocol://base_url/path
        if url.startswith('/'):
            return urljoin(base_url, split_url.path)
        elif url.startswith(base_url):
            return "".join([split_url.scheme, delim, split_url.netloc, split_url.path])

    return None