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

from bs4 import BeautifulSoup
import aiohttp
import urllib.request
import urllib.robotparser
import urllib.error
import gzip
from urllib.parse import (urljoin, urlsplit)


class UrlPool(object):
    '''
    Object to manage a pool of URLs.
    '''

    def __init__(self):
        self.pool = set()

    def check_duplicate(self, new_url):
        '''
        Checks if a URL exists in the current pool.
        '''
        if new_url in self.pool:
            return True
        else:
            return False

    def remove_from_pool(self):
        '''
        Remove a URL from the pool and return it to be crawled.
        '''
        return(self.pool.pop())

    def add_to_pool(self, url):
        self.pool.add(url)

    def list_pool(self):
        pool = self.pool
        return pool


class WebPage(object):
    '''
    Object to manage common operations required to return
    the data from each individual page.
    '''

    # set a sane user-agent and request compression if available.
    headers = {'Accept-Encoding': 'gzip, deflate',
               'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:61.0) Gecko/20100101 Firefox/61.0'}

    def __init__(self, url=None, base_url=None, robots=None):
        self.url = url
        self.base_url = base_url
        self.robots = robots
        self.source = None
        self.urls_to_crawl = set()


    def get_source(self):
        '''
        Retrieve a page's source.
        '''

        request = urllib.request.Request(self.url, headers=self.headers)
        page = urllib.request.urlopen(request, timeout=5)

        # handle the content encoding in case it needs decompressing.
        if 'text/html' in page.info().get('Content-Type'):
            if page.info().get('Content-Encoding'):
                if page.info().get('Content-Encoding') == 'gzip':
                    self.source = gzip.decompress(page.read())
                elif page.info().get('Content-Encoding') == 'deflate':
                    self.source = page.read()
            else:
                self.source = page.read()


    def find_links(self):
        '''
        Find all URLs on a page and ensure they are absolute. If they are
        relative then they will be appended to the base URL.
        '''
        hrefs = set()

        soup = BeautifulSoup(self.source, 'lxml')
        links = soup.find_all('a', href=True)

        for link in links:
            if link['href'].startswith('/'):
                hrefs.add(urljoin(self.url, link['href']))
            else:
                hrefs.add(link['href'])

        self.discovered_hrefs = hrefs


    def parse_urls(self):
        '''
        Iterate through the list of discovered URLs and add them to the
        pool if they start with the base URL.
        '''
        for url in self.discovered_hrefs:
            if url.startswith(self.base_url) and self.robots.check(url):
                sanitised_url = sanitise_url(url=url)
                self.urls_to_crawl.add(sanitised_url)


    def list_urls(self):
        '''
        Returns all valid discovered URLs.
        '''

        return self.urls_to_crawl


    def run(self):
        '''
        Attempt to get the page's source and if successful, iterate through it
        to find any links we can crawl.
        '''
        try:
            self.get_source()
        except Exception:
            # skip if we didn't retrieve the source.
            pass

        if self.source:
            self.find_links()
            self.parse_urls()
            return True
        else:
            return False


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 sanitise_url(url, base_url=False):
    '''
    If `base_url` is True, we attempt to standardise `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 False, we sanitise URLs to strip queries and fragments (we
    don't want to scrape in-page anchors etc).

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

    split_url = urlsplit(url)

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

    return sanitised_url