"""This is a recursive web crawler.  Don't go pointing this at random sites;
it doesn't respect robots.txt and it is pretty brutal about how quickly it
fetches pages.

The code for this is very short; this is perhaps a good indication
that this is making the most effective use of the primitves at hand.
The fetch function does all the work of making http requests,
searching for new urls, and dispatching new fetches.  The GreenPool
acts as sort of a job coordinator (and concurrency controller of
course).
"""
from eventlet.green.urllib.request import urlopen
import eventlet
import re

# http://daringfireball.net/2009/11/liberal_regex_for_matching_urls
url_regex = re.compile(r'\b(([\w-]+://?|www[.])[^\s()<>]+(?:\([\w\d]+\)|([^[:punct:]\s]|/)))')


def fetch(url, seen, pool):
    """Fetch a url, stick any found urls into the seen set, and
    dispatch any new ones to the pool."""
    print("fetching", url)
    data = ''
    with eventlet.Timeout(5, False):
        data = urlopen(url).read().decode()
    for url_match in url_regex.finditer(data):
        new_url = url_match.group(0)
        # only send requests to eventlet.net so as not to destroy the internet
        if new_url not in seen and 'eventlet.net' in new_url:
            seen.add(new_url)
            # while this seems stack-recursive, it's actually not:
            # spawned greenthreads start their own stacks
            pool.spawn_n(fetch, new_url, seen, pool)


def crawl(start_url):
    """Recursively crawl starting from *start_url*.  Returns a set of
    urls that were found."""
    pool = eventlet.GreenPool()
    seen = set()
    fetch(start_url, seen, pool)
    pool.waitall()
    return seen

seen = crawl("http://eventlet.net")
print("I saw these urls:")
print("\n".join(seen))