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August 29, 2015 13:56
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Web crawler
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def compute_ranks(graph): | |
d = 0.8 # damping factor | |
numloops = 10 | |
ranks = {} | |
npages = len(graph) | |
for page in graph: | |
ranks[page] = 1.0 / npages | |
for i in range(0, numloops): | |
newranks = {} | |
for page in graph: | |
newrank = (1 - d) / npages | |
for node in graph: | |
if page in graph[node]: | |
newrank = newrank + (d*ranks[node])/(len(graph[node])) | |
newranks[page] = newrank | |
ranks = newranks | |
return ranks | |
def crawl_web(seed): # returns index, graph of inlinks | |
tocrawl = [seed] | |
crawled = [] | |
graph = {} # <url>, [list of pages it links to] | |
index = {} | |
while tocrawl: | |
page = tocrawl.pop() | |
if page not in crawled: | |
content = get_page(page) | |
add_page_to_index(index, page, content) | |
outlinks = get_all_links(content) | |
graph[page] = outlinks | |
union(tocrawl, outlinks) | |
crawled.append(page) | |
return index, graph | |
def get_page(url): | |
if url in cache: | |
return cache[url] | |
else: | |
return None | |
def get_next_target(page): | |
start_link = page.find('<a href=') | |
if start_link == -1: | |
return None, 0 | |
start_quote = page.find('"', start_link) | |
end_quote = page.find('"', start_quote + 1) | |
url = page[start_quote + 1:end_quote] | |
return url, end_quote | |
def get_all_links(page): | |
links = [] | |
while True: | |
url, endpos = get_next_target(page) | |
if url: | |
links.append(url) | |
page = page[endpos:] | |
else: | |
break | |
return links | |
def union(a, b): | |
for e in b: | |
if e not in a: | |
a.append(e) | |
def add_page_to_index(index, url, content): | |
words = content.split() | |
for word in words: | |
add_to_index(index, word, url) | |
def add_to_index(index, keyword, url): | |
if keyword in index: | |
index[keyword].append(url) | |
else: | |
index[keyword] = [url] | |
def lookup(index, keyword): | |
if keyword in index: | |
return index[keyword] | |
else: | |
return None | |
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