## Probabilistic Counting

I recently got my hands on the common-crawl URL dataset from here and wanted to compute some stats like the number of unique domains contained in the corpus. This was the perfect opportunity to whip up a script that implemented Durande et. al’s log-log paper.

## Tree Edit Distance Enlive Version

In my last post, I presented a code-dump that computed a restricted version of the tree edit distance algorithm. I was able to achieve a decent speed-up using enlive. Here’s a code-dump:

## Tree Edit Distance in Clojure

I had to throw together an implementation of tree-edit distance for clustering web-pages based on their structure. It performs reasonably quickly. The algorithm itself. The repo is here.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 (ns tree-edit-distance.core (:use [clojure.pprint]) (:import (org.htmlcleaner HtmlCleaner DomSerializer CleanerProperties) (org.w3c.dom Document))) (defn init "Perform the correct initialization" [m n c1 c2 del-cost ins-cost] (let [M (make-array Integer/TYPE (inc m) (inc n))] (do (doseq [i (range (inc m)) j (range (inc n))] (aset M i j (int (+ (* del-cost c1) (* ins-cost c2))))) M))) (defn num-children "Expects a html tree" [a-tree] (if(.hasChildNodes a-tree) (.getLength (.getChildNodes a-tree)) 0)) (defn tree-children "Return level 1 children" [a-tree] (let [n (num-children a-tree) cs (.getChildNodes a-tree)] (map #(.item cs %) (range n)))) (defn tree-descendants [a-tree] (if (.hasChildNodes a-tree) (concat (tree-children a-tree) (flatten (map tree-descendants (tree-children a-tree)))) [])) (declare rtdm-edit-distance) (defn invert-cost [t1 t2 del-cost ins-cost sub-cost] (let [t1-desc (tree-descendants t1) t2-desc (tree-descendants t2)] (- (+ (* del-cost (count t1-desc)) (* ins-cost (count t2-desc))) (rtdm-edit-distance t1 t2 del-cost ins-cost sub-cost)))) (defn rtdm-edit-distance "The RTDM algorithm for computing edit-distance. The trees are assumed to be org.w3c.dom.Documents" [tree-1 tree-2 del-cost ins-cost sub-cost] (let [m (num-children tree-1) n (num-children tree-2) t1-children (tree-children tree-1) t2-children (tree-children tree-2) t1-desc (tree-descendants tree-1) t2-desc (tree-descendants tree-2) M (init m n (count t1-desc) (count t2-desc) del-cost ins-cost)] (doseq [i (range m) j (range n)] (let [c-i (nth t1-children i) c-j (nth t2-children j) c-i-desc (tree-descendants c-i) c-j-desc (tree-descendants c-j) del (aget M i (inc j)) ins (aget M (inc i) j) sub-i (- (aget M i j) del-cost ins-cost) sub (if (.isEqualNode c-i c-j) (- sub-i (* ins-cost (count c-j-desc)) (* del-cost (count c-i-desc))) (cond (or (not (.hasChildNodes c-i)) (not (.hasChildNodes c-j))) (+ sub-i sub-cost) (and (= (.getNodeName c-i) (.getNodeName c-j)) (try (= (.getNodeValue (.getNamedItem (.getAttributes c-i) "id")) (.getNodeValue (.getNamedItem (.getAttributes c-j) "id"))) (catch Exception e true)) (try (= (.getNodeValue (.getNamedItem (.getAttributes c-i) "class")) (.getNodeValue (.getNamedItem (.getAttributes c-j) "class"))) (catch Exception e true))) (- sub-i (invert-cost c-i c-j del-cost ins-cost sub-cost)) :else (+ sub-i sub-cost)))] (aset M (inc i) (inc j) (int (min del ins sub))))) (aget M m n))) (defn rtdm-edit-distance-sim [tree-1 tree-2 del-cost ins-cost sub-cost] (let [t1-desc (tree-descendants tree-1) t2-desc (tree-descendants tree-2)] (- 1 (/ (rtdm-edit-distance tree-1 tree-2 del-cost ins-cost sub-cost) (+ (* (+ (count t1-desc) 1) del-cost) (* (+ (count t2-desc) 1) sub-cost)))))) (defn get-xml-tree-body "Downloads a webpage and converts it to an org.w3.dom.Document" [page-src] (let [cleaner (new HtmlCleaner) props (.getProperties cleaner) cleaner-props (new CleanerProperties) dom-serializer (new DomSerializer cleaner-props) tag-node (.clean cleaner page-src)] (.createDOM dom-serializer tag-node))) (defn rtdm-edit-distance-html [pg1 pg2 del-cost ins-cost sub-cost] (rtdm-edit-distance-sim (get-xml-tree-body pg1) (get-xml-tree-body pg2) del-cost ins-cost sub-cost)) 

## A Comment on Dimension-Estimation

I saw this neat comment in a paper I was recently reading. If you have all i.i.d features and you want to estimate its dimension using Grassberger-Procaccia (which estimates dimension using a distance-based metric) or want to classify using a k-NN classifier, it is bad if the data points are mostly pairwise equidistant (for instance, a correlation integral plot will look like a step function and thus will be useless; a k-NN classifier will break because the test point ends up equidistant from all the existing points).

There is a trivial argument using the Hoeffding bound in Chris Burges’ paper that suggests that if the features are all i.i.d, a majority of pairwise distances will end up clustered tightly around a mean which means that k-NN or Grassberger-Procaccia won’t work well. I am going to repeat this argument here so I can remember it for later:

Our vectors are of dimension $d$ and the components are $\pm1$. Assuming all the components are $iid$, the Hoeffding bound gives us:

$$P(||| x_{1} - x_{2} ||^{2} – 2d| > d\epsilon) = P(| x_{1} \cdot x_{2} | > d\epsilon/2) \le 2exp(-\frac{d\epsilon^2}{8})$$

and this shows us that most pairwise distances will end up clustered very tightly around a mean and this means that a majority of pairs of points in the dataset will end up equidistant and thus a $k-NN$ classifier will fail.

This also means that the correlation integral is a good way to determine if a k-NN classifier will work well. If the plot resembles a spike, the distance function needs to change.

The correlation-integral is an immensely powerful tool and here’s an implementation

## The Story of Phineas Gage

I first encountered the story of Phineas Gage in my freshman year in college. Phineas had a projectile drill a hole in his head (while he worked the railroad) and he survived the incident. The incident caused a massive personality change in Phineas and conclusively ruled out phrenology as a reasonable model of how the brain worked.

After the incident Phineas’ life comprised a trip to South America, a gig at P.T. Barnum’s establishment and a massive change in our understanding of the brain. This wikipedia article covers a good chunk of this work.

## Obituary

Earlier this month, my paternal grandfather passed away. I didn’t speak to him before his death since I expected him to recover from his current bout of ill-health and I have since carried some guilt thanks to this.

I think this excerpt from a Pink Floyd song best summarizes his life:

Run, run rabbit run
Dig that hole, forget the sun
And when at last the work is done
Don’t sit down, it’s time to dig another one

-- [Breathe, Pink Floyd](http://rock.rapgenius.com/Pink-floyd-breathe-lyrics)

RIP.

## Web Crawling - Dos and Don’ts

For my SIGIR submission I have been working on finding efficient traversal strategies while crawling websites.

Web crawling is a straightforward graph-traversal problem. My research focuses on discarding unproductive paths and preserving bandwidth to find more information. I will write a post on it once I have my ideas fleshed out and thus that won’t be the focus of this post.

Here, I will describe the finer details needed to make your crawler polite and robust. An impolite crawler will incur the wrath of an admin and might get you banned. A crawler that isn’t robust cannot survive the onslaught of quirks that the WWW is full of.

## Zephyros Racket API

In the recent past, I wanted to control the OS X window manager from racket like I could on Linux using the X11 library. I found a very sweet Github project called zephyros that implemented a large number of vital routines (vital for managing windows anyway) and provided a simple protocol using json. Since it would be convenient to have a racket module, I wrote a wrapper around it.

## Racket Whistlepig Bindings

Whistlepig is a lightweight real-time search engine written in ANSI C. (description and source) I heard about it when Don Metzler plugged it in an answer he wrote on quora. In this post, with very little code, I was able to build an index, query it and write a servlet that talks to the index using the FFI.

## Clueweb12++ Status Report

I recently gave a talk at CMU on the state of the Clueweb12++ crawl. Here are the slides.

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(c) Shriphani Palakodety 2013-2018