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Python Challenges- Code Log

Have been working on Python Challanges by Nadev Samat and yes those are pretty tough and interesting at the same time. I tend to pickup stuff and leave it in the middle so this time I have decided to log my progress by posting code for each challenge I solve and hence keep a code and progress log simultaneously.

Challenge 0:

This one was pretty simple and just required:

>>>pow(2, 38)

on the terminal.
Answer was 274877906944

Challenge 1:

Went dumb(as originally I am) and tried to solve this one by hand, but realization came in after reading the first line and hence switched to code:

>>> import string
>>> str = "g fmnc wms bgblr rpylqjyrc gr zw fylb. rfyrq ufyr amknsrcpq ypc dmp. bmgle gr gl zw fylb gq glcddgagclr ylb rfyr'q ufw rfgq rcvr gq qm jmle. sqgle qrpgle.kyicrpylq() gq pcamkkclbcb. lmu ynnjw ml rfc spj";
>>> intab = "abcdefghijklmnopqrstuvwxyz"
>>> outab = "cdefghijklmnopqrstuvwxyzab"
>>> from string import maketrans
>>> trantab = maketrans(intab, outab)
>>> print str.translate(trantab)

Answer was ocr

Challenge 2:

>>>file = open('temp.txt')
>>>key = """\n!#%$&()+*@[]_^{}"""
>>> new2 =""
>>> for letter in str:
... if letter not in key:
... new2 += letter
>>> print(new2)

Answer was equality

Challenge 3:

>>>file = open('temp.txt')
>>>pat = re.compile('''
... [^A-Z] # any character except a capital letter
... [A-Z]{3} # three capital letters
... ( # the beginning of a capturing group
... [a-z] # one lowercase letter
... ) # the end of the group
... [A-Z]{3} # three capital letters
... [^A-Z] # any character except a capital letter
... ''', re.VERBOSE)
>>>  re.findall(pat,str)

Answer was linkedlist

Had too google a lot for challenges 3 and 4 as my regex and python knowledge are pretty nascent. But yes, I will be completing all the 31 challenges and posting those here.

Challenge 4:

Enjoyed this one, very little google was required and the code was pretty straight forward and this was the first time I used Python to fetch online resources. Here is the final code. This ran with a hiccup as I had to change the value of ‘stringholder’ once as per the instructs, but finally did the job:

>>> stringholder = '12345'
>>> url = ''
>>> for x in range(0,399):
... req = urllib2.Request(url+stringholder)
... response = urllib2.urlopen(req)
... fullStr =
... list = re.findall('\d+', fullStr)
... length = len(list)
... stringholder = list[length - 1]
... print stringholder

Answer was peak

Challenge 5:

Another tough one, finding that banner.p was a task. I got to know about pickling from this one & the output, man I was totally blown away:

>>> import urllib, pickle
>>> url = ''
>>> stream = urllib.urlopen(url)
>>> object = pickle.load(stream)
>>> stream.close()
>>> print object #hint: output of banner program by Unix
>>> for item in object:
... print "".join(i[0]*i[1] for i in item)

Answer was channel

Challenge 6:

Phewww, Man, this puzzle was exhausting. You get started with 3 clues: ‘zip’, ‘Now there are pairs’ and ‘channel’. It gives a weak idea that Python zip module is to be used. Hence, Altering the page url to ‘’ from channel.html gives us a zip file. I unzipped this file to find lots of files with ‘nothings'(remember?) and a readme file. Now, there are 2 hints in the readme file, first is: “Start with 90052” and “Answer is inside the zip”. Being Lazy as I am, I ignored the 2nd advice completely and followed the LinkedList approach of extracting the nothings and following those but now for a much larger dataset(~1000). Here’s the initial code below:

>>> import zipfile
>>> zip = zipfile.ZipFile(open('', 'r'))
>>> nothing = '90052.txt'

>>> while True:
... raw_data =, None)
... print raw_data
... nothing = int(raw_data.split()[-1])
... nothing = str(nothing)+'.txt'

Just when I thought I was done I got this:

Next nothing is 46145
Collect the comments.

I had to modify the code and the final code looked like:

>>> import zipfile
>>> zip = zipfile.ZipFile(open('', 'r'))
>>> nothing = '90052.txt'
>>> comments = []
>>> while True:
...     raw_data =, None)
...     print raw_data
...     nothing = int(raw_data.split()[-1])
...     nothing = str(nothing)+'.txt'
...     comments.append(zip.getinfo(nothing).comment)
print "".join(comments)

This leaves us with the answer, hockey and only if at all it was so easy. I entered this into the url and got this: it’s in the air. look at the letters.

Taking a closer look at the hockey reveals that the answer is oxygen.

Challenge 7:

First things first, through this one I got introduced to the PIL(Python Imaging Library). I had to download and install it as an external module using the help of this link(plus ofcourse a bit of scavenging on stackoverflow). Coming back to the Image being shown in challenge 7, with some pixel manipulation in the middle(once again I omitted the fact that the pixel manipulation was exactly in the middle and tried to play around by printing the random pixel rgba values from the picture).

This was the final code which led to the final answer:

>>> from PIL import Image
>>> original ="oxygen.png")
>>> y = original.size[1]/2
>>> print "".join([chr(original.getpixel((x,y))[0]) for x in range(0, original.size[0],7)])

After running this code, I got the following:

smart guy, you made it. the next level is [105, 110, 116, 101, 103, 114, 105, 116, 121]pe_

The answer was integrity.

Challenge 8:

As compared to the last few puzzles, this one was relatively easy. Our first clue is the text at the bottom of the page: “Where is the missing link?”. Moving the cursor on the image of the fly indicates that the image is actually a hyperlink and clicking on it brings up a username, password authentication box with a cryptic text “The server says: inflate”. From here nowhere to go, I decided to delve into the source and found “un” and “pw”, which are actually encoded bz2 streams(Ok, Ok I admit I did not find about the stream format on my own) and here is some relevent info regarding bz2 streams. Had to use Python to decompress the streams to reveal the username and password:

>>> import bz2
>>> un = 'BZh91AY&SYA\xaf\x82\r\x00\x00\x01\x01\x80\x02\xc0\x02\x00 \x00!\x9ah3M\x07<]\xc9\x14\xe1BA\x06\xbe\x084'
>>> pw = 'BZh91AY&SY\x94$|\x0e\x00\x00\x00\x81\x00\x03$ \x00!\x9ah3M\x13<]\xc9\x14\xe1BBP\x91\xf08'
>>> bz2.decompress(un)
>>> bz2.decompress(pw)

Challenge 9:
Puzzle 9 tells us to “connect the dots”. The Source reveals 2 sets of values which can only be co-ordinate values. Plotting the coordinate values give us the image of a bull(took trial and error, I tried ox too).
Code is given below:

>>> from PIL import Image
>>> im ='RGB', (640, 487))
>>> file = open('first_list.txt')
>>> str_first =
>>> import re
>>> str_first = re.sub('\n', '', str_first)
>>> first_list = str_first.split(",")
>>> flx = first_list[0::2]
>>> fly = first_list[1::2]
>>> file2 = open("second_list.txt")
>>> str_sec =
>>> str_sec = re.sub('\n', '', str_sec)
>>> second_list = str_sec.split(",")
>>> slx = second_list[0::2]
>>> sly = second_list[1::2]
>>> im ='RGB', (640, 487))
>>> for i in range(len(flx)):
...     im.putpixel((int(flx[i]), int(fly[i])), (255, 0, 0))
...     i = i + 1
>>> i = 0
>>> for i in range(len(slx)):
...     im.putpixel((int(slx[i]), int(sly[i])), (255, 0, 0))
...     i = i + 1

Automation- Python to the rescue

Automation, let’s accept it, it is not easy, either it is a product line or getting your code built on the click of a button or as we have all done at some point of time, manually changing those  long and long repeated lines of code and hoping that a robot will do it for you(or maybe that rookie intern ;)).

So, talking in terms of code, what actually is automation and how useful it is and what kind of tools does it use. To analyse all this, let me talk about a scenario(read mundane) that recently occurred during my day job which rather forced me to come up with a small ‘robot’. As the client would have it, While writing an objective-c client for a server, I had some java code lying around, which I was referring to and this Java code had a rather long list(~300) of enums(string based) and as we all know c does not support those string based pixies and would need to be stripped away. So, I was wondering that how can I just copy these into a .m file and quickly get away with it. After pondering for like 5 mins, I realized, that it would be better if I could just write some code which can edit my code and hence save me some time.

Having decided so, I picked up Python as the tool to work with and after 15 or so mins(Sorry, my bad, my Python is really lousy, since I mostly read about it but never practice it, but am sure it won’t be so after this post.), came up with this little piece on terminal:

f = open("xyz.h", 'r')
lines = f.readlines()
f = open("xyz.h", 'w')
import re
for line in lines: p = re.compile('\((.*?)\)'); line = p.sub('', line); f.write(line);

AND, voila, I had a file but this time with only enum constants(Before: ENUM_CONSTANTS(“string”) After: ENUM_CONSTANTS). The little robot just accomplished this in few lines code.

Now, I’m looking forward to use Python as much as possible to create little robots like these to edit my code so that I can create a few more bugs in the meantime 😉

Happy Coding.

Mac Musings- Pulling ‘strings’

‘strings’ is a very powerful good little devil, present in every unix like OS. Its primary job is to hunt down and print text strings embedded in binary strings such as executables. Like every other tool, it is very useful for both hackers and crackers. Pentesters specifically can use this tool to identify vulnerabilities(read static passwords/pass phrases and usernames) during black-box testing of apps.

Usage of ‘strings’ on mac is extremely simple and just requires firing up the terminal and typing in: strings filename

The output is a list of strings present in the binary(if any). So, Coders using static passwords, please beware!

Common usage includes piping it to grep and fold or redirecting the output to a file.


So the thing was, I was tying to setup the adb path on my mac and finally done, I decided to write a little tutorial so that anybody and everybody can simply go through the following instructs and just breeze through it:

  • Fire up the terminal on the mac.
  • Browse to the root directory and create a file named .bash_profile, using the command “touch .bash_profile“.
  • Using the command above can be a little tricky as the user might not have the permissions to create the file under the root directory, so as a solution to that we will use the “sudo chown username file/dirname“, to temporarily change the permissions for a given directory/file.
  • Next, type “open -e .bash_profile” to open it in TextEdit.
  • A TextEdit window will open, copy and past this into that window, export PATH=$PATH:/yoursdkfolderfull path/sdk/platform-tools
  • Save the changes and close text edit.
  • Restart terminal, while in the root directory, type: source .bash_profile
  • ADB should be setup now, check by firing the adb command on the terminal.
  • And Thats pretty much it.

Bits and Nibbles

  • Adding 2 Binary numbers:

One of the popular interview questions, usually in the preliminary or the telephonic rounds. Given 2 binary arrays of length, you need to find their sum?

Ans: The arrays can be initialized as java boolean types,

                boolean A[] = { true, false, false };
		boolean B[] = { true, false, false };
		boolean C[] = { false, false, false, false };
		boolean carry = false;
		for (int j = B.length - 1; j >= 0; j--) 
			C[j + 1] = ((A[j] ^ B[j]) ^ carry);
			if (A[j] && B[j]) 
				carry = true;
				carry = false;

		C[0] = carry;

		return C;

The Sum can be calculated by using the XOR operator with a carry.


  • Finding the unique number from a given array:

Given an array, which has a double of each number(1,1,2,3,2,3,4,4…….). Find a single Number which is unique?

Brute-Force approach: Walk through the array and compare the numbers using a counter variable or something similar.

Simple Approach: XOR the array. Only the unique number will remain.


  • Find if the number ‘n’ i s a power of 2 or not.

n & (n – 1) == 0 should be true.

Custom Android, Custom USB

I have been working with a custom Android device for sometime now. This is a decently powerful 2.3.4 device(It is not a phone). It has USB capabilities too, but for some arcane reasons the vendor does not want to support the USB accessory api and hence one has to rely on native linux to handle the USB drive. The vendor’s app runs as a System app and needs to have the RW access to the USB. 


We were easily able to map the USB to mnt/udisk by having a quick look at the vol.fstab, which showed:

dev_mount usb /mnt/udisk auto /devices/platform/ehci-omap.0/usb1/

Since auto-mount was already enabled so it was not much of a hassle and things looked Vanilla and easy. But, Alas, everything went well until the code responsible for R/W was deployed. The Java code simply refused to acknowledge that the USB was plugged in and had data in it. Other apps, eg: Eclipse File Explorer, And Explorer, shell(OK, let’s leave this one out as it has root privileges), were easily able to acknowledge the existence of the USB. Now, what could have gone wrong with one small piece of File R/W Java code. The permissions looked pretty much Ok(other apps were already accessing the drive).

Few hours and 3 cups of coffee later, we had an aha moment when we released that the app was running as a System App and not a normal one. Putting the theory to test we ran the app as a normal one i.e. we immediately updated the debug.keystore to use the default one plus the manifest and yes everything was sunny again.


Hmmm….The issue at hand was solved now and the Vendor has been notified about the glitch, but This makes us think about the Android Security mechanisms. There should be a way to make system apps access USB but I haven’t found the answer to that yet. Will update as soon as I stumble on something.

Musings of Binary Search

Binary Search is the one of the most simplest difficult algorithms in CS. The first Binary Search Algo was posted in 1946 and the first bug detection in the same happened in 1962. Currently, I have been working various forms of Binary Search Algos. and various applications of the same. One such interesting application is searching for frequency of any given element in a sorted array, which uses a slightly modified version of binary search. Most of the code I found online had minor bugs, so I decided to write my own in Java and post it for future reference.

Here is a basic algo of how you do this,

  • Step 1: Given a sorted array, we find the lowest index of the element in the array, let’s call it L.
  • Step 2: We find the highest index of the element in the array, let’s call it H.
  • Step 3:  We calculate the frequency by, H-L + 1.

Let’s break down these steps into more detailed analysis,

For Step 1, We start from the middle of an array and check for the following,

  1. If the current element is the element we are looking for AND the element to its LEFT is either different OR the current element is at 0th index, then the position of the current element becomes the lowest index.
  2. If above conditions are not met then we readjust our search domain within the array using binary search i.e. we check whether the current element is greater than or less than the element we are looking for.

For Step 2, We start from the middle of an array and check for the following,

  1. If the current element is the element we are looking for AND the element to its RIGHT is either different OR the current element is at highest index, then the position of the current element becomes the highest index.
  2. If above conditions are not met then we readjust our search domain within the array using binary search i.e. we check whether the current element is greater than or less than the element we are looking for.

Here’s the Java code for the same,

public static void main(String[] args) 

        private static void doFrequencyAnalysis() 
		int arr[] = { 1, 3, 3, 3, 5, 5, 10 };
		int l = findLowestIndex(arr, 5, 0, arr.length - 1);
		int h = findHighestIndex(arr, 5, 0, arr.length - 1);

		// System.out.println("L = "+l+"H = "+h);

		if (l == -1) 
			System.out.println("---------------::" + 0);
			System.out.println("---------------::" + (h - l + 1));


        public static int findLowestIndex(int[] arr, int key, int low, 
			int high) 
		int mid;

		while (low <= high) 
			mid = (high + low) / 2;
			if ((mid == 0 || arr[mid - 1] != key) 
					&& arr[mid] == key) 
				return mid;
			else if (arr[mid] >= key) 
				high = mid;
				low = mid;
		return -1;

        public static int findHighestIndex(int[] arr, int key, 
			int low, int high) 
		int mid;
		while (low <= high) 
			mid = (high + low) / 2;
			if ((mid == arr.length - 1 || arr[mid + 1] != key) 
					&& arr[mid] == key) 
				return mid;
			else if (arr[mid] <= key) 
				low = mid;
				high = mid;
		return -1;

Binary Search helps us in accomplishing this task in log2N time, which makes us really appreciate its robustness.

Android Preference Framework is among st the mostly widely used building blocks of Android. Almost every Android application makes use of the Preferences file in some way or the other and our app is no different.

Our app uses the Android Preferences to store some general keys like,

  • Hardware Make
  • Hardware Model
  • Software Version
  • Date
  • Time
  • Connection State

All the above mentioned values are being retrieved from a sensor and are stored by the App in the preferences, to be displayed to the User in a User Friendly Manner.  However, we ran into a weird problem which had its roots in the Android Preferences during one of our test-runs.

Superficially speaking about the problem, whenever the user tried to retrieve the above mentioned data from the Sensor, the process seemingly happened fine, but whenever the app was restarted after any such scenario, all of the previously stored data in the Preferences file was lost, i.e., the App behaved as a freshly installed app and being launched for the first time! We tried various lenses to view this problem, our most prominent lens(wild guess actually) being that somehow something was overwriting the preferences file to default settings. Another programmer also suggested that “Somehow the Preference object was being null“. We tried to be nice and please everybody by incorporating their hypothesis and applying the suggested fixes and ended up wasting like 2 hours over this so called conventional wisdom!

After blaming the Preference Framework for those 2 hours and interrogating the data associated with it, we could not extract anything meaningful out of it except the OnSharedPreferenceChangeListener. So we decided to find somebody else to blame for the mess being created and turned our attention to the Sensor framework(as the bug always popped out whenever the Sensor data was retrieved). After tracking the changes in the preferences via OnSharedPreferenceChangeListener, it became clear that the Sensor Framework was not only retrieving the data, but was also storing like 10 of those values in the Preferences in a rapid succession and the code doing this looked something like,

updatePreferences(key1, value1);

updatePreferences(key2, value2);


updatePreferences(key10, value10);

As soon as this code was commented, the app behaved normally :-).

This made us realize that it was not about Android Preferences but about the manner in which we were updating those and hence we derived a few cardinal rules about the same,

  • Shared Preferences are NOT designed for storing data which changes often and/or large amounts of data.
  • Shared Preferences are just a file – it has to be opened, parsed, updated and closed every time we do an update operation on it. Doing update operation in a rapid succession might just corrupt the file and restore it to default values which was our case exactly.
  • Do not access Shared Preferences from multiple threads.
  • ‘Conventional wisdom’ can be wrong.

Happy Coding!

Android ScrollView Trick

Recently I had a problem with one of my layouts, where I was trying to put a variable length content in a ScrollView. All I wanted was to keep the height of the ScrollView fixed, inspite of the length of the content. I was trying to avoid hardcoding the height parameter due to the obvious choice of multitude of the devices I wanted to cover with my layout(shown in figure below).
My initial attempts with the ScrollView(shown with a green highlight), were about messing with(a combination of) the height/width/fillViewPort parameters, but nothing helped. Since the content inside the ScrollView was getting rendered dynamically, the ScrollView used to extend and occupy more than the top-half of the screen and the price was payed by the controls contained in the bottom LinearLayout which just dissappeared. This was fixed by wrapping the bottom LinearLayout in a ScrollView(shown below) and weirdly, now the top ScrollView always rendered the dynamic content within the top half boundries, which was what I initially intended.

Genesis-1, Enjoying the View!

After uploading 2 casual gaming titles(Powered By And Engine: on the Android Market(Search: Lunar Monk), We at Lunar Monk have been digging into the internals of the Android Graphics and Gaming capabilities(And Engine is good for your basic first game, but native is better). Our adventure begins from Replica Island (, One of the early games written for Android by Chris Pruett, which is open-source ( too. A read only copy can be checked out using Cygwin by giving the following command on Cygwin Bash,

svn checkout replicaisland-read-only
 Just Make sure you have the svn package installed with Cygwin.
Our next step is building the Source, which is a bit of a daunting task due to this error: Conversion to Dalvik format failed with error 1. Let’s see how to eliminate this error.
First thing first, the source which is checked out is setup to be built on android 1.5 and as most of us know that there are not much of those android versions alive today, So we chose to build on android 4.0. But if we are configuring the project in Eclipse, then it is easy to spot that it fails to create, which is due to the 2 anim files  activity_fade_in.xml & activity_fade_out.xml. Both the files use a linear interpolater for animation which is internal to android. To fix this, we need to create another xml with name linear_interpolator.xml. The contents of this xml would be,

<?xml version=”1.0″ encoding=”utf-8″?>
<linearInterpolator xmlns:android=””/&gt;

Moving further we would be linking this xml to our activity_fade_in.xml & activity_fade_out.xml by changing the android:interpolator entry in these two files to, android:interpolator=”@anim/linear_interpolator”.Now cleaning the project and rebuilding it would result in successful compilation, but an attempt to install it on a device will result in the dreaded Conversion to Dalvik format failed with error 1. This is due to the fact because the compiler is confused due to the multiple entries/pointers to few classes being linked from different places in this project which is resulting in a path as well as name space conflict. To fix this, We need to first remove all the other jars included in Replica Island except the android.jar(Can be done by Right Click the Project -> Properties -> Java Build Path). Now we need to open the classpath file present in the project directory and replace its contents by,
<?xml version=”1.0″ encoding=”UTF-8″?>
<classpathentry kind=”src” path=”src”/>
<classpathentry kind=”src” path=”gen”/>
<classpathentry kind=”con” path=””/>
<classpathentry kind=”output” path=”bin”/>

Cleaning the project and rebuilding it now should fix the bug now.

However, If the dreaded error is seen again then this might be the cause(the fix is provided too):