Wednesday, November 23, 2011

Puzzler Solution: Prisoners and Hats and a Jungle

Can’t believe it’s been 2 months since I posted the puzzle. I’ve been involved in test automation using Selenium WebDriver over last couple of months and it has been a great experience designing and coding the test suite. The 1st step is complete - smoke tests for 2 of our applications have been automated and can be run on different browsers concurrently. Now we’re moving to the next steps (which are still TBD). I have lots of ideas in my head but I have to evaluate their feasibility and to break them into different phases so we can realize the added value as we continue to improve the automation suite.

Back to the puzzler solution. For reference, here is the puzzle statement and the code. My implementation of the guessStrategy method is below.

The strategy is that the 1st person to guess speaks the color of the odd hats ahead. So if the (number of black hats ahead) % 2 == 0, the guess is “Black” (lines 4-8). How does it help that person? It doesn’t, that prisoner is unlucky enough to be the 1st one to guess (end of the line) and has a 50% chance. But the rest of the prisoners can guess the color of their hat correctly now that they know the initial guess, all guesses since then and the color of hats ahead. So for example, if the 1st prisoner guesses black, there must be odd black hats ahead. Now if the 2nd prisoner sees odd black hats ahead, s/he must be wearing a white hat because the previous prisoner also saw odd black hats. And the logic goes on like that…if the nth prisoner sees odd number of black hats, and previously, odd number of people have guessed black, then s/he must be wearing a white hat (lines 13-23).
   1: public static char guessStrategy(String prevGuesses, String remArr) {
2: int numB = getNumOfChars(remArr, 'B');
3: int numPrevB;
4: if (prevGuesses.length() == 0) { // initial guess
5: if (numB % 2 == 0)
6: return 'W'; // if B is even, return W
7: else
8: return 'B'; // else return B
9: } else {
10: // strategy: if previously even number of people have said black,
11: // and I see even black hats, I have a white hat, else black
12: numPrevB = getNumOfChars(prevGuesses, 'B');
13: if (numPrevB % 2 == 0) {
14: if (numB % 2 == 0)
15: return 'W';
16: else
17: return 'B';
18: } else {
19: if (numB % 2 == 0)
20: return 'B';
21: else
22: return 'W';
23: }
24: }
25: }
27: /**
28: * @param arr
29: * @param ch
30: * @return number of times the specified char appears in the array
31: */
32: public static int getNumOfChars(String arr, char ch) {
33: int num = 0;
34: for (int i = 0; i < arr.length(); i++) {
35: if (arr.charAt(i) == ch)
36: num++;
37: }
38: return num;
39: }

Thursday, October 13, 2011

Dennis Ritchie 

No doubt, comparisons will be made with Steve Jobs and how much media buzz was created.

iPhone has had a huge impact in my life, but not as much as C. C has defined my thinking. During college, most of the programming I did was in C and K&R was THE reference book I had with me most of the time (I still have it). The precise and succinct way the book is written helped me appreciate the beauty of coding. Every function was defined in clear and concise terms. That not only guided me in my programming adventures, but somehow it also helped me put a different perspective around life and accumulate a no-nonsense attitude of holding myself to much higher standards than anyone else.

From the article linked above:
Colleagues who worked with Mr. Ritchie were struck by his code — meticulous, clean and concise. His writing, according to Mr. Kernighan, was similar. “There was a remarkable precision to his writing,” Mr. Kernighan said, “no extra words, elegant and spare, much like his code.”
In essence, this guy created something wonderful and powerful in the world of computing. And I hope his legacy lives on.

-Gaurav Gupta

Thursday, September 29, 2011

Puzzler: Prisoners and Hats and a Jungle, Oh My!

This puzzler was mentioned in Car Talk sometime ago and I really liked it. In brief, it goes like this:
A prison has 30 prisoners sentenced to be executed and the warden, who has the authority to pardon, decides to give them a chance to escape the punishment. He will stand all the prisoners in a straight line with each prisoner able to see the heads of all prisoners in front of him but not of those behind him. Next, he will put either a white or black hat on each prisoner’s head and ask them to guess the color of their hat one by one (starting with the 1st prisoner in the back of the line who can see all 29 other prisoners’ heads). If he guesses correctly, he’s pardoned. What is the strategy that the prisoners can use to maximize their chances of being pardoned?
The answer I came up with was grossly wrong so when these guys gave the answer (link to which I’m not posting here but can be found easily), I was intrigued. When listening to the answer, it sounded very simple but when I actually thought about it some more, I had to listen to it again to understand the strategy. For example, does the current prisoner need to know all the previous guesses or only the most previous guess?

I decided to write a simple program for this. The line of prisoners here is a StringBuffer of predefined size (in this case, 30) that is randomly filled with ‘B’ or ‘W’ chars. The objective is to write a method that will be called for each character in the StringBuffer with all the previous guesses (as a String) and remaining series (as a String). The method has to return the current character and should be implemented in such a way as to maximize the correct answers. Here’s the code:
   1: import java.util.Random;
2: public class Test {
3: static int SIZE = 30;
5: public static void main(String[] args){
6: int correctGuesses = 0;
7: StringBuffer pRow = new StringBuffer(SIZE);
8: StringBuffer prevGuesses = new StringBuffer(SIZE-1);
9: char currGuess;
11: Random rnd = new Random(System.currentTimeMillis());
12: //---print the series
13: System.out.print("Series:\t");
14: for (int i=0;i<SIZE;i++){
15: if (rnd.nextInt(2) == 0) //0=black, 1=white
16: pRow.append('B');
17: else pRow.append('W');
18: System.out.print(pRow.charAt(i) + " ");
19: }
20: //---strategy
21: System.out.print("\nStrat:\t");
23: for (int i=0;i<SIZE;i++){
24: currGuess = guessStrategy(prevGuesses.toString(),pRow.substring(i+1));
25: System.out.print(currGuess + " ");
26: if(currGuess == pRow.charAt(i)) correctGuesses++;
27: prevGuesses.append(currGuess);
28: }
29: System.out.print("\nCorrect Guesses=" + correctGuesses);
30: }
32: public static char guessStrategy(String prevGuesses, String remArr){
33: //random
34: Random rnd = new Random(System.currentTimeMillis());
35: if (rnd.nextInt(2) == 0) //0=black, 1=white
36: return 'B';
37: else return 'W';
38: }
39: }

First, I generate the series and print it (lines 13-19). And then I call the guessStrategy method for each character in the series with all the previous guesses and the remaining series. But currently, there is no strategy and each time, it randomly returns ‘B’ or ‘W’. And obviously, result is that correct guesses average around 15. Your job, should you decide to accept this assignment is to provide a better implementation of the guessStrategy method which maximizes the chances of guessing correctly. If you’re stuck, you can look at the Car Talk website or search on internet to find the answer and then try to implement it.

I’ll post my implementation of the method in a few days. Hopefully it provides enough of a challenge to some people to work on this besides their otherwise busy life.

Friday, July 1, 2011

Friday Lunch: Content-Type, Content-Disposition HTTP Headers and MIME type handling in IE

We have a scripting tool that is used to create web-based scripts that simulate application scenarios. These scripts are run through a monitoring tool on a continuous basis to assess the health of these applications. The scripts & scripting tool are also used to validate the scenarios if the monitoring tool issues an alert. The scripting tool uses IE to run the HTTP request/response scenarios. When recording the script for a recent new application, it was noticed that after submitting the request which was an HTTP POST with XML data, the tool was bringing up a dialog box to save the response file, something you’d get when clicking on a HTML link in the browser to a file (maybe a word .doc etc) and if you don’t have an application to open that kind of file. We checked the response and it contained a standard XML that should usually be displayed within the browser (like if you click on Amazon’s AWS service). The fact that it was bringing up a download prompt and not displaying the XML in the browser was a problem because the non-technical team that uses it to validate the application will not be able to determine whether the script ran successfully or not.

The application is a servlet that takes the initial POST request and returns an XML response. After further investigation with the tool and looking at the response headers, it was found that the application was returning “application/x-www-form-urlencoded” in “Content-Type” header erroneously. I haven’t found something specific that would say that content type is not correct for HTTP response, but it is more common to use this content type when submitting form-submission data that is URL encoded (see here).

Investigating with WebScarab

My first conjecture was that the file download prompt was being displayed because of this Content-type header’s value. If we could intercept the response and replace its value with something like “text/xml”, the tool or browser will be able to display the content within the application instead of bringing up the file download prompt. And to validate that, I used WebScarab to intercept the request/responses. But for some reason (maybe because of invalid Content-Type header), it didn’t intercept the response. It would intercept the request and after accepting, it would just complete the request and we’ll end up seeing the file download prompt in the tool. Clearing the “Only MIME-Types matching:” didn’t help.

So the workaround was to write a BeanShell script within WebScarab to replace the “Content-Type” header with a proper value instead of manually intercepting the response. That turned out to be quite straightforward. All we needed to do was put:


in WebScarab’s Bean Shell interface under the fetchResponse method. As is obvious, this replaces the value of Content-Type header in the response to “text/xml”.

Once we tried it out, it worked and the browser displayed the XML response without bringing up the file download prompt. This proved that the Content-Type header was the problem. But what exactly was the problem? What makes the browser display a prompt to the user to download a file instead of just displaying the content? Further searching revealed this article:

So basically the Content-Disposition header along with Content-Type specifies the MIME type of the file returned and whether the browser should display it or prompt the user to download it. The RFC has all the details but the gist is that the Content-Disposition header can be set to “attachment” for the browser to display the file download prompt. But the response we were getting didn’t have a Content-Disposition header. And IE was still displaying the file download prompt. Further reading warranted.

How does IE handle MIME types in a response, anyways?

I kept on searching and reading on how and when a browser brings up the file dialog prompt. The Content-Type header still bothered me. Then I came across this MSDN article: MIME Type Detection in IE. It provides details on what steps IE takes to detect the content-type of the content before deciding what to do with the file:

The purpose of MIME type detection, or data sniffing, is to determine the MIME type (also known as content type or media type) of downloaded content using information from the following four sources:

  • The server-supplied MIME type, if available
  • An examination of the actual contents associated with a downloaded URL
  • The file name associated with the downloaded content (assumed to be derived from the associated URL)
  • Registry settings (file name extension/MIME type associations or registered applications) in effect during the download

If you look at the known MIME types for FindMimeFromData, “application/x-www-form-urlencoded” is not one of them. It is also not an ambiguous MIME type ("text/plain," "application/octet-stream," an empty string, or null) either. So with the MIME type as unknown,

If the "suggested" (server-provided) MIME type is unknown (not known and not ambiguous), FindMimeFromData immediately returns this MIME type as the final determination.

And what happens after that is in this MSDN article: Handling MIME Types in IE. It doesn’t mention exactly what happens if the MIME type is unknown, there’s no application associated with it in the registry and there’s no Content-Disposition header. But I’m guessing that the only thing for it left to do is to let the user decide by showing the file download prompt as was happening in our case. And we were able to prove that. I looked up the registry key for “text/xml” under HKEY_CLASSES_ROOT\MIME\Database\Content Type and added it to a new key “application/x-www-form-urlencoded”. The new registry key looked like this:

[HKEY_CLASSES_ROOT\MIME\Database\Content Type\application/x-www-form-urlencoded]

This tells IE that for responses received with Content-Type equal to “application/x-www-form-urlencoded”, use extension “.xml” and open it however XML files are opened (which is to display them inline). Once added, we went back to the tool and ran the script. And this time, it displayed the response xml without bringing up the download prompt.

So through this exercise, I learned about how IE displays different MIME types and about Content-Type and Content-Disposition headers.

Monday, June 6, 2011

GLaDOS is back…

So it's ok if she tries to incinerate me alive for sake of “science” but when I try to escape and torch her in the process, she gets all psyched up and starts singing sarcasm laden song?