31 December 2015

Hotel Automation Controller - Interview coding problem

This is one of the problems I got via a friend who recently faced a company called Sahaj Software a clone company to Thoughtworks.

Problem Statement:

Hotel Automation Controller Problem  Statement

A very prestigious chain of Hotels is facing a problem managing their electronic equipments. Their equipments, like lights, ACs, etc are currently controlled manually, by the hotel staff, using switches. They want to optimise the usage of Power and also ensure that there is no inconvenience caused to the guests and staff.

So the Hotel Management has installed sensors, like Motion Sensors, etc at appropriate places and have approached you to program a Controller which takes inputs from these sensors and controls the various equipments.

The way the hotel equipments are organised and the requirements for the Controller is below:
  • A Hotel can have multiple floors
  • Each floor can have multiple main corridors and sub corridors
  • Both main corridor and sub corridor have one light each
  • Both main and sub corridor lights consume 5 units of power when ON
  • Both main and sub corridor have independently controllable ACs
  • Both main and sub corridor ACs consume 10 units of power when ON
  • All the lights in all the main corridors need to be switched ON between 6PM to 6AM, which is the Night time slot
  • When a motion is detected in one of the sub corridors the corresponding lights need to be switched ON between 6PM to 6AM (Night time slot)
  • When there is no motion for more than a minute the sub corridor lights should be switched OFF
  • The total power consumption of all the ACs and lights combined should not exceed (Number of Main corridors * 15) + (Number of sub corridors * 10) units of per floor. Sub corridor AC could be switched OFF to ensure that the power consumption is not more than the specified maximum value
  • When the power consumption goes below the specified maximum value the ACs that were switched OFF previously must be switched ON

Motion in sub corridors is input to the controller. Controller need to keep track and optimise the power consumption.

Write a program that takes input values for Floors, Main corridors, Sub corridors and takes different external inputs for motion in sub corridors and for each input prints out the state of all the lights and ACs in the hotel. For simplicity, assume that the controller is operating at the night time. Sample input and output below.

Initial input to the controller: Number of floors: 2
Main corridors per floor: 1

Sub corridors per floor: 2





Since the hotel management is trying this for the first time, they would be changing the requirements around which electronic equipments are controlled and the criteria based on which they are controlled, so the solution design should be flexible enough to absorb these requirement changes without significant change to the system.

The solution to this problem involves approaching in an object oriented manner. Also we need to see here that we should use a Command/Strategy Pattern given there could be changes in the behavior based on external factors. I have not included the timings from the problem but from on here it should be easily extensible.

Code below:

Cheers!
Braga

21 November 2015

Cloning remote PG database and loading in Local environment


For projects involving small to medium sized databases one may require to copy the remote (or production) database onto local environment. I was earlier doing this for my production application using custom pg_dump and then restoring with pg_restore. It was relatively straightforward but still consumed good amount of time. I wanted to automate this using capistrano and this is how I did it

You should note that this is extremely fast because it executes the command on the VPS - usually EC2 which has amazing internet speeds. And then copies it over scp as a single file. You can also add a compression step using the --format option in the pg_dump.

Hope this was helpful!

Cheers!
Braga

Dealing with Intermittent Build failures due to Memory - Jenkins + EC2

It is not uncommon to see a Jenkins build failure due to a memory choke now and then while running thousands and thousands of rspec examples. The examples may be too much for the memory allocated within the EC2 instance. One simple solution is to enable Swap memory. Going through the typical EC2 route you need to have a dedicated Swap Partition. However if you feel you don't want to go through that route, you can simply do it via a swap file.

Make sure you have root access and follow these instructions to enable swap memory usage. Typically you could go from 1 to 2 times the allocated RAM but that is not a hard rule. In the current scenario I am going to elicit what I did.

Done! you may check whether this is enabled by typing the free command.


Note: This will only persist until the machine is running. If you reboot this will go away. If you still want to persist the swap after a restart you may do

Hope you find this article helpful. This is a trimmed down version of a wonderful article from DigitalOcean.

Cheers!
Braga

03 November 2015

Stand still - and you are going backwards!



Imagine you are standing on a moving escalator that is supposed to take you from a Ground level to the Top level - only that its moving in the opposite direction. If you stand still on such an escalator you will hit the bottom soon. In order to not hit the bottom - you need to "at least" walk at the -ve speed of the elevator. For an observer it would still be standing still but thats the least one could do especially being in the software industry.

My blog once had crossed a million page views and the most active time I was during 2010. I think looking back - that was the time I had cracked any problems thrown at me. And I enjoyed it. I still could crack em only that I realise recently I've become rusty. However, I developed a variety of different skills down the road.

Alright I think I am diverging too much from what I wanted to say. In the IT world, one needs to constantly keep learning, keep pushing the limits, get hands dirty on a variety of technologies. If you are a manual tester - try poking at Automation testing. If you are an Automation Engineer - learn about performance/scalability testing. If you are an application developer - explore system development. If you are a software architect - see what is going on in that particular space and try to remain on top. That is the only way you could make sure that you are always on the 99th percentile in this industry.

One has to realise doing the mundane and similar work won't scale as we have already headed and running in the First order differentiation of time (speed). Second order differentiation (acceleration) will make us even more redundant. Can't imagine what we differentiated even one more step. The only way to sustain such challenging epochs is to learn. Learn learn learn!

Cheers!
Braga

Printing the permutation of characters from a classical mobile phone

One of the problems I recently faced.

Input: A set of numbers from a mobile keypad
Output: Print all the combinations of characters for that given number.

For example if the input is 26. Then possible outputs are am, an, ao, bm, bn, bo, cm, cn, co

Solution:

There are many ways to solve this problem. One is using a tree. Another way is to simply do it with recursion. I solved this using a linear approach. If you convert the problem from the text domain to number domain - it will seem very simple.

For example - if the input is something like ['abc', 'def', 'pqrs'] (transformed from digits 237). Then the output would have a simple rule - the first character should come from the first element, second from the second and so on. Writing in an incremental number form for the indexes, you would get:

0 0 0
0 0 1
0 0 2
0 0 3
0 1 0
0 1 1
0 1 2
....
2 2 3

That's it - problem solved. Programatically expressed it here

17 August 2015

VIM Cheatsheet

A one page printable cheat sheet for VIM in PDF Format. Direct URL: https://goo.gl/Yu2rpG




Source and credits to RICHARD TORRUELLAS : http://vim.rtorr.com/

Cheers!
Braga

08 August 2015

Ransack namespace collision - The search method

I am a big fan of Ransack, the Rails gem for quickly building a quick and easy Search based on pure ActiveRecord queries. For projects that require some on the fly search without having to set up dedicated Search Engines like Elastic Search, SOLR, ThinkinSphinx - this is a very juicy alternative.

However I recently faced one problem when I introduced this to a project where there already was SOLR integrated. It was because of the collision of the search method.

This simply patch did the job for me. All you have to do is include it to your initializers folder say under config/initializers/ransack.rb

That's it, the Ransack library will no longer collide with your existing suite. As a matter of fact the method search deprecated already and the library itself wants us to use the Alias ransack

Cheers!
Braga

02 June 2015

Good bye Heroku !

I primarily work on Ruby on Rails and I use to deploy one of my pet projects to Production entirely on the Heroku's free tier. It was a wonderful two years to see having my application running there during which time I had around a thumping 10 Million page views! Heroku's free tier is one of the most used around the world for quickly showcasing development applications. However, my Production application was such that all my needs felt well under Heroku's free tier.

Here are some of the freebies I use to enjoy
  • Free Dedicated (techincally not) machine with 512 MB - The reason its technically not because you are allotted a free worker that would go to sleep if the application is not up for 30 minutes. However you could easily overcome this with something like Pingdom.
  • Free Postgres Database - Although there was no specific size limit to this other than a maximum number of 5000 rows, I would still say this is one of the best things Heroku offers. Not only your application can talk to it, anything anywhere from the world can talk to the database directly. And that too 100% free!
  • Free emails via Sendgrid - A maximum 200 emails per day! Sendgrid's free Addon allows me send upto 200 emails per day. My application's requirement was in the range 20-40.
  • Unlimited Job Schedulers - Although they would take one entire and the only worker allotted to you to spin, this is a boon considering you do not have a full fledged linux machine with you. I use to have only one rake task that takes a routine back up of my data as CSV and email it to me.
  • Free PG Backup - Again, if you want to routinely backup your data, you don't have to write any complex code. Heroku provides this out of the box and for free!

Now that Heroku is pulling down the curtain on the 24 hour availability or to pay $7 a month, I have no other option to switch to a much better alternative - that is Digital Ocean. Sign up here for a free $10 credit - https://m.do.co/c/21693a3f9c9e

Sad to depart ways, but you made me learn a lot of stuff in these two + years.

Cheers!
Braga