Category Archives: Tech

Simulating Low Disk Space in iPhone Simulator

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While testing my iPhone application, I realized there is no way to test out my worst-case scenario, when there is no disk space. How do we do that? This StackOverflow post lays it out conceptually:

I bet you could also create your own .dmg file with file system of size … say 2Mb and write to it. If this works, then it is super-easy for testing – you just mount it and switch the path for testing. If the dmg is small enough, you could probably even upload it to the source control.

Let’s do this “in practice”.

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ASIHTTPRequest vs. NSURLRequest

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Support for one of my favorite libraries, ASIHTTPRequest, has been discontinued. I have been slowly but surely making a transition from ASIHTTPRequest (back) to NSURLRequest in some of my projects. I picked ASIHTTPRequest over NSURLRequest because NSURLRequest is verbose. Extremely verbose. ASI also has queueing control, throttling control and much, much more. These are are just some of the things that made ASIHTTPRequest such a pleasure to work with. Alas, I am stuck with NSURLRequest again…

Anyway, one of the more interesting aspects of connection handling is session control. I rely on the backend server to set a session cookie when I log in, and to keep that cookie alive while I do other stuff in the program. But in particular, when I transitioned the login screen to NSURLRequest, I noticed that, despite logging in successfully, my program would think that I’m NOT logged in, in the parts where ASIHTTPRequest is still used. It looks like ASIHTTPRequest is blind to the session cookies that are set using NSURLRequest.

I should also mention that I’m using CodeIgniter on the back end for session control, but in theory it shouldn’t matter (spoiler alert: but it does).

Let’s line up the usual suspects and try to figure out why this happens.

The Usual Suspects
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MySQL Exclusive Row Relationships

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This post will explore how to find exclusive row relationships — rows that may be grouped together by some non-unique id that identifies some kind of grouping of those rows. For example, let’s say that we are managing a team of players, and that each team has to be unique. In other words, players A and B form Team 1, but players A,B,C form Team 2. Searching for existing teams with players A and B should only return Team 1, searching for teams with players A,B,C should return only Team 2, and searching for B,C returns an empty set. How to do this in SQL?

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Stack from Queues

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Ran across a question on how to implement a stack by only using queues on SO, so I decided to give it a go in Java and PHP. It’s actually pretty straight forward:

  • Initialize two queues.
  • On every push operation, if both queues are empty, add to the first queue.
  • If one of the queues is full (one will always be empty), pop off all the elements from the full queue and add them to the empty queue in the same order (as they were popped). This guarantees the full queue will always be sorted like a stack, so when a pop occurs, it is popped in the “LIFO” order, not “FIFO” order.

Code after the break. Continue reading →

Doubly Linked List in C

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Revisiting some of my old Arduino projects over the weekend, I remembered that C is not my strong point (that was almost a pun… close call). Rather than run away from the mountain of ampersands and hail of asterisks that ensued, I decided to improve on my Canon Rebel remote control project (loosely inspired by the awesome Camera Axe) by redoing the menu system for the Arduino-controlled LCD.

I will do the project write-up at another time as this post is largely aimed at higher-level language people like myself who struggle with pointers and memory addressing. But the good news is that there are a lot of tutorials and examples online for linked lists and various data structures in C. Continue reading →

Reversing a Linked List

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Exploring and refreshing data structures and algorithms, you will inevitably find the question “how do you reverse a linked list?”. For this exercise, we assume a singly-linked list, with a head, and the last item pointing to null (as shown).

Linked List

Singly Linked List

The goal is to end up with the following linked list:

Reversed Linked List

Reversed Linked List

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Breadth-First Search and Depth-First Search in Java

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I decided to implement Breadth First Search and Depth First Search in Java. The implementation uses a Directed as the underlying abstract data type, keeps track of visited graph nodes by keeping a separate data structure, and implements BFS by using a Queue.

Some implementations only implement DFS and BFS on trees, but this implementation uses a graph because a tree is really a directed graph with no cycles (i.e. a node cannot reach itself in traversal). However, in the case of a graph, when a node can reach itself, you have to pay special attention to those loops by marking nodes as “visited” so that you don’t “visit” them again and end up in a stack overflow or an infinite loop. There is more than one way to do this: by keeping a separate data structure of the visited nodes (the way I did it) or by adding an extra field to each node, mark it as visited each time it is visited, and reset it once you are done with the search. Since both methods would take linear time but resetting the nodes’ visited flags would make coding more complicated, I chose to keep a separate data structure for that.

Source

UPDATE 2012/10/16: Graph in PHP. Interestingly, PHP arrays use hash maps as the underlying structure, so using key/value pairs might actually speed up the algorithm.