CS488 Final Project: OpenGL Boat Game

Here’s something I’ve been working on for the past few weeks for one of my courses, CS488 – Intro to Computer Graphics. For the final project, you’re allowed to do any OpenGL or raytracing project, as long as it has 10 reasonable graphics related objectives. Here’s a video of mine:

A screenshot:

It’s a simple game where you control a boat and go around a lake collecting coins. When you collect a coin, there’s a bomb that spawns and follows you around. You die when you hit a bomb. Also if two bombs collide then they both explode (although you can’t see that in the video).

Everything is implemented in bare-metal OpenGL, so none of those modern game engines or physics engines. It’s around 1000-ish lines of C++ (difficult to count because there’s a lot of donated code).

Edit (8/10/2016) – I received an Honorable Mention for this project!

Some thoughts about CS488

For those that haven’t heard about CS488, it’s one of the “big three” — fourth year CS courses with the heaviest workload and with large projects (the other two being Real-time and Compilers). It’s one of the hardest courses at Waterloo, but also probably the most rewarding and satisfying course I’ve taken.

There are four assignments, each walking you step by step through graphics techniques, like drawing a cube with OpenGL, or building a puppet with hierarchical modelling, or writing a simple ray tracer. Then there’s the final project, where you can choose to make something with OpenGL or extend your ray tracer. The class is split 50/50, about half the class did OpenGL and the other half did a ray tracer. I personally feel that OpenGL gives you more room to be creative and create something unique whereas ray tracing projects end up implementing a mix of different algorithms.

The first two assignments weren’t too bad (I estimate it took me about 10 hours each), but some time during assignment 3 I realized I was spending a lot of time in the lab, so I got an hours tracking app on my phone to track exactly how much time I was spending working on this course. Assignments 3 and 4 each took me 15 hours. I spent 35 hours on my final project, over a period of 3 weeks. I chose relatively easy objectives that I was confident I could do well, which left time to polish the game and do a few extra objectives. I’m not sure what the average is for time spent on the final project, but it’s common to spend 50-100 hours. Bottom line: you can put in potentially unbounded amounts of time to try to get the gold medal, but the effort actually required to get a good grade is quite reasonable.

Now the bad part about this course (obviously not the instructor’s fault) is OpenGL is so incredibly difficult to work with. Even to draw a line on the screen, you have to deal with a lot of low level concepts like vertex array objects, vertex buffer objects, uniform attributes to pass to shaders, stuff like that. It doesn’t help that when something goes wrong in a shader (which runs on the GPU), there’s no way to pass an error message back to the CPU so you can print out variables and debug it. It also doesn’t help that there’s a lot of incompatible OpenGL versions, and code you find in an online tutorial could be subtly broken for the version you’re using. On the other hand, working with OpenGL really makes you appreciate modern game engines like Unity which takes care of all the low level stuff for you.

Roboroast: upload your photo to get an algorithmically generated insult!

I’d like to share a side project I’ve been working on for the past few weeks. Roboroast is an app that automatically generates humorous insults for you or a friend based on how you look. It was written in collaboration with my friend Andrei Danciulescu.

The basic operation is as follows. There’s a subreddit called /r/RoastMe where random people post a picture of themselves, and other people proceed to “roast” the person with funny comments making fun of his appearance.

Our app takes your photo and uses a face recognition algorithm to find a poster in /r/RoastMe who looks like you. Then we display the comments for your closest matches.

You can try it at roboroast.tk.

Sample Results

Here’s some roasts for myself:

Here’s some for Andrei:

High Level Overview

The project comprises of roughly 3 parts:

Part 1 is the Reddit scraper. We use the PRAW API to go through all posts on the /r/RoastMe subreddit, saving comments to MongoDB and saving images to the filesystem.

Part 2 is the Face++ uploader. Face++ is a cloud service with a REST API that handles our face matching. To use it, we upload all the images from part 1 into a “faceset” which we can query later.

The first two components only need to be run periodically, maybe once a month to update the faceset with new posts from Reddit. Part 3 is the webapp, which is the use facing component. It accepts user uploads, searches for matches using the Face++ API, and renders a list of insults to the user.

Technology Stack

As mentioned before, we used a number of third party APIs; PRAW for scraping Reddit posts, and Face++ for face recognition.

All the backend code is written in Python. The web app uses the Flask web framework, and is wrapped with NGINX and Gunicorn to handle connections and serve static files. We use MongoDB for the database.

The frontend is built with Bootstrap. We also use javascript libraries jQuery and handlebars.js.

The whole thing is hosted on a single AWS EC2 instance.

How good is the face matching?

The face matching is actually decent. Face++ produces reasonable matches most of the time.

To see the matching results for yourself, you can append ?r=1 to the end of the URL (on the results page). This is hidden by default.

Do the insults make sense?

Although the face matching does a decent job, we found that the quality of results were somewhat hit-or-miss.

When we envisioned the concept for this app, we assumed that most insults were going to make fun of the subject’s face. However, many insults refer to their non-facial appearance, or clothing, or objects in the background. Since we only do face matching, these comments will make no sense.

Other times, comments will refer to the title of the post — in other words, an insult depends on both the submission title and the picture. Again, these make no sense with only the picture.

We attempt to mitigate this with heuristics that analyze the comment, in order to exclude roasts which refer to the title or articles of clothing. This approach had limited success because natural language processing is hard.

Conclusion

When Andrei initially proposed this idea for an app, I thought the concept was pretty cool and unique. In a month or so we had a prototype, and I spent a few more weeks polishing the project for release. The quality of results you get is still highly variable, but we’re working on improving our algorithms.

In any case, it’s my first time with a lot of these technologies, and I had fun and learned a lot building it.

Teaching Myself Electronics: Zero to Arduino in 5 Weeks

I’m about to graduate with a degree in computer science, but I can’t describe how a computer works. Okay, maybe that’s an exaggeration. I can tell you all about assembly language and operating system kernels, and I have a good idea of how to build a CPU out of basic logic gates.

That’s where my knowledge ends. I have no idea how to build an AND gate, or how to coerce my 120V power supply to gently power these gates without frying them.

Learning is good, and this is a pretty big knowledge gap. I’m going to teach myself electronics. My plan is to learn by building things. There’s a lot of mathematical theory to learn, much of it is not that useful, and it’s easy to get bogged down in random details. Much better is to just experiment and go back and learn the theory when needed.

Week 1: Electronic Playground

The first problem was getting components. Unlike computer programming, where everything you need is on the internet, for hardware I’ll actually need to buy things. This is difficult when you don’t know exactly what you need. I also didn’t want a million different parts littering my bedroom haphazardly.

Eventually I settled on this all-in-one kit (cost $30).

It has a lot of components: LEDs, resistors, capacitors, even antenna and speakers. All the components are fixed to a board, and to connect them together, you use wires that clip to springs protruding from the board.

The kit comes with an instruction booklet that describes all kinds of things you can wire with it. For example, here’s a “harp” — it makes different tones when you hover your hand over the photoresistor:

This schematic is a bit too advanced for me at this stage — unfortunately the booklet doesn’t attempt to explain how it works.

That’s fine, the following books do an excellent job of starting from the basics:

After playing with this for a while, I learned a lot of basic things like how current / voltage / resistance works, how to read common schematic symbols, and how to decode a resistor.

Week 2: Multimeter

Electricity is invisible, and debugging circuits is difficult without being able to see what’s going on. I went ahead and got a multimeter (cost $20):

It was easy enough to measure resistance and voltage (both AC and DC). The current measurement was not very sensitive though and I could barely register any reading.

Around this time I attended a workshop in Manhattan that taught how to read schematics and build it on a breadboard. We made a 555 timer circuit which made a LED blink on and off:

I can’t understand how it works right now, but breadboards are pretty neat. Much easier than sticking wires into springs on my electronic kit at home.

Week 3: Baby steps with Arduino

By now I was reaching the limits of what my electronic kit could offer, and I needed to graduate to something more serious.

So I went to the nearest electronics shop and got an Arduino Uno kit (cost $90). The Arduino is a microcontroller and lets you prototype circuits easily with a breadboard. The Arduino Uno is only $25, but my kit comes with an assortment of components and sensors.

Before long I had the Arduino up and running. It runs a dialect of C, so I felt at home in the programming environment.

Here’s a program that blinks the onboard LED on and off in a loop (kind of equivalent of hello world):

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin 13 as an output.
  pinMode(13, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);              // wait for a second
  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);              // wait for a second
}

Week 4: Transistor Switching

I didn’t really know what a transistor did, but it’s what logic gates are made of and the backbone of all computers, so it can’t hurt to learn about them, right?

I started off building logic gates from transistors, but couldn’t get it work. It turned out that I misunderstood how a transistor operates (it’s not the most intuitive at first glance). Luckily, I had a friend in electrical engineering and she patiently cleared up my misconceptions.

Transistors are used for logic gates, but I didn’t know that transistors can also amplify a current. I also learned about the many different types of transistors.

Here’s a circuit that I built (from the Arduino kit manual). It uses a transistor as a switch to control a motor:

By the way, here’s how you make an AND gate with two transistors:

While wiring things up, I accidentally burned a LED and a BJT transistor. Apparently 5 volts without a resistor is fatal to many components. In software, if you mess up, you get a segmentation fault in your console or something — never the smell of burnt plastic in your room.

Week 5: Arduino Controlled Desk Lamp

Here’s an idea. Wouldn’t it be nice if your lamp can turn itself on when it gets dark? Useful or not, let’s build it!

This is actually a major milestone for me. Up until now, I’ve been mostly following existing schematics, using parts carefully selected by people who wrote the schematics. But for this project, we improvise everything from scratch. Oh yea, also it’s the first time I’m working with 120V alternating current.

First I got a $15 desk lamp. It’s the kind that plugs into the wall AC socket. I start by using a wire stripper to expose copper wires that I can plug into the breadboard:

I’m a bit nervous working with 120V current, obviously it’s a lot more powerful than the 5V of Arduino. Generally, 120V won’t kill or seriously injure you, but it does deliver an unpleasant shock.

After stripping the cord, the lamp can be plugged into the breadboard. But the voltage is too high for the Arduino to handle directly; instead, I need a relay which acts as a buffer and a switch. A transistor can act as a switch too, but relays can handle more current.

To detect light, I have a separate circuit that uses a photoresistor (changes resistance depending on amount of light shining on it). The Arduino can read the photoresistor by measuring analog voltage.

Here’s the schematic of my design:

Now we do software. It does a loop every 500ms, detects the amount of light, and decides whether the lamp should be on or off. It’s slightly complicated by the fact that when the lamp is on, it produces light and that affects the photoresistor readings. I need to compensate for that but it’s not too bad.

Here’s the code I came up with:

int photoPin = 5;
int lampPin = 3;

bool isLampOn = false;

void setup(){
  pinMode(lampPin, OUTPUT);
  Serial.begin(9600);
}

bool shouldTurnLampOn(){
  int lightLevel = analogRead(photoPin);
  if(isLampOn){
    return lightLevel < 70;
  }
  else{
    return lightLevel < 50;
  }
}

void loop(){
  if(shouldTurnLampOn()){
    digitalWrite(lampPin, true);
    isLampOn = true;
  }
  else{
    digitalWrite(lampPin, false);
    isLampOn = false;
  }
  delay(500);
}

Here’s a video demo:

Onwards

Actually my lamp idea probably isn’t that useful. Nevertheless, I made a lot of progress in just a few weeks and I’m proud of myself for that.

I’ve barely scratched the surface of all the cool things you can do with electronics, but it’s a good start. Now I have all kinds of ideas on what to build next. I’d better get to it!

Four life lessons learned by playing Hearthstone

I’ve played Hearthstone on and off for a few years, since it first came out. As I played more and more, I began to notice parallels between my decision making processes in Hearthstone and in real life. This is a self-reflective post, and probably my first serious attempt to describe the core features of my mentality and decision making process. Although I wanted to write this for a long time, I found it difficult to put my ideas into words because they have been part of my personality for so long.

Why is Hearthstone a good representation of real life? Two reasons:

  • First, it’s a game of imperfect information and chance, so you must take risks and deal with uncertainty. Real life situations are usually like this. Games of perfect information (like chess) lack this probabilistic aspect and behave very differently.
  • Second, Hearthstone is a game about decision making skills, rather than mechanics. Every game has some element of decision making, but many games require doing some mechanical action (eg. last hitting) better than your opponent. Mechanical skills are confined to the specific game and are less likely to be relevant in real life.

By playing Hearthstone, I developed a general internal model for making decisions in uncertain situations. This is a broad criterion and covers many situations in day-to-day life.

Lesson 1: There is always a correct decision, and it’s your job to find it

The goal in Hearthstone is to reduce your opponent’s life to zero. How do you accomplish this? You make a plan, perhaps flooding the board with minions, perhaps unleashing a deadly combination of spells.

For our purposes, it doesn’t matter what your strategy is. At the start of the turn, you look at the cards in your hand, the state of the board, what cards your opponent played before. Call this information the game state. You ponder for a bit and come up with an action that best improves your position.

You execute your action on the board, but you still don’t know what happens next with certainty. There are many things you cannot control, which I will call RNG. RNG is short for Random Number Generator, and I will use it to mean anything you don’t have control over.

I use the term RNG for lack of a better term, but I’m not just talking about random game mechanics. RNG includes any state hidden from you, like your opponent’s hand and strategy.  Think of it as a random variable with a known distribution (eg. you play a card that destroys a random minion, which minion will it hit?) or with an unknown distribution (eg. what is the probability your opponent has two flamestrikes in his deck?). Even if the information is known to your opponent, it’s simpler to treat it as a random variable.

Here’s the model summarized in a diagram:

In any game state, there must be one “correct” action that gives you the highest chance of winning the game. The decision-making player aims to consider all possible actions and choose the best, “correct” one.

As a corollary, decision making should be perfectly rational. Otherwise, if my decision engine generates two different actions depending on my emotional state of mind, they cannot both be correct.

A second corollary is actions should always be justifiable through fundamental values. It’s unacceptable to do things by habit, or because other people are doing it — everything I do should have a positive expected value on the things I want to accomplish.

For me, one of my “meta” goals in life is to make correct decisions as much as possible. This is not to say that I behave like a robot — I still experience emotions like everyone else — but I try to eliminate emotions from my decision making process.

In Hearthstone, doing so gives you the highest chance of winning the game. It makes sense then, by extrapolation, that correct decision making gives you the best shot of getting what you want out of life.

Lesson 2: Information is valuable, treat information gathering as a subgoal

One rule of thumb in Hearthstone is “RNG first”. If you are going to play a sequence of cards, one of which has a random effect, it’s better to play the random effect first. This way you extract information out of the RNG pool of unknowns, and with this extra information you might be able to make a better play.

Another useful thing is to keep track of enemy secrets. Imagine you have this on the board:

You want to play a giant, but you’re worried that the secret is “mirror entity”, which summons a copy of the next minion you play.

Without any other information, you’re in a tough spot. But what if you played a minion last turn and the secret did not activate? Then you know that the secret isn’t mirror entity, and confidently play the giant.

Alternatively, suppose that you don’t have this information handy. One tactic is you can “test out” the secret by playing a small minion, and seeing whether the secret activates. You are paying a price with a normally inferior move, but the information you obtain is valuable for future decisions.

A similar concept occurring in real life is flirting. You’re at a party and you see a cute girl walk by. At first, you make a few playful comments, and observe her reaction and assess if she is interested in you. Flirting isn’t just an arbitrary social custom; it makes sense logically as a way of gathering information.

While information isn’t the final end-goal by itself, even a little information can greatly improve decision making, by eliminating vast swathes of possibilities that no longer need to be considered. Whether it be playing a giant, making a big purchase, or asking someone out on a date, gathering information is a useful subgoal.

Lesson 3: Focus on things you have control over, RNG evens out in the long run

Often in Hearthstone, luck is just not on your side. Have you ever seen your opponent topdeck the pyroblast and instantly win the game? Or that mad bomber that hits you three times in the face? How do you feel?

It’s natural to feel angry when this happens to you, especially if it ends up losing you the game. But eventually I realized how pointless it was to get upset at unlucky RNG. What’s the use of worrying about things you have no control over?

I see this all the time — people getting visibly upset when the bus is late, or when a player on your team goes AFK in a game of league. I try to adopt the opposite mindset: worry about my own decision making and simply accept random events beyond my control.

Let me give you an example. Last term, during an important phone interview, my phone stopped working during the middle of the interview. Calmly I got up and notified the CECA front desk, and waited as they spent the next 20 minutes troubleshooting the problem. Most people would be stressed out at this point, but I didn’t feel stressed at all. Rather than getting upset, my mind was relaxed, because I took comfort in knowing that I did everything that could be done; whatever happens next was out of my control.

The law of large numbers says that when you repeat a random event many times, the average outcome will surely converge to the expected value. Hearthstone is so random that a legend player will beat a rank 5 player no more than 55% of the time. Any single game is close to a coin flip, just marginally in favor of the stronger player. But over the long run, it’s a mathematical guarantee that the better player will end up on top.

Lesson 4: Separate the outcome of a decision from the decision itself

In real life and in Hearthstone, you can’t directly tell if a decision was good or not. You only know the outcome, and you can decide if the outcome is good or bad. But the outcome is a function of the decision and RNG, which adds noise to the process.

In other words, the correct decision does not always produce a good outcome, and sometimes a bad decision produces a good outcome. It would be a mistake to retroactively label a decision as “correct” simply because you got lucky.

Here’s a Hearthstone example:

Your opponent is a mage, and on turn 6 you flood the board with a lot of small minions. If he has flamestrike, playing it deals 4 damage to each of your minions, instantly killing your whole board.

Turn 7 comes and it turns out he doesn’t have flamestrike, so you win the game easily. You conclude that playing all your minions was a great idea because he didn’t have flamestrike.

This logic is fallacious: it fails to separate decision from outcome. A correct action is the one that maximizes the win probability, given the information available at the time. Therefore it makes no sense to look at the outcome and retroactively judge the correctness of the initial decision.

So in this example, playing all these minions was a mistake because there’s a high chance the mage has flamestrike. It doesn’t matter if he actually has flamestrike or not, the mistake is equally bad. (A better play would be to play fewer minions, thus mitigating the risk).

Now here’s a real life example. Last term, I had multiple job offers for software engineering internships and I had trouble deciding which one to accept. So I tried to negotiate: I picked one of the companies, told them about my other offers, and asked for a 20% raise in salary. My request was denied.

Does this mean that negotiating was a waste of time? Absolutely not. I know friends who successfully negotiated a higher salary by doing something similar. My particular outcome was not successful, but this doesn’t indicate my attempt was a mistake; if I found myself again with multiple offers, I would do the same thing.

Alfred Tennyson wrote the following about romance:

‘Tis better to have loved and lost

Than never to have loved at all

There are many ways to interpret this quote, here’s mine. Even if the outcome of a romantic encounter is unfavorable (to have loved and lost), it does not mean the decision to pursue the relationship was a mistake.

Why I still do stupid things

Alas, despite my best efforts, I still find myself doing stupid things — quite frequently even. Mistakes happen for a variety of reasons, but after analyzing some, I group them into three broad categories.

The first type of mistake happens when the situation is complicated, and the amount of data available exceeds my brain’s capacity to process it. In theory, I should never lose at chess — all the information is known. Of course, the number of positions explodes combinatorially and in reality I’m a mediocre chess player. Chess grandmasters group information in “chunks” and can reason about positions more efficiently — but this requires experience. In general, humans are prone to making mistakes in complicated situations.

The second category of mistake is having an incorrect model of the world. When we evaluate possible actions, we “simulate” the effects with a simplified version of the world. Problems arise when there is a discrepancy between the model in our heads and the real world.

This discrepancy can manifest itself in several related ways. We may incorrectly value subgoals, for example, a newbie Hearthstone player, knowing the objective is to reduce the enemy’s health to zero, thus decides to deal the maximum damage to the enemy’s hero every turn and ignoring everything else. We may overlook important factors, for example, leaving a Gadgetzan Auctioneer on the board, not realizing its potential, and being surprised next turn when your opponent draws 10 spells using its special ability. Or we may simply miss a possible play that never even occurred to us.

This type of mistake is the most common, but fortunately the most fixable of the three. As you gain more experience with the domain, your model of the world becomes a more accurate representation of the real thing. Then you learn to correctly assign values to things, and generate the full set of possibilities for a situation. For me, this gradual process of learning and self-improvement is one of the most satisfying things in life.

The third and final category of mistake is making decisions without thinking, thereby short-circuiting the entire decision making process. This could be when you’re stressed, emotional, or just tired. An example of this is when you casually trade some minions in Hearthstone, then realizing you had lethal. If only you thought more carefully, you would have easily found the correct play.

It’s not necessarily bad to do things without thinking too hard: it would be silly to invoke the full mechanism to choose between a burrito or a sandwich for lunch. It’s important, however, to realize when a decision is likely to have far-reaching consequences. In that case, it’s wise to defer making the decision until you had time to think things through.

There’s a lot more I could talk about, but this post is getting quite long so I’ll stop here. Whether you agree or disagree with my view of the world, please leave a comment!

Why is it so rainy in El Yunque – travels in Puerto Rico

This week, the entire engineering team at Yext went on a trip to Puerto Rico. Three nights at a beach resort, all expenses paid for.

What?! As an intern? No way! That was my reaction when I first heard about it. Friends at other software companies boasted about corporate housing, gourmet meals, and pantries stocked full of snacks of every kind, but Yext’s Puerto Rico offsite takes the cake.

The Resort

San Juan, the main city in Puerto Rico, is a 4 hour flight from New York. Puerto Rico is a popular destination because it’s a US territory, so you don’t need to worry about things like visas or international currencies. Also the drinking age is 18, rather than 21 for most of the US.

The resort was located 1 hour from San Juan, in the Fajardo region. I had never been to a Caribbean resort before, but the experience was more or less identical to my preconception of what a resort should be like. Along with my fellow engineers, we had a good time swimming in the beach, playing beach volleyball, and drinking lots of mojitos.

Here’s me on the beach:

Since this is a company offsite, there were some serious activities too. For half the day, senior Yext engineers gave tech talks on things like domain driven design and how to write integration tests.

After the Resort

For me, the amount of fun I have at a resort is not constant. The first day at the resort is the most amazing thing ever. Then on the second and third day, when you redo the same resort activities, the excitement wears off. I think if I spent a week at the resort, I’d be pretty bored by the end of it.

After the 3 days that were officially scheduled, some of the engineers decided to stay at the resort for the weekend. I joined a group that rented a car and drove to El Yunque — a tropical rainforest not too far away. After that, I spent another day exploring the city of San Juan by myself before getting on the plane back to New York.

El Yunque was surprisingly rainy. Even though we knew it was a rainforest, the amount of rain caught all of us off guard.

Standing on a lush green mountainside, you could see the dark clouds releasing a constant downpour of rain. Yet in the distance, the beach resort remained warm and sunny. The skies cleared up the moment we left the rainforest.

It seemed all the rain was concentrated within the boundaries of El Yunque national park, as if artificially constrained by a force field.

So why is it rainy in El Yunque?

The curious climate of El Yunque intrigued me. When I got home, I did some research on why it behaved this way.

A quick Google search gave me this precipitation map, which confirmed my suspicions:

Figure: Mean Annual Precipitation of Puerto Rico in 1963-1996

The purple region in the northeast is El Yunque. It receives 120 inches of rainfall a year, which is 3 times more than San Juan.

It might also be worth looking at a relief map of Puerto Rico:

The rainforest area is on a higher elevation than the surrounding region. So the rain falls where there are mountains. Gotcha.

This phenomenon is called orographic precipitation (orographic means relating to mountains). When warm and humid air is forced up a mountain, it cools and forms clouds, which then precipitates. The other side of the mountain experiences a rain shadow effect as the descending is devoid of moisture.

Also, in the Caribbeans, the trade winds tend to blow from east to west. This explains why El Yunque is a rainforest, but there are higher mountains in other parts of the island which are not rainforests.

Actually, in retrospect this seems like a fact we all learned in grade school. I don’t know what explanation I was expecting, something fancier?

In any case, this mix of geographical and weather conditions gives us a unique and beautiful landscape — and the only tropical rainforest in the US.

Achievement Unlocked: publish app on iOS App Store without testing it on a device

This week marks the end of a hobby project I’ve been working on for the last few months. It’s called WATisRain, and here’s a link to github. Initially an Android app, I ported the app to iOS over a period of two months. Yesterday, the app was approved on the app store, you can download it here.

Some background

I was never an Apple person. I do not own a Macbook, iPod, iPhone, or any Apple device.

My first mobile platform is Android. In this post I talk about my first impressions as an Android developer. That’s a story for another day.

Last year I started work on an app to navigate the tunnels between buildings of my university campus. The network of buildings, tunnels, and bridges were not very well known, even among upper years, so I figured it would be a cool idea for an Android app.

I worked on this idea on and off for a few months, then released version 1.0 to Google Play. The app quickly got about 2000 downloads and a couple dozen positive reviews. I was pretty happy.

An obvious next step to take is port this to iOS. The campus population is split between Android and iOS, so an Android-only app locks out a significant fraction of the user base. Unfortunately, I didn’t build my app with any of the cross-platform technologies, so this involves porting the entire codebase (2k lines of Java) into objective-C. I also didn’t have a Macbook or iPhone, both of which happens to be pretty crucial for iOS development.

Few months later, I landed an internship at Minted, in San Francisco. My company lent me a Macbook Pro, so I finally have the hardware to work on an iOS port. I still didn’t have an iPhone, but no matter. Surely the simulator is sufficient, right?

Motivated by a hard deadline (I had to return my Macbook at the end of my internship), I worked evenings and weekends to finish the iOS port. I ignored all coding conventions and translated my Java code, literally line by line, into objective-C.

It only took a few weeks to port over all the features and get it on the app store. I called a few of my friends who had iPhones, and asked them to download my app. They confirmed the app works. Mission accomplished.

Impressions on iOS development

My overall impression on iOS development so far is mixed.

I’m impressed with the technical aspects of Apple’s products, from the iPhone devices themselves to the IDE, Xcode, that Apple provides for developers. Compared to Android development, I was faced with far fewer random IDE glitches, inconsistencies between devices, and the like. Developing on the simulator worked amazingly well — enough to get me to the app store. For comparison, it would be unimaginable to develop the same Android app entirely on the emulator.

What I really disliked is the closed and proprietary approach Apple takes for its products. First of all, you need a Mac of some sort to develop for iOS, period. I can happily develop Android on any platform I want, but I cannot run Xcode on Windows.

Next, you need to enroll in Apple’s developer program, at a cost of $119 per year. At the end of the year, if you don’t renew your membership, your app is removed from the store. Even if you just want to develop for fun, without submitting to the app store, you still need this license to push your app to your device. In contrast, Google Play charges a $25 lifetime fee for the same thing.

One last thing I have to mention is the app review process takes 1-2 weeks. This is incredibly frustrating, since any bugfix will take a week to push to users.

In practice all these factors combined leads to a high barrier of entry for a hobbyist like me. Let’s calculate. If you spend $2500 on a Macbook Pro, $500 for some sort of iPhone, $119 for the developer program, that’s already over 3000 dollars before you can even start coding.

Can you really develop without a device?

All across internet forums, people advocate that you should test your app thoroughly across many devices before submitting to the app store. It seems that trying to develop without a device is an edge case, often the instructions for a task assumes you have a device, and you have to find a workaround if you don’t.

In my case, it was successful, in the sense that I produced an app that didn’t crash and got past app review. But I don’t know if I got lucky, because things could have turned out badly.

Throughout the whole process, I was worried that running the app on a real device would exhibit bugs that aren’t producible in the simulator. In that case, I’d have no way to debug the problem, and the project would be done. My app uses nothing but the most basic functionality, so I had a good chance of dodging this bullet. But still, the possibility loomed over me, threatening to kill the project just as it crosses the finish line.

A second problem is by copying my original Android app feature by feature, the resulting iOS app looks and feels like an Android app. A friend pointed this out when I sent him the app. I hadn’t noticed it, but after looking at some other iOS apps, I have to agree with him. Actually in hindsight it shouldn’t surprise anyone that without seeing other iOS apps, I don’t really know how an iOS app should behave. But it just never occurred to me that the natural way to do things for me might be unnatural for iOS users.

Finally, this is subjective, but for me it wasn’t very fun to develop for a simulator. Without the tactile sensation of your creation running on an actual phone, the whole experience feels detached from reality. You feel like an unwelcome foreigner in a country where the customs are different, and you begin to question yourself, why am I doing this iOS port anyway?

Part of what kept me going was sunk cost fallacy. I paid $119 to be an iOS developer, so I’d better get at least something on the app store, have something to show for it.

Now that the app is finished, I think I’m done with iOS development. Perhaps the app store is fertile ground for developers and startups looking to make a profit, but the cost of entry is unreasonable for someone making a few open source apps for fun.

What’s the hardest bug you’ve ever debugged?

In a recent interview, I was asked this question: “what’s the most difficult bug you’ve encountered, and how did you fix it?” I thought this was an interesting question because there are so many answers you could give to this question, and the sort of answer you give demonstrated your level of experience with developing software.

I thought for a moment, recalling all the countless bugs I had seen and fixed. Which one was the most difficult and interesting? In this article I’m going to describe my most difficult bug to date.

It was an iOS app. I was working as a four-month intern at the time. “We’ve been seeing reports from our users that the app randomly display a black screen,” my boss explained one afternoon. “No error message, no crash log, nothing. The app is simply stuck at a black screen state until you kill it.”

“Fair enough. How do I reproduce it?”

He shrugged. “I don’t know. Users are reporting it happens randomly. Here’s what you gotta do: grab an iPad, download the game off the app store. Create an account and play the game until you hit the bug.”

So I did. I was reduced to one of these typewriter monkeys, banging away mindlessly at the keyboard until I stumble upon the sequence of button presses to trigger the undiscovered bug by sheer coincidence.

For an afternoon I monkeyed away, but no matter what buttons I pressed, the mythical black screen would not appear. I left the office, defeated and mentally exhausted.

The next morning I checked into the office, picked up the iPad, and resumed my monkeying. But this time my fortune was different: within 15 minutes, lo and behold, the screen flashed white, followed by an unrepentant screen of black.

What did I do to trigger this? I retraced my steps, trying to repeat the miracle. It happened again. Methodically I searched for a deterministic sequence of actions that brought our app to its knees. Go to the profile page. Hit button X. Go to page Y and back to the profile page. Hit button Z. The screen flickered for a millisecond, the black. Ten times out of ten.

With a sigh of relief, I jotted down this strange choreography and went for a walk. Returning with a fresh mind ready to tackle the next stage of the problem, I executed the sequence one more time, just to make sure. But the bug was nowhere to be seen.

I racked my brain for an explanation. The same sequence of actions now produced different results, I reasoned. Which meant something must have changed. But what?

It occurred to me that the page looked a little different now from when I was able to reproduce the bug. In the morning, when I came in, there was a little countdown timer in the corner of the screen that indicated the time until an upcoming event. The timer was not there anymore. Could it be the culprit…

To test this hypothesis, I produced a build that pointed the game to the dev server, and fired up a system event. The timer appeared. I executed my sequence — profile, tap, home screen, back to profile, tap, and sure enough, with a flicker the black screen appeared. I turned off the timer, repeated the sequence — profile, tap, home, profile, tap — no black screen. I had finally discovered the heart of the matter. There was some strange interaction going on between the timer and other things on the page.

At this point, with 100% reproducibility, the worst was over. It took a few more hours for me to investigate the issue and come up with a fix. My patch was quickly rolled out to production, and users stopped complaining about random black screens. Then my team went out for some celebratory beer.

I will now describe exactly what happened — and why did a timer cause such an insidious bug.

The timer widget was implemented using an NSTimer which made a callback every second. To do this, the timer holds a reference to the parent view which contains it. This is not too unusual, and is generally innocent and harmless — until you combine it with Objective C’s garbage collection system.

Objective C’s garbage collection system uses a reference counting algorithm. I’ll remind you what this means. The garbage collector maintains, for each object, a count of how many references lead to it. When this reference count reaches zero, it means your object is dead, since there is no way to reach it from anywhere in the system. Thus the garbage collector is free to delete it.

This doesn’t work for NSTimer, though. When two objects hold references to each other, their reference count remain at least 1, which means they can never get garbage collected. In our app, this meant that whenever the view with the timer goes out of view, it doesn’t get disposed, but remains in the background forever. A memory leak.

A memory leak, by itself, can go unnoticed for a long time with no impact. The last part of the puzzle that brought everything crashing down had to do with the way a certain button was implemented. This button, when pressed, broadcasted a message, which would then be received by the profile view.

When the timer is active, it is possible to get the system into a state with two profile views — a real one and a zombie one kept alive by a reference cycle with the timer.

Then when the message is broadcasted, both the real and zombie views receive the message in parallel. The button logic is executed twice in rapid succession, which understandably causes the whole system to give in.

With this mechanism in mind, the fix was easy. Just invalidate the timer when the view goes out of view. Without the reference cycle, the profile view is disposed of correctly and all is well again.

I think this story demonstrates a fundamental truth about debugging: in order to debug effectively, you need to have a deep understanding of your technology stack. This is not always true of programming in general — quite often you can write code that works yet not really understand what it’s doing. When developing a feature in an unfamiliar technology, the typical workflow is, if you don’t know how to do something, copy something similar from StackOverflow or a different part of your code base, make some changes until it works. And that’s a fine way to do things.

But debugging requires a more structured methodology. When many things are breaking in haphazard ways, you need to narrow down the problem to its very core, to identify precisely which component is broken. In this case it was a reference cycle that wouldn’t get released. The core of the problem may be buried within layers upon layers of an API, even an API you believe to be bulletproof. It might require digging into assembly code, even hardware.

To find that core requires an understanding of a mind-boggling stack of technologies that software today sits upon. That’s what it takes to become a master debugger.

So, what’s the hardest bug you’ve ever debugged?