Advent of code/testing 2016

This year I participated in the “Advent of Code” challenge. Every day two small programming questions were posted on adventofcode.com that (depending on your skill) could be solved within 30 minutes to 1-2 hours. During this challenge I tried something new: test-driven development. This post is a writedown of how this went and what I learned.

During the first few challenges I noticed that each challenge included a wide range of examples on how to solve a single problem. This greatly helped in understanding the problem, and helped you debug your own code. Only after the samples worked for me I would input the posed problem to get the answer (and the shiny star).

At the start of the december I started with my normal debugging method: writing print statements at the places I wanted to see were correct (and places I suspected had a bug). However, if you are unsure where your bug is, this can lead to a long messy output. In this long list of output you have to find the ‘wrong’ output, and have to track down where this came from (usually with more print statements).

To fix this problem I started writing down some of the examples using the Unittest framework. This had the advantage of verifying step by step that your example works (using AssertEquals). For each function I wrote I had at least one testcase to verify that the code I wrote was working for the example given in the question.

The first few days this slowed me down a little. Finding out what tools to use for Python (my go to language for quickly solving small problems) took some time. Although I used unittests in Java, testing Python code was something I never did before. After a few days I did notice that the amount of time spent to type in the test-cases started to outweigh the time necessary to gradually walk through the code to find that one bug in my code (or my thinking).

Later during the month I started using Doctests instead of Unittests. With this I knew that each function I wrote worked in the cases specified in the problem statement. After I verified this it was easy to link them together to (finally) solve the question for that day.

An example of how my code looked at day 21:

def swap_letter(input_str,letterx,lettery):
    """
    >>> swap_letter('hallo','a','l')
    'hlaao'
    >>> swap_letter('ebcda','d','b')
    'edcba'
    """
    input_str = input_str.replace(letterx,"@")
    input_str = input_str.replace(lettery,letterx)
    input_str = input_str.replace("@",lettery)
    return input_str

Overall I really enjoyed Advent of code 2016! Some of my personal stats are:

• Solved 50 problems (2 per day, including the last “free” one)
• Got 29 points on the public leaderboard (solved one problem as one of the first 100 people, had to wake up at 6 in the morning to do so)
• Got 537 points on the private leaderboard and finished first of my friends!
• Learned more about regular expressions (sometimes the text to parse required some extra power than I normally use)
• Became a better test-driven developer, using both Unittests and Doctests in Python

Instant update: the solutions of Peter Norvig can be found here: https://nbviewer.jupyter.org/url/norvig.com/ipython/Advent%20of%20Code.ipynb . It is great to see that he too continuously tests his code using assert statements in a iPython notebook. Next up: comparing his solutions to mine to learn more about efficiently solving these kind of puzzles.

Do you have an opinion on test-driven development, or want to show how you solved the Advent of Code 2016? Please post them in the comments or send them to me!

Merry christmas everybody!