# Advent of Code, Data Structures, and Hidden Complexity

Since 2015, Eric Wastl has gifted us each December with Advent of Code - a 25 day programming competition that I very much enjoy. This year I haven't been able to get to too many of the problems. I only completed the first two days on the day they were released, problem three a day late and then I didn't get back to the problems until almost 12/25 - the final day of the competition. I've spent a bit of time over the break working through the problems. I'm currently through 9. I started working through the problems in Clojure - part of my yearly attempt to dive into the language but then did a few in Python just to speed up my progress.

Yesterday, I did question 9 which I found interesting as a teacher - so much so that I think I'll assign at some point in the future. At the core of the problem you have to maintain a list of items inserting and deleting items at assorted locations. I threw together this solution:

```
num_players =431 # 9
last_score = 70950 # 255
players = [0 for x in range(num_players+1)]
player=3
board=[0,2,1]
next_marble = 3
index=1
while next_marble < last_score:
if next_marble % 23 != 0:
#regular insert
L = len(board)
index=(index+1)%L
index=(index+1)%L
board.insert(index,next_marble)
else:
players[player]=players[player]+ next_marble
index=(index-7)%len(board)
players[player]=players[player]+board[index]
del board[index]
player=(player+1)%num_players
next_marble = next_marble + 1
print(max(players))
```

It worked and I got my first star. This unlocked part 2 which changed the number of marbles in the game from 70950 to 70950<b>00</b>. This brings me to my first observation - hidden complexity. Seeing my input I knew my part 2 would be crazy slow. The solution requires you insert and delete into a list over and over again and I did it using a Python list and a Python list is backed by an array. This means that every insert and delete will be linear and I had a whole bunch of them.

It's another great simple example of hidden complexity. Students and beginners use Python lists as if they were linked structures with direct indexing - the best of all worlds. It's great if you're dealing with small data sets but unless you're careful things can get very slow very fast. It's important that students learn about this and it appears that many don't.

Even though I knew the program would take forever to run it was my bed time so I started it on part two and turned in for the night.

When I woke up I had an answer to part 2, entered it and earned my next star.

Of course I knew that the solution should really be written using a doubly linked list. The funny thing is that if you're comfortable with creating dynamic data structures like linked lists, writing a solution using a doubly linked list and running it on part 2 actually takes a fraction of the time of writing the original solution and running it. To make sure, I did it. I decided to code it up in Java since I might assign something like this to my students in C++. You can check it out here.

The second take away was that it's actually beneficial for students to be able comfortable with pointers and dynamic memory and while creating a linked list let alone a tree can be tricky at first, it does get easier.

With all the debate going on over coding vs computational thinking vs CS it's things like this - thinking about things like this that marks one of the ways a computer scientist is different from a programmer.