Advent 2019 Day - 8

# COMMENTS

I spent most of last week up in Albany working on the NY State K12 CS Standards so fell a bit behind. I had to go back to complete day 5 but still haven't finished day 7 which builds on day 5 which in turn builds on day 2. I might not get to finishing 7 for a while but it looks like a good chance to play with core.async - Clojure's facilities for concurrency. I also want to write up day 6 but today let's look at day 8.

Day 8 is image processing problem so it fits nicely into an APCS-A type class. You're given the height and width of the image and you're told that the image comes in layers. The input is a long string of digits. So, if you're told that the width is 5 and height is 3 you might get this as your input representing an image with 3 layers:

100210120100120001102020112100000121002110102

Adding commas and spaces:

10021,01201,00120   00110,20201,12100   00012,10021,10102

The first part of the task is to find the layer with the most 0s and return the number of 1s multiplied with the number of 2s.

This is just a nice little data parsing problem. You have to break the input into width*height chunks, see which has the most zeros, count the ones and the twos and return the result.

Part 2 is where the image stuff comes in. We're told that a color value of 0 is black, 1 is white, and 2 is transparent. We're also told that the image's actual color is the first color it sees (black or white) when going through the layers. Depending on how you read in your data, this could be easy or hard. You have to scan down the layers and keep the first non-transparent color.

For part 1, I read my data into a list of lists where each item was a layer. I did this in Clojure, but the Python representation of the sample image above might look something like this:

[ [1,0,0,2,1,0,1,2,0,1,0,0,1,2,0],
  [0,0,1,1,0,2,0,2,0,1,1,2,1,0,0],
  [0,0,0,1,2,1,0,0,2,1,1,0,1,0,2]]

There was no reason to subdivide each layer into rows and columns.

For part 1, I wanted to have lists where each list was all the values of each pixel across the layers. This meant transposing the matrix - tat is, if you look at the above list of lists as a matrix, switching the rows and columns. This was pretty easy to do in Clojure using ~(apply mapv vector matrix)~ but it shouldn't be too hard in other languages.

Next, we have to keep the first non-transparent color we see in each list.

The final fun part is then taking the colors and printing out the final image.

Here's what I got:

XXX  XXXX  XX   XX  X  X 
X  X X    X  X X  X X X  
X  X XXX  X    X  X XX   
XXX  X    X    XXXX X X  
X    X    X  X X  X X X  
X    X     XX  X  X X  X 

What's nice about this problem from a teacher's point of view? First, it's the puzzle aspect of finding the hidden text. Second, it's an image problem which in APCS-A usually means 2D arrays but you can also solve it without any 2D matrices. My solution involved a bunch of filtering of data and the matrix transpose. Both interesting concepts to bring to your classes.

If you want to see my Clojure solution, you can find it here.