day3.md

Day 3

Part 1

Problem

In a grid of chars (which loops horizontally), find how many tree characters we will encounter when moving down 1, right 1.

Solution

Like the other puzzles, this can be solved easily with streams. I first convert the data into a 2d array. I could have left it as a list of strings, but this just felt right.

Then I did an indexed map to take each line at a time, moving 3 positions for each line. I mod the index by the width of the input to effectively loop it around rather than go off the edge. Finally, I take a count of '#' characters found.

Part 2

Problem

Same as above, but for additional slopes (down and right values).

Solution

To solve for different down amounts in the slope was simple, I just filtered out rows using a mod on the index.

I refactored the tree calculation into its own function so that I could iterate over a list of slopes.

Finally, I used a reduce to calculate the product. I had some issues here as I initially got overflow and had to convert to BigInt. (Later I realized Long was enough.)