day03.md

Day 3: Squares With Three Sides

Approach and Reflections

We're given a list of numbers in three columns. For part 1, we simply calculate if each set of numbers represents a valid triangle. For part 2, we need to work downwards in columns, with each grouping of three numbers in a column representing a triangle.

This was tricky, but I used Elixir's pattern matching to "hardcode" the solution. First, I used the built in chunk_every to split rows into groups of three, then I made a rotate function like so:

def rotate_three([[a, b, c], [d, e, f], [g, h, i]]) do
  [
    [a, d, g],
    [b, e, h],
    [c, f, i]
  ]
end

Maybe there's a better way to do this? I found this to be sufficient and cliear, though.

Solutions

• Elixir

Problem Description

2016 Day 03 on AdventOfCode.com

Part 1

Now that you can think clearly, you move deeper into the labyrinth of hallways and office furniture that makes up this part of Easter Bunny HQ. This must be a graphic design department; the walls are covered in specifications for triangles.

Or are they?

The design document gives the side lengths of each triangle it describes, but... 5 10 25? Some of these aren't triangles. You can't help but mark the impossible ones.

In a valid triangle, the sum of any two sides must be larger than the remaining side. For example, the "triangle" given above is impossible, because 5 + 10 is not larger than 25.

In your puzzle input, how many of the listed triangles are possible?

Part 2

Now that you've helpfully marked up their design documents, it occurs to you that triangles are specified in groups of three vertically. Each set of three numbers in a column specifies a triangle. Rows are unrelated.

For example, given the following specification, numbers with the same hundreds digit would be part of the same triangle:

101 301 501
102 302 502
103 303 503
201 401 601
202 402 602
203 403 603

In your puzzle input, and instead reading by columns, how many of the listed triangles are possible?