Advent of Code: Day 1, 2021

Mix.install([
  {:kino, "~> 0.7.0"}
])

Parse Input

input = Kino.Input.textarea("Input file:")

Part 1

input
|> Kino.Input.read()
|> String.split("\n", trim: true)
|> Enum.map(&String.to_integer/1)
|> Enum.chunk_every(2, 1, :discard)
|> Enum.count(fn [left, right] -> left < right end)

Part 2

input
|> Kino.Input.read()
|> String.split("\n", trim: true)
|> Enum.map(&String.to_integer/1)
|> Enum.chunk_every(3, 1, :discard)
|> Enum.chunk_every(2, 1, :discard)
|> Enum.count(fn [left, right] -> Enum.sum(left) < Enum.sum(right) end)

For a small improvement, we can note that we actually don’t need to do any sums. When comparing two adjacent sums, say $A = A_1 + A_2 + A_3$ and $B = B_1 + B_2 + B_3$ then $A_2 = B_1$ and $A_3 = B_2$. It follows that $A < B$ if, and only if $A_1 < B_3$.

input
|> Kino.Input.read()
|> String.split("\n", trim: true)
|> Enum.map(&String.to_integer/1)
|> Enum.chunk_every(4, 1, :discard)
|> Enum.count(fn [left, _, _, right] -> left < right end)