Thought about rewriting part 1 to use Cramer’s rule like part 2 does but I quite like my comprehension use, actually, even if it’s not terribly efficient (part 2 finishes faster than part 1 lol). And no, I didn’t know Cramer’s rule. Reddit to the rescue again. I like it when I learn something new. Or something I may have known 30 years ago and forgot all about since linear algebra is not exactly prominent in my day to day life.
defmodule Day13 do
@re ~r/(?<=[A|B|Prize][:|=] X[\+|\=])(\d*).*(?<=^|Y[\+|\=])(\d*)/
@real File.read!(__DIR__ <> "/input.txt") |> String.trim()
def run(input \\ @real) do
data =
input
|> parse()
:timer.tc(fn -> solve_1(data) end) |> IO.inspect(label: :part_1)
:timer.tc(fn -> solve_2(data) end) |> IO.inspect(label: :part_2)
end
defp parse(input) do
input
|> String.split("\n\n", trim: true)
|> Enum.map(fn machine -> Regex.scan(@re, machine) end)
|> Enum.map(fn machine ->
machine |> Enum.map(fn line -> line |> tl() |> Enum.map(&String.to_integer/1) end)
end)
end
defp solve_1(machines, limit \\ 100) do
machines
|> Task.async_stream(fn machine -> calc_min_cost(machine, limit) end, timeout: 10_000_000)
|> Enum.filter(fn {:ok, cost} -> is_integer(cost) end)
|> Enum.sum_by(&elem(&1, 1))
end
@doc """
[[ax, ay],[bx,by],[px,py]] = machine
A * ax + B * bx == px AND A * ay + B * by == py
solve for all values of A and B
get_b = fn a -> (px + py - a * (ax + ay)) / (bx + by) end
"""
def calc_min_cost([[ax, ay], [bx, by], [px, py]], limit) do
max_moves_a = min(div(px, ax), div(py, ay))
max_moves_b = min(div(px, bx), div(py, by))
for a <- 0..min(limit, max_moves_a),
b <- 0..min(limit, max_moves_b),
a * ax + b * bx == px,
a * ay + b * by == py,
reduce: :infinity do
acc -> min(3 * a + b, acc)
end
end
defp solve_2(machines) do
machines
|> Enum.map(fn [axay, bxby, [px, py]] ->
[axay, bxby, [px + 10_000_000_000_000, py + 10_000_000_000_000]]
end)
|> Enum.map(&cramers_rule/1)
|> Enum.filter(&is_integer/1)
|> Enum.sum()
end
defp cramers_rule([[ax, ay], [bx, by], [px, py]]) do
a = div(px * by - py * bx, ax * by - ay * bx)
b = div(py * ax - px * ay, ax * by - ay * bx)
if a * ax + b * bx == px and a * ay + b * by == py do
3 * a + b
else
:infinity
end
end
end
I also finally decided to structure this day with the test data as an actual test that is run with mix test. Rather than running it as a script I’m compiling it and running from iex with iex -S mix and then calling the Day13.run function from there.