Links
Highlights
- So… I got stuck. I didn’t think it was possible for any knot to be a full diagonal away from the preceding knot, but I was wrong. I had to reach out on Reddit to ask for help.
- Other than that, I’m pretty okay with my code. I used
map_reduce for the first time, and it was the perfect tool for the job.
defmodule Day09 do
use AOC
def step("R"), do: {1, 0}
def step("L"), do: {-1, 0}
def step("U"), do: {0, 1}
def step("D"), do: {0, -1}
def new_tail({head_x, head_y}, {tail_x, tail_y}) do
case {tail_x - head_x, tail_y - head_y} do
{-2, -2} -> {head_x - 1, head_y - 1}
{-2, 2} -> {head_x - 1, head_y + 1}
{2, -2} -> {head_x + 1, head_y - 1}
{2, 2} -> {head_x + 1, head_y + 1}
{-2, _} -> {head_x - 1, head_y}
{2, _} -> {head_x + 1, head_y}
{_, -2} -> {head_x, head_y - 1}
{_, 2} -> {head_x, head_y + 1}
_ -> {tail_x, tail_y}
end
end
def part1 do
input(9)
|> String.split("\n")
|> Enum.reduce({{0, 0}, {0, 0}, []}, fn motions, state ->
[direction, steps] = String.split(motions)
make_move_1(step(direction), String.to_integer(steps), state)
end)
|> elem(2)
|> Enum.uniq
|> Enum.count
end
def make_move_1({step_x, step_y}, move, state) do
Enum.reduce(1..move, state, fn _, {{head_x, head_y}, tail, tail_positions} ->
new_head = {head_x + step_x, head_y + step_y}
new_tail = new_tail(new_head, tail)
{new_head, new_tail, [new_tail | tail_positions]}
end)
end
def part2 do
input(9)
|> String.split("\n", trim: true)
|> Enum.reduce({[], (for _ <- 1..10, do: {0, 0})}, fn motions, state ->
[direction, steps] = String.split(motions)
make_move_2(step(direction), String.to_integer(steps), state)
end)
|> elem(0)
|> Enum.uniq
|> Enum.count
end
def make_move_2({step_x, step_y}, move, state) do
Enum.reduce(1..move, state, fn _, {tail_positions, [{head_x, head_y} | knots]} ->
new_head = {head_x + step_x, head_y + step_y}
{new_knots, new_tail} = Enum.map_reduce(knots, new_head, fn knot, last_knot ->
new_knot = new_tail(last_knot, knot)
{new_knot, new_knot}
end)
{[new_tail | tail_positions], [new_head | new_knots]}
end)
end
end