defmodule Day14 do
defmodule Board do
@derive Inspect
defstruct [:w, :h, :robots]
def from_data(data, w, h) do
robots = data
|> String.split("\n", trim: true)
|> Enum.map(fn line ->
Regex.scan(~r/-?\d+/, line)
|> List.flatten()
|> Enum.map(&String.to_integer/1)
|> List.to_tuple()
end)
%Board{w: w, h: h, robots: robots}
end
def move(board, moves) do
board.robots
|> Enum.map(fn {x, y, vx, vy} ->
{
posrem(x+moves*vx, board.w),
posrem(y+moves*vy, board.h)
}
end)
end
defp posrem(n, m) do
rem(rem(n, m)+m, m)
end
def print(board, pos) do
pos = MapSet.new(pos)
for i <- 0..board.w-1 do
for j <- 0..board.h-1 do
if MapSet.member?(pos, {j, i}) do
"X"
else
"."
end
end
end
|> Enum.map(&List.to_string/1)
|> Enum.join("\n")
|> IO.puts()
end
end
def part1(data) do
board = Board.from_data(data, 101, 103)
Board.move(board, 100)
|> Enum.map(fn {x, y} ->
cond do
x < div(board.w, 2) and y < div(board.h, 2) ->
{1, 0, 0, 0}
x < div(board.w, 2) and y > div(board.h, 2) ->
{0, 1, 0, 0}
x > div(board.w, 2) and y < div(board.h, 2) ->
{0, 0, 1, 0}
x > div(board.w, 2) and y > div(board.h, 2) ->
{0, 0, 0, 1}
true ->
{0, 0, 0, 0}
end
end)
|> Enum.reduce({0, 0, 0, 0}, fn {i1, i2, i3, i4}, {q1, q2, q3, q4} ->
{q1+i1, q2+i2, q3+i3, q4+i4}
end)
|> then(fn {q1, q2, q3, q4} -> q1*q2*q3*q4 end)
end
def part2(data) do
board = Board.from_data(data, 101, 103)
Stream.map(0..10000, fn i ->
pos = Board.move(board, i)
most = pos
|> Enum.group_by(&elem(&1, 0), &(elem(&1, 1)))
|> Enum.map(&Enum.sort(elem(&1, 1)))
|> Enum.map(fn ys ->
Enum.zip(ys, tl(ys))
|> Enum.map(fn {a, b} -> b-a end)
end)
|> List.flatten()
|> Enum.frequencies()
|> Enum.max_by(&elem(&1, 1))
if elem(most, 0) == 1 and elem(most, 1) > 100 do
dbg({i, most})
Board.print(board, pos)
i
end
end)
|> Stream.filter(& &1 != nil)
|> Enum.take(1)
|> hd()
end
end
data = IO.read(:stdio, :eof)
{time1, ans1} = :timer.tc(fn -> Day14.part1(data) end)
IO.puts("Time : #{time1 / 1000000}")
IO.puts("Answer: #{ans1}")
{time2, ans2} = :timer.tc(fn -> Day14.part2(data) end)
IO.puts("Time : #{time2 / 1000000}")
IO.puts("Answer: #{ans2}")
