109 lines
3 KiB
Elixir
109 lines
3 KiB
Elixir
defmodule Day9 do
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defmodule Rope do
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defstruct head: {0, 0},
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tail: {0, 0},
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knots: for(_ <- 1..8, do: {0, 0}),
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visited: MapSet.new([{0, 0}])
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end
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def part1(input) do
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input
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|> Enum.reduce(%Rope{}, &motion/2)
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|> then(fn %Rope{visited: visited} -> MapSet.size(visited) end)
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end
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defp motion({_dir, 0}, rope), do: rope
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defp motion({dir, amount}, rope) do
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head = step_head(rope.head, dir)
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tail = step_tail(head, rope.tail)
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visited = MapSet.put(rope.visited, tail)
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motion({dir, amount - 1}, %Rope{head: head, tail: tail, visited: visited})
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end
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defp step_head({x, y}, "R"), do: {x + 1, y}
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defp step_head({x, y}, "L"), do: {x - 1, y}
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defp step_head({x, y}, "U"), do: {x, y + 1}
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defp step_head({x, y}, "D"), do: {x, y - 1}
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defp step_tail({hx, hy}, {tx, ty}) when abs(hx - tx) <= 1 and abs(hy - ty) <= 1, do: {tx, ty}
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defp step_tail({tx, hy}, {tx, ty}) when hy - ty > 0, do: {tx, ty + 1}
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defp step_tail({tx, hy}, {tx, ty}) when hy - ty < 0, do: {tx, ty - 1}
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defp step_tail({hx, ty}, {tx, ty}) when hx - tx > 0, do: {tx + 1, ty}
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defp step_tail({hx, ty}, {tx, ty}) when hx - tx < 0, do: {tx - 1, ty}
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defp step_tail({hx, hy}, {tx, ty}) do
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dx = if hx - tx > 0, do: 1, else: -1
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dy = if hy - ty > 0, do: 1, else: -1
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{tx + dx, ty + dy}
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end
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def part2(input) do
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input
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|> Enum.reduce(%Rope{}, &motion2/2)
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|> then(fn %Rope{visited: visited} -> MapSet.size(visited) end)
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end
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defp motion2({_dir, 0}, rope), do: rope
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defp motion2({dir, amount}, rope) do
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head = step_head(rope.head, dir)
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{knots, leader} =
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Enum.map_reduce(rope.knots, head, fn knot, leader ->
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new_knot = step_tail(leader, knot)
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{new_knot, new_knot}
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end)
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tail = step_tail(leader, rope.tail)
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visited = MapSet.put(rope.visited, tail)
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motion2({dir, amount - 1}, %Rope{head: head, tail: tail, knots: knots, visited: visited})
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end
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def input do
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with [input_filename] <- System.argv(),
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{:ok, input} <- File.read(input_filename) do
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input
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|> String.trim()
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|> String.split("\n")
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|> Enum.map(fn <<dir::binary-1, " ", amount::binary>> ->
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{dir, String.to_integer(amount)}
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end)
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else
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_ -> :error
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end
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end
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#######################
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# HERE BE BOILERPLATE #
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#######################
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def run do
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case input() do
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:error -> print_usage()
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input -> run_parts_with_timer(input)
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end
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end
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defp run_parts_with_timer(input) do
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run_with_timer(1, fn -> part1(input) end)
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run_with_timer(2, fn -> part2(input) end)
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end
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defp run_with_timer(part, fun) do
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{time, result} = :timer.tc(fun)
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IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
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IO.puts("#{result}\n")
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end
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defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
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defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
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defp format_time(μsec), do: "#{μsec / 1_000_000}s"
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defp print_usage do
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IO.puts("Usage: elixir day9.exs input_filename")
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end
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end
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Day9.run()
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