Advent of Code 2021 - Day 21

bjorng · December 21, 2021, 7:53am

This topic is about Day 21 of the Advent of Code 2021.

We have a private leaderboard (shared with users of Erlang Forums):

https://adventofcode.com/2021/leaderboard/private/view/370884

The entry code is:
370884-a6a71927

bjorng · December 21, 2021, 7:56am

Fun puzzle today.

Here is my solution:

github.com

bjorng/advent-of-code-2021/blob/a81d0de617e7c909eb7910d96f52d61d0d01705c/day21/lib/day21.ex

defmodule Day21 do
  def part1(input) do
    parse(input)
    |> Day21_part1.solve
  end

  def part2(input) do
    parse(input)
    |> Day21_part2.solve
  end

  defp parse(input) do
    Enum.map(input, fn line ->
      [pos] = Regex.run(~r/(\d+)$/, line, capture: :all_but_first)
      String.to_integer(pos)
    end)
    |> List.to_tuple
  end
end

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hauleth · December 21, 2021, 10:00am

For now only Part 1, as I am still working on Part 2:

round =
  Stream.unfold(0, &{&1, &1 + 1})
  |> Stream.scan({{0, p1}, {0, p2}}, fn n, {{s, p}, other} ->
    p =
      case rem(p + 6 + n * 9, 10) do
        0 -> 10
        v -> v
      end

    {other, {s + p, p}}
  end)
  |> Enum.take_while(fn {{s, _}, {_, _}} -> s < 1000 end)

{rounds, {{loser, _}, _}} = {length(round), List.last(round)}

rounds * 3 * loser

princemaple · December 21, 2021, 10:09am

github.com

princemaple/aoc/blob/main/2021/day21.exs

# Title: Day21

# ── Untitled ──

[p1start, p2start] =
  "input"
  |> IO.getn(1_000_000)
  |> String.trim()
  |> String.split(["\r\n", "\n"], trim: true)
  |> Enum.map(&(&1 |> String.split(": ") |> List.last() |> String.to_integer()))

defmodule D21 do
  def play(pos, inc) do
    sum = pos + rem(inc, 10)
    if sum > 10, do: sum - 10, else: sum
  end

  def solve1(p1start, p2start) do
    Stream.iterate(1, fn n -> rem(n, 100) + 1 end)
    |> Stream.chunk_every(3, 3, :discard)

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Oh sorry, wrong reply button

wasi0013 · December 21, 2021, 10:23am

Enjoyed this one!

github.com

wasi0013/advent_of_code/blob/master/lib/y2021/day_21.ex

defmodule Aoc.Y2021.Day21 do
  @moduledoc """
  Solved https://adventofcode.com/2021/day/21
  """
  import Aoc.Helper.IO
  use Agent
  defp start, do: Agent.start_link(fn -> %{} end, name: __MODULE__)

  def run_part1(), do: get_input() |> solve_part1()
  def run_part2(), do: get_input() |> solve_part2()

  def solve_part1([_p1, num1, _p2, num2]) do
    play(num1, num2, 1, 100, 1000)
  end

  def play(num1, num2, low_dice, high_dice, winning_score) do
    Stream.cycle(low_dice..high_dice)
    |> Stream.chunk_every(6, 6)
    |> Enum.reduce_while(
      {num1 - 1, num2 - 1, 0, 0, 0},

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rhruiz · December 21, 2021, 9:56pm

First star with only streams: advent-of-code-2021/first.ex at main · rhruiz/advent-of-code-2021 · GitHub

Still figuring out why my second star does not generate enough branching universes

camilleryr · December 22, 2021, 2:23am

Really enjoyed todays puzzle - had to step away and work between parts 1 and 2 that gave me enough time to work out a good portion of the solution.

github.com

camilleryr/advent-2021/blob/main/elixir/lib/puzzles/day_21.ex

defmodule Day21 do
  defprotocol Dice do
    def roll(data)
  end

  defmodule DirecDie do
    defstruct value: nil

    defimpl Dice, for: DirecDie do
      def roll(%{value: value}) when value in 3..9 do
        {value, %DirecDie{}}
      end
    end
  end

  defmodule OneHundredSidedDie do
    defstruct rolls: 0

    defimpl Dice, for: OneHundredSidedDie do
      def roll(%{rolls: rolls} = one_hundred_sided_die) do

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