Day 13: Transparent Origami

Intro

https://adventofcode.com/2021/day/13

Input

defmodule Input do
  def prepare(i) do
    [dots, folds] = String.split(i, "\n\n", trim: true)

    dots =
      dots
      |> String.split("\n", trim: true)
      |> Enum.map(fn s ->
        [x, y] = String.split(s, ",")
        {String.to_integer(x), String.to_integer(y)}
      end)
      |> MapSet.new()

    folds =
      folds
      |> String.split("\n", trim: true)
      |> Enum.map(fn s -> String.trim_leading(s, "fold along ") end)
      |> Enum.map(fn s -> String.split(s, "=") end)
      |> Enum.map(fn [axis, line] -> %{"axis" => axis, "at" => String.to_integer(line)} end)

    {dots, folds}
  end
end

{:module, Input, <<70, 79, 82, 49, 0, 0, 10, ...>>, {:prepare, 1}}

{dots_test, folds_test} =
  """
  6,10
  0,14
  9,10
  0,3
  10,4
  4,11
  6,0
  6,12
  4,1
  0,13
  10,12
  3,4
  3,0
  8,4
  1,10
  2,14
  8,10
  9,0

  fold along y=7
  fold along x=5
  """
  |> Input.prepare()

{#MapSet<[
   {0, 3},
   {0, 13},
   {0, 14},
   {1, 10},
   {2, 14},
   {3, 0},
   {3, 4},
   {4, 1},
   {4, 11},
   {6, 0},
   {6, 10},
   {6, 12},
   {8, 4},
   {8, 10},
   {9, 0},
   {9, 10},
   {10, 4},
   {10, 12}
 ]>, [%{"at" => 7, "axis" => "y"}, %{"at" => 5, "axis" => "x"}]}

{dots, folds} =
  File.read!("input13.txt")
  |> Input.prepare()

{#MapSet<[
   {950, 833},
   {904, 861},
   {1078, 245},
   {169, 73},
   {264, 740},
   {1019, 880},
   {1260, 267},
   {805, 772},
   {907, 812},
   {711, 523},
   {691, 432},
   {341, 676},
   {947, 686},
   {447, 658},
   {131, 780},
   {1288, 313},
   {705, 724},
   {880, 472},
   {490, 305},
   {663, 12},
   {283, 591},
   {887, 143},
   {457, 515},
   {251, 707},
   {1223, 788},
   {239, 778},
   {264, 154},
   {1191, 322},
   {883, 894},
   {1121, 709},
   {1232, 413},
   {333, 194},
   {209, 312},
   {557, 887},
   {545, 219},
   {1193, 669},
   {1129, 84},
   {1235, 564},
   {104, 159},
   {895, 113},
   {33, 239},
   {442, 670},
   {525, 379},
   {693, 807},
   {1131, 870},
   {1275, 238},
   {1275, 443},
   {855, ...},
   {...},
   ...
 ]>,
 [
   %{"at" => 655, "axis" => "x"},
   %{"at" => 447, "axis" => "y"},
   %{"at" => 327, "axis" => "x"},
   %{"at" => 223, "axis" => "y"},
   %{"at" => 163, "axis" => "x"},
   %{"at" => 111, "axis" => "y"},
   %{"at" => 81, "axis" => "x"},
   %{"at" => 55, "axis" => "y"},
   %{"at" => 40, "axis" => "x"},
   %{"at" => 27, "axis" => "y"},
   %{"at" => 13, "axis" => "y"},
   %{"at" => 6, "axis" => "y"}
 ]}

Part 1

defmodule F do
  def fold(dots, %{"axis" => axis, "at" => line}) do
    dots
    |> Enum.reduce(MapSet.new(), fn {x, y}, ds ->
      if axis == "x" do
        # IO.puts("X")
        cond do
          x < line -> MapSet.put(ds, {x, y})
          x == line -> ds
          x > line -> MapSet.put(ds, {line * 2 - x, y})
        end
      else
        # IO.puts("Y")
        cond do
          y < line -> MapSet.put(ds, {x, y})
          y == line -> ds
          y > line -> MapSet.put(ds, {x, line * 2 - y})
        end
      end
    end)
  end
end

f0 = Enum.at(folds_test, 0)
f = folds_test
d = dots_test

Enum.reduce(f, d, fn fold, map -> F.fold(map, fold) end) |> MapSet.size()
# F.fold(d, f0) |> MapSet.size()

16

f0 = Enum.at(folds_test, 0)
f = folds_test
d = dots_test

# Enum.reduce(f, d, fn fold, map -> F.fold(map, fold) end) |> MapSet.size()
F.fold(d, f0) |> MapSet.size()

17

f0 = Enum.at(folds, 0)
d = dots

# Enum.reduce(f, d, fn fold, map -> F.fold(map, fold) end) |> MapSet.size()
F.fold(d, f0) |> MapSet.size()

770

Correct: 770

Part 2

defmodule F2 do
  def print(dots) do
    xmax = dots |> Enum.max_by(fn {x, _y} -> x end) |> then(fn {x, _y} -> x end)
    ymax = dots |> Enum.max_by(fn {_x, y} -> y end) |> then(fn {_x, y} -> y end)

    m =
      for y <- 0..ymax do
        r =
          for x <- 0..xmax do
            if {x, y} in dots do
              "#"
            else
              "."
            end
          end

        List.to_string(r)
      end

    Enum.map(m, fn r -> IO.puts(r) end)
    m
  end
end

d = dots
f = folds
Enum.reduce(f, d, fn fold, map -> F.fold(map, fold) end) |> F2.print()

####.###..#..#.####.#....###..###..###.
#....#..#.#..#.#....#....#..#.#..#.#..#
###..#..#.#..#.###..#....#..#.###..#..#
#....###..#..#.#....#....###..#..#.###.
#....#....#..#.#....#....#....#..#.#.#.
####.#.....##..####.####.#....###..#..#

["####.###..#..#.####.#....###..###..###.", "#....#..#.#..#.#....#....#..#.#..#.#..#",
 "###..#..#.#..#.###..#....#..#.###..#..#", "#....###..#..#.#....#....###..#..#.###.",
 "#....#....#..#.#....#....#....#..#.#.#.", "####.#.....##..####.####.#....###..#..#"]

Correct: EPUELPBR