<!-- livebook:{"persist_outputs":true} -->
# --- Day 18: Boiling Boulders ---
```elixir
Mix.install([])
```
<!-- livebook:{"output":true} -->
```
:ok
```
## Part 1
You and the elephants finally reach fresh air. You've emerged near the base of a large volcano that seems to be actively erupting! Fortunately, the lava seems to be flowing away from you and toward the ocean.
Bits of lava are still being ejected toward you, so you're sheltering in the cavern exit a little longer. Outside the cave, you can see the lava landing in a pond and hear it loudly hissing as it solidifies.
Depending on the specific compounds in the lava and speed at which it cools, it might be forming obsidian! The cooling rate should be based on the surface area of the lava droplets, so you take a quick scan of a droplet as it flies past you (your puzzle input).
Because of how quickly the lava is moving, the scan isn't very good; its resolution is quite low and, as a result, it approximates the shape of the lava droplet with 1x1x1 cubes on a 3D grid, each given as its x,y,z position.
To approximate the surface area, count the number of sides of each cube that are not immediately connected to another cube. So, if your scan were only two adjacent cubes like 1,1,1 and 2,1,1, each cube would have a single side covered and five sides exposed, a total surface area of 10 sides.
Here's a larger example:
```elixir
example_input = """
2,2,2
1,2,2
3,2,2
2,1,2
2,3,2
2,2,1
2,2,3
2,2,4
2,2,6
1,2,5
3,2,5
2,1,5
2,3,5
"""
```
<!-- livebook:{"output":true} -->
```
"2,2,2\n1,2,2\n3,2,2\n2,1,2\n2,3,2\n2,2,1\n2,2,3\n2,2,4\n2,2,6\n1,2,5\n3,2,5\n2,1,5\n2,3,5\n"
```
In the above example, after counting up all the sides that aren't connected to another cube, the total surface area is 64.
What is the surface area of your scanned lava droplet?
## Part 2
Something seems off about your calculation. The cooling rate depends on exterior surface area, but your calculation also included the surface area of air pockets trapped in the lava droplet.
Instead, consider only cube sides that could be reached by the water and steam as the lava droplet tumbles into the pond. The steam will expand to reach as much as possible, completely displacing any air on the outside of the lava droplet but never expanding diagonally.
In the larger example above, exactly one cube of air is trapped within the lava droplet (at 2,2,5), so the exterior surface area of the lava droplet is 58.
What is the exterior surface area of your scanned lava droplet?
## Code
```elixir
defmodule Face do
defstruct [:plane, :coord, :vertices, :adjacent_cube, :exposed]
end
defmodule Cube do
defstruct [:faces, :type]
end
defmodule Day18 do
defp parse_input(input) do
for l <- String.split(input, "\n", trim: true), t = String.split(l, ",") do
Enum.map(t, fn i -> String.to_integer(i) end)
end
end
defp cubes_to_faces(cubes, type \\ :solid) do
for c <- cubes, reduce: %{} do
acc ->
f = faces(c, type)
Map.put(acc, c, f)
end
end
defp faces([x, y, z], type) do
%Cube{
type: type,
faces: [
%Face{
plane: :x,
coord: x,
vertices: Enum.sort([[y, z], [y + 1, z], [y, z + 1], [y + 1, z + 1]]),
adjacent_cube: nil,
exposed: true
},
%Face{
plane: :x,
coord: x + 1,
vertices: Enum.sort([[y, z], [y + 1, z], [y, z + 1], [y + 1, z + 1]]),
adjacent_cube: nil,
exposed: true
},
%Face{
plane: :y,
coord: y,
vertices: Enum.sort([[x, z], [x + 1, z], [x, z + 1], [x + 1, z + 1]]),
adjacent_cube: nil,
exposed: true
},
%Face{
plane: :y,
coord: y + 1,
vertices: Enum.sort([[x, z], [x + 1, z], [x, z + 1], [x + 1, z + 1]]),
adjacent_cube: nil,
exposed: true
},
%Face{
plane: :z,
coord: z,
vertices: Enum.sort([[x, y], [x + 1, y], [x, y + 1], [x + 1, y + 1]]),
adjacent_cube: nil,
exposed: true
},
%Face{
plane: :z,
coord: z + 1,
vertices: Enum.sort([[x, y], [x + 1, y], [x, y + 1], [x + 1, y + 1]]),
adjacent_cube: nil,
exposed: true
}
]
}
end
defp find_box(cubes) do
[first | rest] = Map.keys(cubes)
for coord <- rest, reduce: {first, first} do
acc ->
{curr_min, curr_max} = acc
v_min = List.zip([coord, curr_min])
v_max = List.zip([coord, curr_max])
n_min =
for {a, b} <- v_min do
min(a, b)
end
n_max =
for {a, b} <- v_max do
max(a, b)
end
{n_min, n_max}
end
end
defp mark_adjacent(cubes) do
for {v, cube} <- cubes, reduce: cubes do
acc ->
for f <- cube.faces, reduce: acc do
acc ->
w = cube_from_adj_face(v, f)
cond do
is_map_key(acc, w) -> touching_cubes(acc, v, w)
true -> acc
end
end
end
end
defp cube_from_adj_face(cube, face) do
x1 = face.coord
[x2, x3] = List.first(face.vertices)
index = %{x: 0, y: 1, z: 2}
cond do
Enum.at(cube, index[face.plane]) == x1 -> face_plane(face.plane, x1 - 1, x2, x3)
true -> face_plane(face.plane, x1, x2, x3)
end
end
defp face_plane(plane, x1, x2, x3) do
cond do
plane == :x -> [x1, x2, x3]
plane == :y -> [x2, x1, x3]
plane == :z -> [x2, x3, x1]
end
end
defp touching_cubes(cubes, v, w) do
for {{xv, xw}, ind} <- List.zip([v, w]) |> Enum.with_index(), xv != xw, reduce: cubes do
acc ->
cond do
xv > xw -> share_face(acc, v, w, ind)
xv < xw -> share_face(acc, w, v, ind)
end
end
end
defp share_face(cubes, v, w, index) do
axis = %{0 => :x, 1 => :y, 2 => :z}
v_faces = update_faces(cubes[v].faces, axis[index], Enum.at(v, index), w, cubes[w].type)
w_faces = update_faces(cubes[w].faces, axis[index], Enum.at(w, index) + 1, v, cubes[v].type)
Map.update!(cubes, v, fn c -> %Cube{c | faces: v_faces} end)
|> Map.update!(w, fn c -> %Cube{c | faces: w_faces} end)
end
defp update_faces(faces, plane, coord, touching_cube, touching_type) do
for f <- faces do
if f.plane == plane and f.coord == coord do
cond do
touching_type == :solid or touching_type == :inside_air ->
%Face{f | adjacent_cube: touching_cube, exposed: false}
touching_type == :air ->
%Face{f | adjacent_cube: touching_cube, exposed: true}
end
else
f
end
end
end
defp create_air_cubes(cubes) do
{[x_min, y_min, z_min], [x_max, y_max, z_max]} = find_box(cubes)
for x <- (x_min - 1)..(x_max + 1),
y <- (y_min - 1)..(y_max + 1),
z <- (z_min - 1)..(z_max + 1),
reduce: cubes do
acc ->
w = [x, y, z]
cond do
not is_map_key(acc, w) -> Map.put(acc, w, faces(w, :air))
true -> acc
end
end
end
defp flood_fill(cubes, c, set) do
cond do
MapSet.member?(set, c) -> set
true -> flood_fill_int(cubes, c, set)
end
end
defp flood_fill_int(cubes, c, set) do
s = MapSet.put(set, c)
for f <- cubes[c].faces, reduce: s do
acc ->
adj = f.adjacent_cube
cond do
is_nil(adj) -> MapSet.put(acc, [:inf, :inf, :inf])
cubes[adj].type == :air -> MapSet.union(acc, flood_fill(cubes, adj, acc))
true -> acc
end
end
end
defp count_exposed_faces(cubes) do
for {_c, cube} <- cubes,
f <- cube.faces,
cube.type == :solid and f.exposed,
reduce: 0 do
acc -> acc + 1
end
end
defp update_air_pockets(cubes) do
for {c, cube} <- cubes, cube.type == :air, reduce: {MapSet.new(), MapSet.new()} do
acc ->
{outside_set, inside_set} = acc
if MapSet.member?(outside_set, c) or MapSet.member?(inside_set, c) do
acc
else
s = flood_fill(cubes, c, MapSet.new())
cond do
MapSet.member?(s, [:inf, :inf, :inf]) -> {MapSet.union(outside_set, s), inside_set}
true -> {outside_set, MapSet.union(inside_set, s)}
end
end
end
end
defp mark_inside(cubes, set) do
for c <- set, f <- cubes[c].faces, adj = f.adjacent_cube, reduce: cubes do
acc ->
upd = Map.update!(acc, c, fn x -> %Cube{x | type: :inside_air} end)
if upd[adj].type == :solid do
touching_cubes(upd, adj, c)
else
acc
end
end
end
defp create_solid_cubes(input) do
input
|> parse_input()
|> cubes_to_faces()
end
def part1(input) do
input
|> create_solid_cubes()
|> mark_adjacent()
|> count_exposed_faces()
end
def part2(input) do
cubes =
input
|> create_solid_cubes()
|> create_air_cubes()
|> mark_adjacent()
{_outside_set, inside_set} = update_air_pockets(cubes)
mark_inside(cubes, inside_set)
|> count_exposed_faces()
end
end
```
<!-- livebook:{"output":true} -->
```
{:module, Day18, <<70, 79, 82, 49, 0, 0, 59, ...>>, {:part2, 1}}
```
Checking that the example input produces the right result:
```elixir
Day18.part1(example_input) == 64
```
<!-- livebook:{"output":true} -->
```
true
```
```elixir
input = File.read!("Day18/input.txt")
Day18.part1(input)
```
<!-- livebook:{"output":true} -->
```
3526
```
```elixir
Day18.part2(example_input)
```
<!-- livebook:{"output":true} -->
```
58
```
```elixir
Day18.part2(input)
```
<!-- livebook:{"output":true} -->
```
2090
```
