I’m a few days late.
Day 10 was not too hard, and surprisingly, I had instant part2 resolution, after struggling a bit on part1’s recursion
Part 1
defmodule Advent.Y2024.Day10.Part1 do
import Enum
def run(puzzle) do
map = parse(puzzle)
map |> find_zeros() |> map(&hike(map, &1)) |> map(&elem(&1, 0)) |> sum()
end
def parse(puzzle) do
for {row, y} <- puzzle |> String.split("\n") |> with_index(),
{height, x} <- row |> String.graphemes() |> with_index(),
into: %{},
do: {{x, y}, String.to_integer(height)}
end
def find_zeros(map), do: for({{x, y}, h} <- map, h == 0, do: {x, y})
defp hike(map, pos, height \\ 0, visited \\ MapSet.new())
defp hike(_map, pos, 9, visited), do: {1, MapSet.put(visited, pos)}
defp hike(map, pos, height, visited) do
visited = MapSet.put(visited, pos)
case neighbours(map, pos, height, visited) do
[] ->
{0, visited}
neighbours ->
for {n, h} <- neighbours, reduce: {0, visited} do
{total, visited} ->
{score, visited} = hike(map, n, h, visited)
{total + score, visited}
end
end
end
defp neighbours(map, {x, y}, height, visited) do
for {dx, dy} <- [{-1, 0}, {1, 0}, {0, -1}, {0, 1}],
{nx, ny} = {x + dx, y + dy},
not MapSet.member?(visited, {nx, ny}),
new_height = Map.get(map, {nx, ny}, -1),
new_height == height + 1,
do: {{nx, ny}, new_height}
end
end
Part 2
defmodule Advent.Y2024.Day10.Part2 do
alias Advent.Y2024.Day10.Part1
def run(puzzle) do
map = Part1.parse(puzzle)
map |> Part1.find_zeros() |> Enum.map(&hike(map, &1, 0)) |> Enum.sum()
end
defp hike(_map, _pos, 9), do: 1
defp hike(map, pos, height) do
case neighbours(map, pos, height) do
[] -> 0
ns -> ns |> Enum.map(fn {pos, h} -> hike(map, pos, h) end) |> Enum.sum()
end
end
defp neighbours(map, {x, y}, height) do
for {dx, dy} <- [{-1, 0}, {1, 0}, {0, -1}, {0, 1}],
{nx, ny} = {x + dx, y + dy},
new_height = Map.get(map, {nx, ny}, -1),
new_height == height + 1,
do: {{nx, ny}, new_height}
end
end
Part 2 is basically a simplified version of part 1, in which I don’t track already visited nodes.
