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-<!-- vim: set syntax=markdown: -->
-<!-- livebook:{"persist_outputs":true} -->
-
-# Advent of Code 2021, Day 9
-
-## Short problem summary
-
-
-
-**Part 1.** Find the total "risk level" of all the local minima on a relief map, represented by a 
-100 $\times$ 100 array of integers from 1 to 9.  Only orthogonal neighbors count.
-The risk level is the elevation plus one.
-
-**Part 2.** Find the product of the areas of the three largest basins on the relief map.  Basins are regions
-bordered by the edges of the map or by points with elevation 9.
-Again, only orthogonal neighbors count.
-
-## Setup
-
-I'm using [Nx](https://github.com/elixir-nx/nx/tree/main/nx#readme) tensors
-since this problem will require a lot of arbitrary indexing.
-
-```elixir
-Mix.install([
-  {:nx, "~> 0.1.0-dev", github: "elixir-nx/nx", sparse: "nx", override: true}
-])
-```
-
-```output
-:ok
-```
-
-Bringing in the data as a new 2D tensor:
-
-```elixir
-floor =
-  with {:ok, data} <- File.read("input.txt") do
-    data
-    |> String.trim()
-    |> String.split("\n")
-    |> Enum.flat_map(&String.graphemes/1)
-    |> Enum.map(&String.to_integer/1)
-    |> Enum.chunk_every(100)
-    |> Nx.tensor(names: [:y, :x])
-  end
-```
-
-```output
-#Nx.Tensor<
-  s64[y: 100][x: 100]
-  [
-    [7, 6, 5, 9, 9, 9, 1, 0, 9, 8, 9, 9, 9, 8, 7, 6, 5, 7, 9, 9, 1, 0, 1, 2, 9, 8, 7, 9, 9, 9, 9, 8, 7, 6, 4, 3, 2, 1, 2, 3, 4, 5, 9, 8, 7, 4, 3, 4, 5, 5, ...],
-    ...
-  ]
->
-```
-
-## Part 1
-
-For a given coordinate $(x, y)$, we want to examine its orthogonal neighbors, 
-but only the ones that exist within the map.
-
-```elixir
-defmodule Day09.Part1 do
-  def neighbors({x, y}) do
-    [{x + 1, y}, {x - 1, y}, {x, y + 1}, {x, y - 1}]
-    |> Enum.filter(fn {x, y} -> x >= 0 && x < 100 && y >= 0 && y < 100 end)
-  end
-end
-```
-
-```output
-{:module, Day09.Part1, <<70, 79, 82, 49, 0, 0, 7, ...>>, {:neighbors, 1}}
-```
-
-Now scan the whole array to check each cell's neighbors, 
-and return the "risk level" for each local minimum, then accumulate the sum.
-
-```elixir
-risk_level =
-  for x <- 0..99,
-      y <- 0..99,
-      reduce: 0 do
-    acc ->
-      Day09.Part1.neighbors({x, y})
-      |> Enum.all?(fn {xn, yn} -> floor[x][y] < floor[xn][yn] end)
-      |> if(do: acc + Nx.to_number(floor[x][y]) + 1, else: acc)
-  end
-```
-
-```output
-591
-```
-
-## Part 2
-
-Now we need to recursively walk outwards from each previously-unwalked point
-until we can't walk any further.  An agent will keep track of the visited points.
-
-```elixir
-defmodule Day09.Part2 do
-  def walkable?({x, y} = coords, tensor, pid) do
-    Nx.to_number(tensor[x][y]) < 9 && not Agent.get(pid, fn m -> Map.has_key?(m, coords) end)
-  end
-
-  def walk_it(coords, tensor, pid) do
-    if walkable?(coords, tensor, pid) do
-      Agent.update(pid, fn m -> Map.put(m, coords, true) end)
-
-      for c <- Day09.Part1.neighbors(coords) do
-        walk_it(c, tensor, pid)
-      end
-      |> Enum.reduce(1, fn x, acc -> acc + x end)
-    else
-      0
-    end
-  end
-end
-```
-
-```output
-{:module, Day09.Part2, <<70, 79, 82, 49, 0, 0, 11, ...>>, {:walk_it, 3}}
-```
-
-```elixir
-{:ok, tracker} = Agent.start_link(fn -> %{} end)
-
-for x <- 0..99, y <- 0..99 do
-  Day09.Part2.walk_it({x, y}, floor, tracker)
-end
-|> Enum.sort(:desc)
-|> Enum.take(3)
-|> Enum.reduce(fn x, acc -> acc * x end)
-```
-
-```output
-1113424
-```