bjorng

Erlang Core Team

Advent of Code 2021 - Day 9

This topic is about Day 9 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

13 comments

#advent-of-code

7 2193 13

2021-12-19 06:57:34 UTC

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bjorng

Erlang Core Team

I solved part two with a simple union-find algorithm. Since it finished in less than two seconds on my computer, I didn’t need to do any further optimizations.

github.com

bjorng/advent-of-code-2021/blob/49f0d8eff0110e6e5d823bf314236c6bf7dcc991/day09/lib/day09.ex

defmodule Day09 do
  def part1(input) do
    map = parse(input)
    Enum.reduce(map, 0, fn {coord, h}, acc ->
      nh = neighbors(map, coord)
      |> Enum.map(&(Map.fetch!(map, &1)))
      |> Enum.min
      if h < nh do
        acc + 1 + h
      else
        acc
      end
    end)
  end

  # Find the largest basin using a simple union-find algorithm.
  # The run time on my computer is less than 2 seconds.
  def part2(input) do
    map = parse(input)
    |> Enum.reject(fn {_key, value} -> value === 9 end)

This file has been truncated. show original

Post #2

deadbeef

For part two I did a sort-of breadth-first search (I think?). Feels a bit messy, but got the job done.
Also, felt my file’s a bit cruft-y with how many small functions I made, but was trying to make things reusable between parts.

github.com

ed-flanagan/advent-of-code-solutions-elixir/blob/main/lib/advent/y2021/d09.ex

defmodule Advent.Y2021.D09 do
  @moduledoc """
  https://adventofcode.com/2021/day/9
  """

  @typep point :: {non_neg_integer(), non_neg_integer()}
  @typep grid :: %{point() => non_neg_integer()}

  @doc """
  Find all of the low points on your heightmap. What is the sum of the risk
  levels of all low points on your heightmap?
  """
  @spec part_one(Enumerable.t()) :: non_neg_integer()
  def part_one(input) do
    grid = parse_input(input)

    grid
    |> find_low_points()
    |> Enum.map(&risk_level(grid, &1))
    |> Enum.sum()

This file has been truncated. show original

Post #3

wasi0013

Here’s my solution: y2021/day_09.ex
For part 2 used bfs

Post #4

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View thread on forum (has 13 responses!)

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#advent-of-code

7 2194 13

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Advent of Code 2021 - Day 9

bjorng

Advent of Code 2021 - Day 9

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bjorng

deadbeef

wasi0013

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