Advent of Code 2023 - Day 9

Greetings, fantastic folks! Ukrainian Erlanger reporting in :metal:! As we transition into the weekend, today marks the advent of Day 9 in our thrilling journey through the Advent of Code. A fresh puzzle awaits, ready to challenge and inspire! Don’t forget to share your ingenious code solutions in the comments below. Embrace the spirit of discovery and coding joy. Happy Saturday, and happy coding, everyone! :blush:

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My day 9 solution:

defmodule Aoc.Nine do
  def parse(string) do
    string
    |> String.split("\n", trim: true)
    |> Enum.map(fn line ->
      line |> String.split(" ", trim: true) |> Enum.map(&String.to_integer/1)
    end)
  end

  def part_one(input) do
    input
    |> Enum.map(&reduce/1)
    |> Enum.unzip()
    |> elem(0)
    |> Enum.sum()
  end

  def part_two(input) do
    input
    |> Enum.map(&reduce/1)
    |> Enum.unzip()
    |> elem(1)
    |> Enum.sum()
  end

  def reduce([first|_] = list) do
    reduce(list, [], first)
  end

  def reduce([a, b | rest], acc, c) do
    reduce([b | rest], [b - a | acc], c)
  end

  def reduce([a], [b | _] = acc, c) do
    if Enum.all?(acc, &(&1 == a)) do
      {a + b, c - b}
    else
      {new_b, new_c} = reduce(Enum.reverse(acc))
      {a + new_b, c - new_c}
    end
  end
end

input = File.read!("priv/9.txt") |> Aoc.Nine.parse()

input |> Aoc.Nine.part_one() |> IO.inspect(label: "part 1")
input |> Aoc.Nine.part_two() |> IO.inspect(label: "part 2")

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My Day 9 solution: https://github.com/wfvining/aoc2023/blob/main/day9/day9.erl

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Day 9

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