2024 day 9, part 1

README.md

@@ -5,8 +5,8 @@
 My Elixir solutions for [Advent of Code](https://adventofcode.com/) (all years).
 
 <!-- stars start -->
-<p><img src="https://img.shields.io/static/v1?label=Total&message=435%20stars&style=for-the-badge&color=green" alt="435 stars" /></p>
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+<p><a href="./lib/y2024/"><img src="https://img.shields.io/static/v1?label=2024&message=17%20stars&style=for-the-badge&color=orange" alt="17 stars" /></a><br />
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lib/y2024/README.md

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 My Elixir solutions for [Advent of Code 2024](https://adventofcode.com/2024).
 
-<!-- stars 2024 start --><img src="https://img.shields.io/static/v1?label=2024&message=16%20stars&style=for-the-badge&color=orange" alt="16 stars" /><!-- stars 2024 end -->
+<!-- stars 2024 start --><img src="https://img.shields.io/static/v1?label=2024&message=17%20stars&style=for-the-badge&color=orange" alt="17 stars" /><!-- stars 2024 end -->
 
 ## Benchmarks
 
@@ -30,4 +30,5 @@ day 07, part 1        103.05        9.70 ms     ±2.34%        9.69 ms       10.
 day 07, part 2         14.76       67.73 ms     ±1.35%       67.51 ms       71.95 ms
 day 08, part 1        1.07 K        0.93 ms     ±6.91%        0.90 ms        1.07 ms
 day 08, part 2        0.83 K        1.20 ms     ±7.78%        1.17 ms        1.38 ms
+day 09, part 1          9.27      107.87 ms     ±0.84%      107.61 ms      111.83 ms
 ```

lib/y2024/day09.ex

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+defmodule Y2024.Day09 do
+  use Advent.Day, no: 09
+
+  @doc """
+  iex> Day09.part1(Integer.digits(2333133121414131402))
+  1928
+  """
+  def part1(input) do
+    # This is how long the final list should be - the sum of all of the lengths of non-gaps
+    finished_length = input |> Enum.take_every(2) |> Enum.sum()
+
+    # build up the 00..111.222.. string
+    list =
+      input
+      |> Enum.reduce({0, true, []}, fn num, {file_id, fill?, list} ->
+        fill_value = if fill?, do: file_id, else: nil
+        next_file_id = if fill?, do: file_id + 1, else: file_id
+        list = if num == 0, do: list, else: [for(_i <- 1..num, do: fill_value) | list]
+        {next_file_id, !fill?, list}
+      end)
+      |> elem(2)
+      |> Enum.reverse()
+      |> List.flatten()
+
+    # Shorten that list and reverse it to use while backfilling
+    to_use = Enum.reject(list, &(!&1)) |> Enum.reverse()
+
+    Enum.reduce_while(list, {0, [], to_use}, fn num, {length, built, list} ->
+      if length == finished_length do
+        {:halt, Enum.reverse(built)}
+      else
+        if num do
+          {:cont, {length + 1, [num | built], list}}
+        else
+          {:cont, {length + 1, [hd(list) | built], tl(list)}}
+        end
+      end
+    end)
+    |> Enum.reduce({0, 0}, fn num, {index, acc} ->
+      {index + 1, acc + index * num}
+    end)
+    |> elem(1)
+  end
+
+  # @doc """
+  # iex> Day09.part2("update or delete me")
+  # "update or delete me"
+  # """
+  # def part2(input) do
+  #   input
+  # end
+
+  def parse_input(input) do
+    input
+    |> String.trim()
+    |> String.to_integer()
+    |> Integer.digits()
+  end
+
+  def part1_verify, do: input() |> parse_input() |> part1()
+  # def part2_verify, do: input() |> parse_input() |> part2()
+end

test/y2024/day09_test.exs

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+defmodule Y2024.Day09Test do
+  use ExUnit.Case, async: true
+  alias Y2024.Day09
+  doctest Day09
+
+  test "verification, part 1", do: assert(Day09.part1_verify() == 6_241_633_730_082)
+  # test "verification, part 2", do: assert(Day09.part2_verify() == "update or delete me")
+end