Day 1 - Advent of Code 2025

Mix.install([:kino, :benchee])

Links

• Advent of Code (AoC)
• AoC Puzzle Input
• tylerj GitHub - Livebook

Prompt

— Day 1: Secret Entrance —

You arrive at the secret entrance to the North Pole base ready to start decorating. Unfortunately, the password seems to have been changed, so you can’t get in. A document taped to the wall helpfully explains:

“Due to new security protocols, the password is locked in the safe below. Please see the attached document for the new combination.”

The safe has a dial with only an arrow on it; around the dial are the numbers 0 through 99 in order. As you turn the dial, it makes a small click noise as it reaches each number.

The attached document (your puzzle input) contains a sequence of rotations, one per line, which tell you how to open the safe. A rotation starts with an L or R which indicates whether the rotation should be to the left (toward lower numbers) or to the right (toward higher numbers). Then, the rotation has a distance value which indicates how many clicks the dial should be rotated in that direction.

So, if the dial were pointing at 11, a rotation of R8 would cause the dial to point at 19. After that, a rotation of L19 would cause it to point at 0.

Because the dial is a circle, turning the dial left from 0 one click makes it point at 99. Similarly, turning the dial right from 99 one click makes it point at 0.

So, if the dial were pointing at 5, a rotation of L10 would cause it to point at 95. After that, a rotation of R5 could cause it to point at 0.

The dial starts by pointing at 50.

You could follow the instructions, but your recent required official North Pole secret entrance security training seminar taught you that the safe is actually a decoy. The actual password is the number of times the dial is left pointing at 0 after any rotation in the sequence.

For example, suppose the attached document contained the following rotations:

L68
L30
R48
L5
R60
L55
L1
L99
R14
L82

Following these rotations would cause the dial to move as follows:

• The dial starts by pointing at 50.
• The dial is rotated L68 to point at 82.
• The dial is rotated L30 to point at 52.
• The dial is rotated R48 to point at 0.
• The dial is rotated L5 to point at 95.
• The dial is rotated R60 to point at 55.
• The dial is rotated L55 to point at 0.
• The dial is rotated L1 to point at 99.
• The dial is rotated L99 to point at 0.
• The dial is rotated R14 to point at 14.
• The dial is rotated L82 to point at 32.

Because the dial points at 0 a total of three times during this process, the password in this example is 3.

Analyze the rotations in your attached document. What’s the actual password to open the door?

To begin, get your puzzle input.

— Part Two —

You’re sure that’s the right password, but the door won’t open. You knock, but nobody answers. You build a snowman while you think.

As you’re rolling the snowballs for your snowman, you find another security document that must have fallen into the snow:

“Due to newer security protocols, please use password method 0x434C49434B until further notice.”

You remember from the training seminar that “method 0x434C49434B” means you’re actually supposed to count the number of times any click causes the dial to point at 0, regardless of whether it happens during a rotation or at the end of one.

Following the same rotations as in the above example, the dial points at zero a few extra times during its rotations:

• The dial starts by pointing at 50.
• The dial is rotated L68 to point at 82; during this rotation, it points at 0 once.
• The dial is rotated L30 to point at 52.
• The dial is rotated R48 to point at 0.
• The dial is rotated L5 to point at 95.
• The dial is rotated R60 to point at 55; during this rotation, it points at 0 once.
• The dial is rotated L55 to point at 0.
• The dial is rotated L1 to point at 99.
• The dial is rotated L99 to point at 0.
• The dial is rotated R14 to point at 14.
• The dial is rotated L82 to point at 32; during this rotation, it points at 0 once.

In this example, the dial points at 0 three times at the end of a rotation, plus three more times during a rotation. So, in this example, the new password would be 6.

Be careful: if the dial were pointing at 50, a single rotation like R1000 would cause the dial to point at 0 ten times before returning back to 50!

Using password method 0x434C49434B, what is the password to open the door?

Although it hasn’t changed, you can still get your puzzle input.

Input

input = Kino.Input.textarea("Please paste your input file:")

input = input |> Kino.Input.read()

"R9\nL8\nL26\nR45\nR40\nL45\nR13\nL20\nL8\nR5\nR47\nR38\nL22\nL26\nL22\nL19\nL25\nL21\nL15\nR19\nR3\nR33\nR46\nL9\nR48\nL21\nR13\nR10\nL20\nR48\nL43\nR44\nR35\nR23\nL48\nL21\nL7\nR26\nL23\nL50\nL34\nR22\nL14\nR21\nL47\nR16\nR21\nL5\nL48\nR85\nR87\nL82\nR53\nL22\nL49\nL92\nL8\nR36\nR72\nR92\nR36\nL13\nR55\nL85\nL14\nL74\nL28\nL65\nL12\nR24\nR76\nR13\nL15\nL31\nL81\nL86\nL62\nR37\nL76\nR8\nR30\nL84\nL51\nR67\nR31\nR14\nR86\nL89\nR76\nL16\nR29\nR47\nR46\nR14\nR69\nR24\nR82\nR18\nR1\nR92\nL51\nR44\nL13\nR87\nR76\nR64\nL90\nR66\nL73\nL26\nR78\nR86\nL15\nR74\nL390\nL510\nR899\nL67\nR32\nL935\nL2\nL27\nL11\nL889\nL138\nL6\nL56\nR25\nL25\nL2\nR2\nL13\nL87\nR68\nL17\nR49\nR78\nR17\nL68\nL14\nL1\nL780\nR68\nL29\nL59\nR971\nL19\nR57\nL25\nR33\nL6\nL81\nR258\nL3\nL88\nL909\nR24\nR53\nL53\nL24\nR278\nR37\nR85\nL17\nR417\nL58\nR42\nR45\nR871\nR48\nR67\nL715\nR15\nL77\nR62\nR50\nR50\nR363\nL27\nL9\nR261\nL62\nR2\nR99\nR57\nL684\nR399\nL447\nL94\nR97\nR97\nR75\nR44\nR42\nL149\nL536\nL64\nL11\nR43\nL196\nL32\nL75\nL40\nL360\nR824\nR912\nL64\nR135\nR54\nL33\nR89\nL30\nL80\nR82\nR12\nL90\nL4\nL56\nR956\nL662\nL62\nL43\nL8\nR76\nL98\nL8\nR61\nR344\nL78\nL37\nL85\nR73\nL73\nR260\nR40\nR33\nL236\nL11\nR89\nR25\nL9\nL91\nL410\nR97\nR2\nR261\nR85\nL957\nL24\nR95\nL94\nR41\nR4\nR112\nR188\nL34\nR34\nR32\nL13\nL26\nL8\nR17\nR6\nL8\nL28\nL28\nR56\nR79\nR15\nR606\nR76\nR224\nL47\nL77\nR29\nL5\nL5\nR773\nR50\nR82\nR355\nL77\nL25\nL43\nR16\nL726\nR69\nR16\nL85\nR86\nL71\nL82\nR60\nL11\nL8\nR524\nL24\nR26\nR760\nL60\nL12\nL88\nL72\nR317\nR154\nR96\nL65\nR66\nL27\nR3\nL4\nR36\nL53\nL51\nL90\nR90\nR40\nR89\nL18\nR34\nR40\nR15\nR70\nL270\nL58\nL42\nL71\nL27\nL2\nR942\nL930\nR19\nR40\nR29\nR9\nR54\nL171\nR8\nL814\nL86\nL716\nL84\nL59\nL41\nL25\nR25\nL68\nR51\nR70\nR39\nL892\nR60\nR74\nR93\nR26\nR47\nR62\nL55\nR436\nR31\nR1\nR25\nR35\nR65\nL62\nL38\nR76\nL678\nL91\nR93\nL91\nL9\nL10\nL90\nL13\nR92\nL379\nR54\nR46\nL81\nL29\nR95\nL58\nR61\nR12\nL12\nR373\nL71\nL12\nR77\nL955\nR6\nR71\nL19\nL20\nR475\nL22\nR9\nR944\nR2\nL42\nL85\nR39\nR38\nL112\nR16\nR75\nL375\nR68\nR32\nL48\nR98\nR82\nL60\nR107\nR358\nL40\nL97\nR34\nR66\nL426\nR9\nL23\nL66\nL70\nR7\nR39\nR30\nL31\nR548\nL317\nR39\nR171\nL59\nR449\nR61\nL15\nL97\nL49\nR78\nL4\nR26\nR84\nR893\nL37\nR32\nR728\nL93\nL64\nR357\nL270\nL69\nR39\nR76\nL87\nR94\nR28\nL620\nR658\nL49\nL193\nR4\nL211\nL43\nR702\nL29\nL43\nR20\nL209\nR34\nL72\nR3\nR37\nL89\nL67\nR783\nL27\nL52\nR20\nR37\nR19\nR476\nR33\nR56\nL6\nL58\nL48\nL9\nR51\nR64\nR17\nL7\nL64\nL29\nR45\nL845\nL726\nL12\nL162\nL91\nR29\nR38\nL72\nR96\nR50\nR53\nL29\nL63\nL311\nR201\nR999\nR88\nR12\nL593\nL6\nR51\nR17\nR434\nL48\nR999\nR46\nR79\nL553\nL26\nL80\nR836\nR744\nL905\nL51\nL69\nL9\nL85\nR67\nR52\nL78\nL822\nL14\nL8\nL3\nL75\nL57\nL49\nR6\nL75\nR81\nR694\nL59\nR959\nR67\nL67\nR853\nR47\nR42\nL919\nL23\nR11\nL94\nL78\nR85\nL39\nL85\nR26\nL30\nR4\nL93\nR51\nL211\nR53\nR10\nR90\nR51\nR95\nR42\nL103\nL382\nL3\nL25\nL75\nL69\nL766\nR95\nL260\nL78\nL23\nR41\nR66\nL43\nR78\nL14\nR94\nL321\nR23\nR99\nR76\nR180\nR58\nL10\nL26\nL65\nL35\nR58\nR42\nR20\nR41\nR39\nR8\nL60\nR22\nL74\nL72\nR76\nL31\nR87\nR44\nR59\nR69\nL5\nL57\nL813\nR881\nR80\nL14\nL98\nL60\nR21\nR37\nL73\nR66\nR61\nL54\nL612\nL56\nR57\nL26\nR37\nR745\nR36\nL76\nL5\nR152\nL68\nL714\nL61\nL32\nR97\nR49\nL48\nL75\nR27\nR88\nR61\nR506\nL47\nL90\nL89\nR45\nL3\nL20\nR52\nR70\nR40\nR867\nL93\nR82\nL37\nL5\nR46\nR30\nR97\nR73\nL40\nR60\nL20\nL705\nL22\nL73\nL66\nR713\nL947\nL39\nR83\nL54\nR322\nR97\nR97\nR49\nR13\nL68\nL49\nL73\nR3\nL50\nL31\nR83\nR70\nL85\nL593\nR37\nR60\nR28\nL91\nR91\nR184\nR16\nR46\nR33\nL79\nL80\nL20\nR19\nL17\nL27\nR25\nR9\nL9\nR21\nR79\nL20\nL10\nR72\nL542\nL43\nR56\nR87\nL68\nL32\nR18\nR82\nL306\nL68\nL26\nL51\nR51\nR79\nL277\nR45\nR19\nR44\nR38\nL233\nL71\nL17\nL58\nR703\nR728\nR26\nR75\nL44\nL357\nL31\nR667\nR64\nR57\nL57\nL883\nL817\nR54\nR10\nL86\nL75\nL903\nL55\nR52\nL97\nR51\nR49\nR91\nR15\nL1\nR95\nR21\nL22\nL99\nR51\nR449\nL45\nR19\nR26\nL223\nL677\nL1\nL399\nL41\nL59\nR224\nL24\nR228\nR26\nL68\nR238\nR76\nR85\nL85\nR99\nR595\nR872\nR234\nR24\nL90\nL34\nR69\nR58\nL27\nL99\nL67\nR66\nL74\nL26\nL988\nL12\nR734\nL43\nL91\nR46\nR66\nL12\nL30\nR53\nL77\nR54\nR90\nR10\nL399\nL16\nL20\nR85\nL77\nR841\nR86\nR13\nL33\nR3\nR31\nL85\nR170\nL99\nR86\nR14\nL17\nL83\nL41\nR16\nL475\nR946\nR28\nR66\nR85\nL25\nL15\nL85\nR684\nR65\nL49\nR69\nR92\nL72\nR7\nR4\nR52\nL55\nR40\nL356\nL81\nR348\nL26\nR778\nL45\nL30\nR60\nR17\nR98\nR435\nR65\nL618\nR37\nR81\nR80\nR20\nR84\nL77\nL10\nL40\nL37\nL20\nL847\nL47\nL62\nR32\nL58\nR716\nR66\nR63\nR93\nR44\nR33\nR60\nL9\nR70\nL654\nL48\nL7\nL70\nL75\nR96\nR94\nR60\nL655\nL795\nR44\nL44\nL84\nR55\nR12\nR17\nL92\nL51\nR43\nR54\nR646\nL987\nR87\nR34\nR66\nL80\nL947\nR320\nL51\nR79\nL21\nR90\nL2\nL32\nR44\nR12\nR87\nL4\nR534\nR265\nL69\nL47\nL28\nL60\nR10\nR54\nR80\nL35\nR1\nR91\nR9\nL7\nR98\nL55\nL18\nR782\nR968\nL383\nL510\nL178\nL354\nL40\nR97\nL39\nL92\nR31\nL22\nL23\nL655\nR33\nL478\nR25\nL588\nL15\nL565\nL50\nR47\nL10\nL99\nL2\nL95\nL582\nL529\nR8\nR80\nL51\nR41\nL55\nL45\nR898\nR32\nR411\nL692\nR248\nR33\nR41\nR664\nR95\nR24\nL74\nL2" <> ...

Solution

defmodule Day01 do
  defdelegate parse(input), to: __MODULE__.Input

  def part1(input) do
    input
    |> parse()
    |> Enum.reduce([50], &turn_dial/2)
    |> Enum.count(&(&1 == 0))
  end

  def part2(input) do
    input
    |> parse()
    |> Enum.reduce({50, 0}, &turn_dial_part_2/2)
    |> elem(1)
  end

  defp turn_dial({"L", amount}, [h | _] = list) do
    [normalize_dial(h - rem(amount, 100)) | list]
  end

  defp turn_dial({"R", amount}, [h | _] = list) do
    [normalize_dial(h + rem(amount, 100)) | list]
  end

  defp normalize_dial(x) when x < 0, do: 100 + x
  defp normalize_dial(x) when x > 99, do: x - 100
  defp normalize_dial(x), do: x

  defp turn_dial_part_2({"L", amount}, acc) do
    1..amount
    |> Enum.reduce(acc, fn 
      _, {1, count} -> {0, count + 1}
      _, {0, count} -> {99, count}
      _, {dial, count} -> {dial - 1, count}
    end)
  end

  defp turn_dial_part_2({"R", amount}, acc) do
    1..amount
    |> Enum.reduce(acc, fn
      _, {99, count} -> {0, count + 1}
      _, {dial, count} -> {dial + 1, count}
    end)
  end

  defmodule Input do
    def parse(input) when is_binary(input) do
      input
      |> String.splitter("\n", trim: true)
      |> parse()
    end

    def parse(input) do
      Stream.map(input, &amp;parse_line/1)
    end

    def parse_line(line) do
      {dir, x} = String.split_at(line, 1)

      {dir, String.to_integer(x)}
    end
  end
end

{:module, Day01, <<70, 79, 82, 49, 0, 0, 16, ...>>,
 {:module, Day01.Input, <<70, 79, 82, ...>>, {:parse_line, 1}}}

Analyze the rotations in your attached document. What’s the actual password to open the door?

Your puzzle answer was 1034.

Day01.part1(input)

1034

Using password method 0x434C49434B, what is the password to open the door?

Your puzzle answer was 6166.

Day01.part2(input)

6166

Both parts of this puzzle are complete! They provide two gold stars: **

At this point, you should return to your Advent calendar and try another puzzle.

If you still want to see it, you can get your puzzle input.

Tests

ExUnit.start(auto_run: false)

defmodule Day01Test do
  use ExUnit.Case, async: false

  setup_all do
    [
      input: "L68\nL30\nR48\nL5\nR60\nL55\nL1\nL99\nR14\nL82"
    ]
  end

  describe "part1/1" do
    test "returns expected value", %{input: input} do
      assert Day01.part1(input) == 3
    end
  end

  describe "part2/1" do
    test "returns expected value", %{input: input} do
      assert Day01.part2(input) == 6
    end
  end
end

ExUnit.run()

Running ExUnit with seed: 647656, max_cases: 28

..
Finished in 0.00 seconds (0.00s async, 0.00s sync)
2 tests, 0 failures

%{total: 2, failures: 0, excluded: 0, skipped: 0}

Benchmarking

defmodule Benchmarking do
  # https://github.com/bencheeorg/benchee
  def run(input) do
    Benchee.run(
      %{
        "Part 1" => fn -> Day01.part1(input) end,
        "Part 2" => fn -> Day01.part2(input) end
      },
      memory_time: 2,
      reduction_time: 2
    )

    nil
  end
end

{:module, Benchmarking, <<70, 79, 82, 49, 0, 0, 8, ...>>, {:run, 1}}

Benchmarking.run(input)

Operating System: macOS
CPU Information: Apple M4 Pro
Number of Available Cores: 14
Available memory: 48 GB
Elixir 1.18.3
Erlang 27.3
JIT enabled: true

Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 2 s
reduction time: 2 s
parallel: 1
inputs: none specified
Estimated total run time: 22 s
Excluding outliers: false

Benchmarking Part 1 ...
Benchmarking Part 2 ...
Calculating statistics...
Formatting results...

Name             ips        average  deviation         median         99th %
Part 1        2.38 K        0.42 ms     ±9.77%        0.41 ms        0.51 ms
Part 2        0.49 K        2.02 ms    ±17.05%        1.96 ms        4.40 ms

Comparison: 
Part 1        2.38 K
Part 2        0.49 K - 4.82x slower +1.61 ms

Memory usage statistics:

Name      Memory usage
Part 1         2.14 MB
Part 2        16.44 MB - 7.68x memory usage +14.30 MB

**All measurements for memory usage were the same**

Reduction count statistics:

Name   Reduction count
Part 1         0.140 M
Part 2          1.68 M - 11.95x reduction count +1.54 M

**All measurements for reduction count were the same**

nil