Mixifying

.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

.gitignore

@@ -0,0 +1,24 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Temporary files, for example, from tests.
+/tmp/
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+advent_of_code-*.tar
+

README.md

@@ -0,0 +1,21 @@
+# AdventOfCode
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `advent_of_code` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:advent_of_code, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/advent_of_code>.
+

lib/advent_of_code.ex

@@ -0,0 +1,18 @@
+defmodule AdventOfCode do
+  @moduledoc """
+  Documentation for `AdventOfCode`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> AdventOfCode.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lib/year_2025/day1.exs

@@ -0,0 +1,8 @@
+filepath = Path.join([File.cwd!, "lib", "year_2025", "day1_input.txt"])
+ops = File.stream!(filepath)
+|>Stream.map(&String.trim_trailing/1)
+|>Enum.to_list
+
+{_, zeroes} = AdventOfCode.Year2025.Dial.rotate(ops)
+IO.puts(zeroes)
+

lib/year_2025/dial.ex

@@ -1,13 +1,13 @@
-defmodule Dial do
+defmodule AdventOfCode.Year2025.Dial do
   # Initial conditions
   def rotate(operations), do: rotate(50, operations, 0)
 
   # When no operations remain, return the number of zeroes
-  def rotate(_, [], zeroes), do: zeroes
+  def rotate(index, [], zeroes), do: { index, zeroes }
 
   # Compute the new dial position
   def rotate(index, [head | tail], zeroes) do
-    new_index = case head do
+    { new_index, _crosses } = case head do
       "R" <> number -> rotate_right(index, String.to_integer(number))
       "L" <> number -> rotate_left(index, String.to_integer(number))
     end
@@ -19,26 +19,20 @@ defmodule Dial do
   end
 
   defp rotate_right(index, number) do
+    crosses = div(number, 100)
     diff = index + rem(number, 100)
     case diff do
-      diff when diff <= 99 -> diff
+      diff when diff <= 99 -> { diff, crosses }
-      diff when diff > 99 -> diff - 100
+      diff when diff > 99 -> { diff - 100, crosses + 1 }
     end
   end
 
   defp rotate_left(index, number) do
+    crosses = div(number, 100)
     diff = index - rem(number, 100)
     case diff do
-      diff when diff >= 0 -> diff
+      diff when diff >= 0 -> { diff, crosses }
-      diff when diff < 0 -> diff + 100
+      diff when diff < 0 -> { diff + 100, crosses + 1 }
     end
   end
 end
-
-ops = File.stream!("./day1.txt")
-  |> Stream.map(&String.trim_trailing/1)
-  |> Enum.to_list()
-
-dial = Dial.rotate(ops)
-
-IO.puts dial

mix.exs

@@ -0,0 +1,28 @@
+defmodule AdventOfCode.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :advent_of_code,
+      version: "0.1.0",
+      elixir: "~> 1.19",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()

test/year_2025/dial_test.exs

@@ -0,0 +1,25 @@
+defmodule AdventOfCode.Year2025.DialTest do
+  use ExUnit.Case, async: true
+
+  import AdventOfCode.Year2025.Dial
+
+  test "dial starts at 50" do
+    { index, _ } = rotate([])
+    assert index == 50
+  end
+
+  test "rotate right increments index" do
+    {index, _} = rotate(["R1"])
+    assert index == 51
+  end
+
+  test "rotate left decrements index" do
+    {index, _} = rotate(["L1"])
+    assert index ==49 
+  end
+
+  test "increments zero crossings when landing on zero" do
+    {_, zeroes} = rotate(["R50"])
+    assert zeroes == 1
+  end
+end