diff --git a/2021/.formatter.exs b/2021/.formatter.exs
new file mode 100644
index 0000000..fea6a6c
--- /dev/null
+++ b/2021/.formatter.exs
@@ -0,0 +1,5 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
+  locals_without_parens: [assert_solution: 2]
+]
diff --git a/2021/.gitignore b/2021/.gitignore
new file mode 100644
index 0000000..7eeb530
--- /dev/null
+++ b/2021/.gitignore
@@ -0,0 +1,27 @@
+# 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/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# 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
+
+
+# Temporary files for e.g. tests
+/tmp
diff --git a/2021/.tool-versions b/2021/.tool-versions
new file mode 100644
index 0000000..eda65d9
--- /dev/null
+++ b/2021/.tool-versions
@@ -0,0 +1,2 @@
+erlang 24.1.4
+elixir 1.12.3-otp-24
diff --git a/2021/README.md b/2021/README.md
new file mode 100644
index 0000000..f246dcc
--- /dev/null
+++ b/2021/README.md
@@ -0,0 +1,22 @@
+# Advent of Code 2021
+
+<details>
+  <summary>Setup</summary>
+
+  Using [asdf]:
+
+  ```sh
+  asdf plugin add erlang
+  asdf plugin add elixir
+  asdf install
+  ```
+</details>
+
+|  S  |  M  |  T  |  W  |  T  |  F  |  S  |
+| :-: | :-: | :-: | :-: | :-: | :-: | :-: |
+|     |     |     |  1  |  2  |  3  |  4  |
+|  5  |  6  |  7  |  8  |  9  | 10  | 11  |
+| 12  | 13  | 14  | 15  | 16  | 17  | 18  |
+| 19  | 20  | 21  | 22  | 23  | 24  | 25  |
+
+[asdf]: https://asdf-vm.com/#/
diff --git a/2021/lib/advent_of_code.ex b/2021/lib/advent_of_code.ex
new file mode 100644
index 0000000..f8016fb
--- /dev/null
+++ b/2021/lib/advent_of_code.ex
@@ -0,0 +1,12 @@
+defmodule AdventOfCode do
+  @moduledoc """
+  Solutions to the 2021 Advent of Code puzzles
+  """
+
+  def solver(selector) do
+    [day, part] = String.split(selector, ".")
+    day_module = Macro.camelize("Day#{day}")
+    part_module = Macro.camelize("Part#{part}")
+    Module.concat([AdventOfCode, day_module, part_module])
+  end
+end
diff --git a/2021/lib/advent_of_code/day0.ex b/2021/lib/advent_of_code/day0.ex
new file mode 100644
index 0000000..e69de29
diff --git a/2021/lib/advent_of_code/day0/part0.ex b/2021/lib/advent_of_code/day0/part0.ex
new file mode 100644
index 0000000..6c32387
--- /dev/null
+++ b/2021/lib/advent_of_code/day0/part0.ex
@@ -0,0 +1,11 @@
+defmodule AdventOfCode.Day0.Part0 do
+  alias AdventOfCode.PuzzleSolver
+
+  use PuzzleSolver
+
+  @impl PuzzleSolver
+  def solve(stream) do
+    Stream.run(stream)
+    "42"
+  end
+end
diff --git a/2021/lib/advent_of_code/puzzle_solver.ex b/2021/lib/advent_of_code/puzzle_solver.ex
new file mode 100644
index 0000000..212df08
--- /dev/null
+++ b/2021/lib/advent_of_code/puzzle_solver.ex
@@ -0,0 +1,18 @@
+defmodule AdventOfCode.PuzzleSolver do
+  @moduledoc """
+  Behaviour for a puzzle solution.
+  """
+
+  @doc """
+  Given the input as a stream, return the solution as a string
+  """
+  @callback solve(Enumerable.t()) :: String.t()
+
+  def solve(mod, stream), do: apply(mod, :solve, [stream])
+
+  defmacro __using__(_) do
+    quote do
+      @behaviour AdventOfCode.PuzzleSolver
+    end
+  end
+end
diff --git a/2021/lib/mix/advent_of_code.ex b/2021/lib/mix/advent_of_code.ex
new file mode 100644
index 0000000..a012629
--- /dev/null
+++ b/2021/lib/mix/advent_of_code.ex
@@ -0,0 +1,13 @@
+defmodule Mix.AdventOfCode do
+  @moduledoc """
+  Helpers for `AdventOfCode` mix tasks.
+  """
+
+  def day_module(day) do
+    Module.concat(AdventOfCode, Macro.camelize("Day#{day}"))
+  end
+
+  def part_module(day, part) do
+    Module.concat(day_module(day), Macro.camelize("Part#{part}"))
+  end
+end
diff --git a/2021/lib/mix/tasks/advent_of_code.gen.solution.ex b/2021/lib/mix/tasks/advent_of_code.gen.solution.ex
new file mode 100644
index 0000000..f1d1165
--- /dev/null
+++ b/2021/lib/mix/tasks/advent_of_code.gen.solution.ex
@@ -0,0 +1,94 @@
+defmodule Mix.Tasks.AdventOfCode.Gen.Solution do
+  use Mix.Task
+  import Mix.Generator
+  import Mix.AdventOfCode
+
+  @shortdoc "Generate a new solution module"
+
+  @moduledoc """
+  #{@shortdoc}.
+
+  Includes new solution module, test, and empty problem input.
+
+  ## Examples
+
+      # Generate solution modules, tests, and empty input for Day 2
+      $ mix advent_of_code.gen.solution 2
+  """
+
+  @switches []
+
+  @impl Mix.Task
+  def run(args) do
+    case OptionParser.parse!(args, switches: @switches) do
+      {_, [day]} ->
+        day_module = day_module(day)
+        day_contents = day_template(mod: day_module) |> Code.format_string!()
+        day_path = Path.join("lib", Macro.underscore(day_module))
+        day_tests_path = Path.join("test", Macro.underscore(day_module))
+        day_file = "#{day_path}.ex"
+
+        create_directory(day_path)
+        create_directory(day_tests_path)
+        create_file(day_file, day_contents)
+
+        for part <- 1..2 do
+          part_module = part_module(day, part)
+
+          part_contents =
+            part_template(mod: part_module, day_mod: day_module) |> Code.format_string!()
+
+          part_file = Path.join("lib", "#{Macro.underscore(part_module)}.ex")
+          create_file(part_file, part_contents)
+
+          part_test_module = Module.concat(day_module, Macro.camelize("Part#{part}Test"))
+
+          part_test_contents =
+            part_test_template(mod: part_module, test_mod: part_test_module)
+            |> Code.format_string!()
+
+          part_test_file =
+            Path.join(day_tests_path, "#{Macro.underscore("part_#{part}_test")}.exs")
+
+          create_file(part_test_file, part_test_contents)
+        end
+
+      _ ->
+        Mix.raise("Unknown arguments.")
+    end
+  end
+
+  embed_template(:day, ~S"""
+    defmodule <%= inspect(@mod) %> do
+
+    end
+  """)
+
+  embed_template(:part, ~S"""
+    defmodule <%= inspect(@mod) %> do
+      alias AdventOfCode.PuzzleSolver
+      use PuzzleSolver
+
+      import <%= inspect(@day_mod) %>, warn: false
+
+      @impl PuzzleSolver
+      def solve(_input_stream) do
+        :ok |> to_string()
+      end
+    end
+  """)
+
+  embed_template(:part_test, ~S[
+    defmodule <%= inspect(@test_mod) %> do
+      use AdventOfCode.PuzzleCase, module: <%= inspect(@mod) %>
+
+      test "returns :ok" do
+        input = ~S"""
+        input
+        """
+
+        assert_solution input, "ok"
+      end
+    end
+  ])
+end
diff --git a/2021/lib/mix/tasks/advent_of_code.solve.ex b/2021/lib/mix/tasks/advent_of_code.solve.ex
new file mode 100644
index 0000000..5c506c5
--- /dev/null
+++ b/2021/lib/mix/tasks/advent_of_code.solve.ex
@@ -0,0 +1,73 @@
+defmodule Mix.Tasks.AdventOfCode.Solve do
+  use Mix.Task
+
+  @shortdoc "Runs solution code with problem input"
+
+  @moduledoc """
+  #{@shortdoc}.
+
+  ## Options
+
+  `--input`
+    - name of a `.input` file in `priv/inputs/` without the `.input` extension or `-` to read from stdin
+
+  ## Examples
+
+      # Run Day 2, Part 1 solution program against the `2.1.input` file
+      $ mix advent_of_code.solve 2.1
+
+      # Run Day 1, Part 2 with a special input
+      $ mix advent_of_code.solve 1.2 --input day-1-part-2-small # priv/inputs/day-1-part-2-small.input
+
+      # Run Day 17, Part 1 with input from stdin
+  #   $ mix advent_of_code.solve 17.1 --input -
+  """
+
+  @switches [input: :string]
+
+  @impl Mix.Task
+  def run(args) do
+    case OptionParser.parse!(args, strict: @switches) do
+      {opts, [selector]} ->
+        case stream_for_input(selector, opts) do
+          {:indeterminate, stream} ->
+            ProgressBar.render_spinner(
+              [text: "Processing indeterminate data...", done: "Done."],
+              fn -> solve(selector, stream) end
+            )
+            |> IO.puts()
+
+          {size, stream} ->
+            stream =
+              stream
+              |> Stream.with_index(1)
+              |> Stream.each(fn {_, i} -> ProgressBar.render(i, size) end)
+              |> Stream.map(fn {l, _} -> l end)
+
+            solve(selector, stream)
+            |> IO.puts()
+        end
+
+      _ ->
+        nil
+    end
+  end
+
+  defp solve(selector, stream) do
+    selector
+    |> AdventOfCode.solver()
+    |> AdventOfCode.PuzzleSolver.solve(stream)
+  end
+
+  defp stream_for_input(selector, opts) do
+    input = Keyword.get(opts, :input, selector)
+
+    if input == "-" do
+      {:indeterminate, IO.stream()}
+    else
+      file = Path.join("priv/inputs", "#{input}.input")
+      stream = File.stream!(file)
+      {Enum.count(stream), stream}
+    end
+  end
+end
diff --git a/2021/mix.exs b/2021/mix.exs
new file mode 100644
index 0000000..a8ad075
--- /dev/null
+++ b/2021/mix.exs
@@ -0,0 +1,31 @@
+defmodule AdventOfCode.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :advent_of_code,
+      version: "0.1.0",
+      elixir: "~> 1.11",
+      start_permanent: Mix.env() == :prod,
+      elixirc_paths: elixirc_paths(Mix.env()),
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger, :eex]
+    ]
+  end
+
+  defp elixirc_paths(:test), do: ["lib", "test/support"]
+  defp elixirc_paths(_), do: ["lib"]
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      {:progress_bar, "~> 2.0"}
+    ]
+  end
+end
diff --git a/2021/mix.lock b/2021/mix.lock
new file mode 100644
index 0000000..e43918e
--- /dev/null
+++ b/2021/mix.lock
@@ -0,0 +1,4 @@
+%{
+  "decimal": {:hex, :decimal, "2.0.0", "a78296e617b0f5dd4c6caf57c714431347912ffb1d0842e998e9792b5642d697", [:mix], [], "hexpm", "34666e9c55dea81013e77d9d87370fe6cb6291d1ef32f46a1600230b1d44f577"},
+  "progress_bar": {:hex, :progress_bar, "2.0.1", "7b40200112ae533d5adceb80ff75fbe66dc753bca5f6c55c073bfc122d71896d", [:mix], [{:decimal, "~> 2.0", [hex: :decimal, repo: "hexpm", optional: false]}], "hexpm", "2519eb58a2f149a3a094e729378256d8cb6d96a259ec94841bd69fdc71f18f87"},
+}
diff --git a/2021/priv/inputs/0.0.input b/2021/priv/inputs/0.0.input
new file mode 100644
index 0000000..86e041d
--- /dev/null
+++ b/2021/priv/inputs/0.0.input
@@ -0,0 +1,3 @@
+foo
+bar
+baz
diff --git a/2021/test/advent_of_code/day0/part_0_test.exs b/2021/test/advent_of_code/day0/part_0_test.exs
new file mode 100644
index 0000000..cd53141
--- /dev/null
+++ b/2021/test/advent_of_code/day0/part_0_test.exs
@@ -0,0 +1,7 @@
+defmodule AdventOfCode.Day0.Part0Test do
+  use AdventOfCode.PuzzleCase, module: AdventOfCode.Day0.Part0
+
+  test "returns the answer to live the universe and everything" do
+    assert_solution "life the universe and everything", "42"
+  end
+end
diff --git a/2021/test/advent_of_code_test.exs b/2021/test/advent_of_code_test.exs
new file mode 100644
index 0000000..589fa60
--- /dev/null
+++ b/2021/test/advent_of_code_test.exs
@@ -0,0 +1,13 @@
+defmodule AdventOfCodeTest do
+  use ExUnit.Case
+
+  import AdventOfCode
+
+  describe "solver/1" do
+    test "returns a module for a solver" do
+      assert solver("1.1") == AdventOfCode.Day1.Part1
+      assert solver("2.2") == AdventOfCode.Day2.Part2
+      assert solver("3.1") == AdventOfCode.Day3.Part1
+    end
+  end
+end
diff --git a/2021/test/support/puzzle_case.ex b/2021/test/support/puzzle_case.ex
new file mode 100644
index 0000000..c0d7e09
--- /dev/null
+++ b/2021/test/support/puzzle_case.ex
@@ -0,0 +1,21 @@
+defmodule AdventOfCode.PuzzleCase do
+  @moduledoc """
+  Defines tests for an `AdventOfCode.PuzzleSolver` module.
+  """
+
+  use ExUnit.CaseTemplate
+
+  using module: module do
+    quote bind_quoted: [module: module] do
+      @module module
+
+      defp assert_solution(input, desired_output) when is_binary(input) do
+        {:ok, stream_pid} = StringIO.open(input)
+        stream_input = IO.stream(stream_pid, :line)
+        actual_output = AdventOfCode.PuzzleSolver.solve(@module, stream_input)
+        @module.solve(stream_input)
+        assert actual_output == desired_output
+      end
+    end
+  end
+end
diff --git a/2021/test/test_helper.exs b/2021/test/test_helper.exs
new file mode 100644
index 0000000..869559e
--- /dev/null
+++ b/2021/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()