initial commit.

.formatter.exs

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

.gitignore

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+# 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").
+yt_ext-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/

lib/yt_ext.ex

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+defmodule YtExt do
+  @moduledoc """
+  Documentation for `YtExt`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> YtExt.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lib/yt_ext/application.ex

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+defmodule YtExt.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      # Starts a worker by calling: YtExt.Worker.start_link(arg)
+      # {YtExt.Worker, arg}
+    ]
+
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: YtExt.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

lib/yt_ext/youtube_id.ex

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+defmodule YouTubeID do
+  @youtube_chars "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+  @base length(String.graphemes(@youtube_chars))
+
+  def to_integer(<<video_id::binary-size(11)>>) do
+    video_id
+    |> String.graphemes()
+    |> Enum.reverse()
+    |> Enum.with_index()
+    |> Enum.reduce(0, fn {char, index}, acc ->
+      char_value = contains_at(@youtube_chars, char)
+      acc + char_value * trunc(:math.pow(@base, index))
+    end)
+  end
+
+  defp contains_at(string, char) do
+    String.split(string, "", trim: true)
+    |> Enum.find_index(&(&1 == char))
+  end
+end

mix.exs

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+defmodule YtExt.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :yt_ext,
+      version: "0.1.0",
+      elixir: "~> 1.17",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {YtExt.Application, []}
+    ]
+  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

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+ExUnit.start()

test/yt_ext_test.exs

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+defmodule YtExtTest do
+  use ExUnit.Case
+  doctest YtExt
+
+  test "greets the world" do
+    assert YtExt.hello() == :world
+  end
+end