Add Dominoes exercise for Elixir track

elixir/dominoes/.exercism/config.json

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+{
+  "authors": [
+    "Tuxified"
+  ],
+  "contributors": [
+    "angelikatyborska",
+    "Cohen-Carlisle",
+    "devonestes",
+    "neenjaw",
+    "sotojuan"
+  ],
+  "files": {
+    "solution": [
+      "lib/dominoes.ex"
+    ],
+    "test": [
+      "test/dominoes_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  },
+  "blurb": "Make a chain of dominoes."
+}

elixir/dominoes/.exercism/metadata.json

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+{"track":"elixir","exercise":"dominoes","id":"c3029f8eea4743b88999e49c85863daa","url":"https://exercism.org/tracks/elixir/exercises/dominoes","handle":"negrienko","is_requester":true,"auto_approve":false}

elixir/dominoes/.formatter.exs

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

elixir/dominoes/.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").
+dominoes-*.tar
+

elixir/dominoes/HELP.md

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+# Help
+
+## Running the tests
+
+From the terminal, change to the base directory of the exercise then execute the tests with:
+
+```bash
+$ mix test
+```
+
+This will execute the test file found in the `test` subfolder -- a file ending in `_test.exs`
+
+Documentation:
+
+* [`mix test` - Elixir's test execution tool](https://hexdocs.pm/mix/Mix.Tasks.Test.html)
+* [`ExUnit` - Elixir's unit test library](https://hexdocs.pm/ex_unit/ExUnit.html)
+
+## Pending tests
+
+In test suites of practice exercises, all but the first test have been tagged to be skipped.
+
+Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.
+
+For example:
+
+```elixir
+# @tag :pending
+test "shouting" do
+  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
+end
+```
+
+If you wish to run all tests at once, you can include all skipped test by using the `--include` flag on the `mix test` command:
+
+```bash
+$ mix test --include pending
+```
+
+Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the file `test/test_helper.exs`.
+
+```elixir
+# ExUnit.configure(exclude: :pending, trace: true)
+```
+
+## Useful `mix test` options
+
+* `test/<FILE>.exs:LINENUM` - runs only a single test, the test from `<FILE>.exs` whose definition is on line `LINENUM`
+* `--failed` - runs only tests that failed the last time they ran
+* `--max-failures` - the suite stops evaluating tests when this number of test failures
+is reached
+* `--seed 0` - disables randomization so the tests in a single file will always be ran
+in the same order they were defined in
+
+## Submitting your solution
+
+You can submit your solution using the `exercism submit lib/dominoes.ex` command.
+This command will upload your solution to the Exercism website and print the solution page's URL.
+
+It's possible to submit an incomplete solution which allows you to:
+
+- See how others have completed the exercise
+- Request help from a mentor
+
+## Need to get help?
+
+If you'd like help solving the exercise, check the following pages:
+
+- The [Elixir track's documentation](https://exercism.org/docs/tracks/elixir)
+- The [Elixir track's programming category on the forum](https://forum.exercism.org/c/programming/elixir)
+- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
+- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
+
+Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
+
+If you're stuck on something, it may help to look at some of the [available resources](https://exercism.org/docs/tracks/elixir/resources) out there where answers might be found.

elixir/dominoes/README.md

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+# Dominoes
+
+Welcome to Dominoes on Exercism's Elixir Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+
+## Introduction
+
+In Toyland, the trains are always busy delivering treasures across the city, from shiny marbles to rare building blocks.
+The tracks they run on are made of colorful domino-shaped pieces, each marked with two numbers.
+For the trains to move, the dominoes must form a perfect chain where the numbers match.
+
+Today, an urgent delivery of rare toys is on hold.
+You've been handed a set of track pieces to inspect.
+If they can form a continuous chain, the train will be on its way, bringing smiles across Toyland.
+If not, the set will be discarded, and another will be tried.
+
+The toys are counting on you to solve this puzzle.
+Will the dominoes connect the tracks and send the train rolling, or will the set be left behind?
+
+## Instructions
+
+Make a chain of dominoes.
+
+Compute a way to order a given set of domino stones so that they form a correct domino chain.
+In the chain, the dots on one half of a stone must match the dots on the neighboring half of an adjacent stone.
+Additionally, the dots on the halves of the stones without neighbors (the first and last stone) must match each other.
+
+For example given the stones `[2|1]`, `[2|3]` and `[1|3]` you should compute something
+like `[1|2] [2|3] [3|1]` or `[3|2] [2|1] [1|3]` or `[1|3] [3|2] [2|1]` etc, where the first and last numbers are the same.
+
+For stones `[1|2]`, `[4|1]` and `[2|3]` the resulting chain is not valid: `[4|1] [1|2] [2|3]`'s first and last numbers are not the same.
+4 != 3
+
+Some test cases may use duplicate stones in a chain solution, assume that multiple Domino sets are being used.
+
+## Source
+
+### Created by
+
+- @Tuxified
+
+### Contributed to by
+
+- @angelikatyborska
+- @Cohen-Carlisle
+- @devonestes
+- @neenjaw
+- @sotojuan

elixir/dominoes/lib/dominoes.ex

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+defmodule Dominoes do
+  @type domino :: {1..6, 1..6}
+
+  @doc """
+  chain?/1 takes a list of domino stones and returns boolean indicating if it's
+  possible to make a full chain
+  """
+  @spec chain?(dominoes :: [domino]) :: boolean
+  def chain?([]), do: true
+  def chain?([{first, second}]) when first == second, do: true
+
+  def chain?([{first, second} | dominoes]) do
+    Enum.any?(
+      dominoes,
+      fn
+        {^first, x} = domino -> chain?([{second, x} | List.delete(dominoes, domino)])
+        {x, ^first} = domino -> chain?([{second, x} | List.delete(dominoes, domino)])
+        _ -> false
+      end
+    )
+  end
+end

elixir/dominoes/mix.exs

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+defmodule Dominoes.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :dominoes,
+      version: "0.1.0",
+      # elixir: "~> 1.8",
+      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
+    [
+      {:lettuce, "~> 0.3.0", only: ~w(dev)a}
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

elixir/dominoes/test/dominoes_test.exs

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+defmodule DominoesTest do
+  use ExUnit.Case
+
+  test "empty input = empty output" do
+    assert Dominoes.chain?([]) == true
+  end
+
+  test "singleton input = singleton output" do
+    assert Dominoes.chain?([{1, 1}]) == true
+  end
+
+  test "singleton that can't be chained" do
+    assert Dominoes.chain?([{1, 2}]) == false
+  end
+
+  test "three elements" do
+    assert Dominoes.chain?([{1, 2}, {3, 1}, {2, 3}]) == true
+  end
+
+  test "can reverse dominoes" do
+    assert Dominoes.chain?([{1, 2}, {1, 3}, {2, 3}]) == true
+  end
+
+  test "can't be chained" do
+    assert Dominoes.chain?([{1, 2}, {4, 1}, {2, 3}]) == false
+  end
+
+  test "disconnected - double loop" do
+    assert Dominoes.chain?([{1, 2}, {2, 1}, {3, 4}, {4, 3}]) == false
+  end
+
+  test "disconnected - single isolated" do
+    assert Dominoes.chain?([{1, 2}, {2, 3}, {3, 1}, {4, 4}]) == false
+  end
+
+  test "need backtrack" do
+    # a variation in which we have to turn but no duplicates
+    assert Dominoes.chain?([{1, 2}, {2, 3}, {3, 1}, {2, 4}, {2, 4}]) == true
+  end
+
+  test "separate loops" do
+    assert Dominoes.chain?([{1, 2}, {2, 3}, {3, 1}, {1, 1}, {2, 2}, {3, 3}]) == true
+  end
+
+  test "nine elements" do
+    assert Dominoes.chain?([
+             {1, 2},
+             {5, 3},
+             {3, 1},
+             {1, 2},
+             {2, 4},
+             {1, 6},
+             {2, 3},
+             {3, 4},
+             {5, 6}
+           ]) == true
+  end
+
+  test "separate three-domino loops" do
+    refute Dominoes.chain?([{1, 2}, {2, 3}, {3, 1}, {4, 5}, {5, 6}, {6, 4}])
+  end
+
+  test "disconnected - simple" do
+    refute Dominoes.chain?([{1, 1}, {2, 2}])
+  end
+
+  test "first and last not matching" do
+    assert Dominoes.chain?([{1, 2}, {2, 3}, {3, 4}]) == false
+  end
+
+  test "wrong starting order" do
+    assert Dominoes.chain?([{2, 1}, {2, 3}, {3, 1}]) == true
+  end
+end

elixir/dominoes/test/test_helper.exs

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+ExUnit.start()
+ExUnit.configure(exclude: :pending, trace: true)