Implement run-length-encoding in Elixir

elixir/run-length-encoding/.formatter.exs

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

elixir/run-length-encoding/.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").
+run_length_encoding-*.tar
+

elixir/run-length-encoding/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/run_length_encoder.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/run-length-encoding/README.md

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+# Run-Length Encoding
+
+Welcome to Run-Length Encoding on Exercism's Elixir Track.
+If you need help running the tests or submitting your code, check out `HELP.md`.
+
+## Instructions
+
+Implement run-length encoding and decoding.
+
+Run-length encoding (RLE) is a simple form of data compression, where runs (consecutive data elements) are replaced by just one data value and count.
+
+For example we can represent the original 53 characters with only 13.
+
+```text
+"WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"  ->  "12WB12W3B24WB"
+```
+
+RLE allows the original data to be perfectly reconstructed from the compressed data, which makes it a lossless data compression.
+
+```text
+"AABCCCDEEEE"  ->  "2AB3CD4E"  ->  "AABCCCDEEEE"
+```
+
+For simplicity, you can assume that the unencoded string will only contain the letters A through Z (either lower or upper case) and whitespace.
+This way data to be encoded will never contain any numbers and numbers inside data to be decoded always represent the count for the following character.
+
+## Source
+
+### Created by
+
+- @Teapane
+
+### Contributed to by
+
+- @angelikatyborska
+- @CoderDennis
+- @Cohen-Carlisle
+- @dalexj
+- @daveyarwood
+- @devonestes
+- @lex57ukr
+- @lpil
+- @neenjaw
+- @parkerl
+- @sotojuan
+- @waiting-for-dev
+
+### Based on
+
+Wikipedia - https://en.wikipedia.org/wiki/Run-length_encoding

elixir/run-length-encoding/lib/run_length_encoder.ex

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+defmodule RunLengthEncoder do
+  @doc """
+  Generates a string where consecutive elements are represented as a data value and count.
+  "AABBBCCCC" => "2A3B4C"
+  For this example, assume all input are strings, that are all uppercase letters.
+  It should also be able to reconstruct the data into its original form.
+  "2A3B4C" => "AABBBCCCC"
+  """
+  @spec encode(String.t()) :: String.t()
+  def encode(string) do
+    string
+    |> String.codepoints()
+    |> Enum.reduce({"", 1, ""}, fn
+      char, {current_elem, count, result} when current_elem == char ->
+        {current_elem, count + 1, result}
+
+      char, {current_elem, 1, result} ->
+        {char, 1, result <> current_elem}
+
+      char, {current_elem, count, result} ->
+        {char, 1, "#{result}#{count}#{current_elem}"}
+    end)
+    |> then(fn
+      {current_elem, 1, result} ->
+        result <> current_elem
+
+      {current_elem, count, result} ->
+        "#{result}#{count}#{current_elem}"
+    end)
+  end
+
+  @spec decode(String.t()) :: String.t()
+  def decode(string) do
+    string
+    |> String.codepoints()
+    |> Enum.reduce({"0", ""}, fn
+      char, {current_num, result} ->
+        case Integer.parse(char) do
+          :error ->
+            repeat =
+              case Integer.parse(current_num) do
+                {0, _} -> 1
+                {num, _} -> num
+              end
+
+            {"0", result <> String.duplicate(char, repeat)}
+
+          _ ->
+            {current_num <> char, result}
+        end
+    end)
+    |> then(fn {_, result} -> result end)
+  end
+end

elixir/run-length-encoding/mix.exs

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+defmodule RunLengthEncoder.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :run_length_encoder,
+      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
+    [
+      # {: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/run-length-encoding/test/run_length_encoder_test.exs

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+defmodule RunLengthEncoderTest do
+  use ExUnit.Case
+
+  test "encode empty string" do
+    assert RunLengthEncoder.encode("") === ""
+  end
+
+  test "encode single characters only are encoded without count" do
+    assert RunLengthEncoder.encode("XYZ") === "XYZ"
+  end
+
+  test "encode string with no single characters" do
+    assert RunLengthEncoder.encode("AABBBCCCC") == "2A3B4C"
+  end
+
+  test "encode single characters mixed with repeated characters" do
+    assert RunLengthEncoder.encode("WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB") ===
+             "12WB12W3B24WB"
+  end
+
+  test "encode multiple whitespace mixed in string" do
+    assert RunLengthEncoder.encode("  hsqq qww  ") === "2 hs2q q2w2 "
+  end
+
+  test "encode lowercase characters" do
+    assert RunLengthEncoder.encode("aabbbcccc") === "2a3b4c"
+  end
+
+  test "decode empty string" do
+    assert RunLengthEncoder.decode("") === ""
+  end
+
+  test "decode single characters only" do
+    assert RunLengthEncoder.decode("XYZ") === "XYZ"
+  end
+
+  test "decode string with no single characters" do
+    assert RunLengthEncoder.decode("2A3B4C") == "AABBBCCCC"
+  end
+
+  test "decode single characters with repeated characters" do
+    assert RunLengthEncoder.decode("12WB12W3B24WB") ===
+             "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"
+  end
+
+  test "decode multiple whitespace mixed in string" do
+    assert RunLengthEncoder.decode("2 hs2q q2w2 ") === "  hsqq qww  "
+  end
+
+  test "decode lower case string" do
+    assert RunLengthEncoder.decode("2a3b4c") === "aabbbcccc"
+  end
+
+  test "encode followed by decode gives original string" do
+    original = "zzz ZZ  zZ"
+    encoded = RunLengthEncoder.encode(original)
+    assert RunLengthEncoder.decode(encoded) === original
+  end
+end

elixir/run-length-encoding/test/test_helper.exs

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