Add intergalactic-transmission exercise (#1566)

* Add intergalactic-transmission exercise

* Add further prerequisites from example solution

Co-authored-by: Angelika Cathor <angelikacathor@fastmail.com>

* Address review comments

* Rename module in mix.exs

---------

Co-authored-by: Angelika Cathor <angelikacathor@fastmail.com>

Author: kahgoh

config.json

@@ -2147,6 +2147,28 @@
         ],
         "difficulty": 5
       },
+      {
+        "slug": "intergalactic-transmission",
+        "name": "Intergalactic Transmission",
+        "uuid": "3ba8f12a-a873-4882-8eef-e44ebc3ff946",
+        "practices": [
+          "bit-manipulation",
+          "bitstrings"
+        ],
+        "prerequisites": [
+          "atoms",
+          "bit-manipulation",
+          "bitstrings",
+          "case",
+          "enum",
+          "if",
+          "pattern-matching",
+          "recursion",
+          "tail-call-recursion",
+          "tuples"
+        ],
+        "difficulty": 5
+      },
       {
         "slug": "luhn",
         "name": "Luhn",

exercises/practice/intergalactic-transmission/.docs/instructions.md

@@ -0,0 +1,54 @@
+# Instructions
+
+Your job is to help implement
+
+- the transmitter, which calculates the transmission sequence, and
+- the receiver, which decodes it.
+
+A parity bit is simple way of detecting transmission errors.
+The transmitters and receivers can only transmit and receive _exactly_ eight bits at a time (including the parity bit).
+The parity bit is set so that there is an _even_ number 1s in each transmission and is always the first bit from the right.
+So if the receiver receives `11000001`, `01110101` or `01000000` (i.e. a transmission with an odd number of 1 bits), it knows there is an error.
+
+However, messages are rarely this short, and need to be transmitted in a sequence when they are longer.
+
+For example, consider the message `11000000 00000001 11000000 11011110` (or `C0 01 C0 DE` in hex).
+
+Since each transmission contains exactly eight bits, it can only contain seven bits of data and the parity bit.
+A parity bit must then be inserted after every seven bits of data:
+
+```text
+11000000 00000001 11000000 11011110
+      ↑       ↑       ↑       ↑          (7th bits)
+```
+
+The transmission sequence for this message looks like this:
+
+```text
+1100000_ 0000000_ 0111000_ 0001101_ 1110
+       ↑        ↑        ↑        ↑      (parity bits)
+```
+
+The data in the first transmission in the sequence (`1100000`) has two 1 bits (an even number), so the parity bit is 0.
+The first transmission becomes `11000000` (or `C0` in hex).
+
+The data in the next transmission (`0000000`) has zero 1 bits (an even number again), so the parity bit is 0 again.
+The second transmission thus becomes `00000000` (or `00` in hex).
+
+The data for the next two transmissions (`0111000` and `0001101`) have three 1 bits.
+Their parity bits are set to 1 so that they have an even number of 1 bits in the transmission.
+They are transmitted as `01110001` and `00011011` (or `71` and `1B` in hex).
+
+The last transmission (`1110`) has only four bits of data.
+Since exactly eight bits are transmitted at a time and the parity bit is the right most bit, three 0 bits and then the parity bit are added to make up eight bits.
+It now looks like this (where `_` is the parity bit):
+
+```text
+1110 000_
+     ↑↑↑   (added 0 bits)
+```
+
+There is an odd number of 1 bits again, so the parity bit is 1.
+The last transmission in the sequence becomes `11100001` (or `E1` in hex).
+
+The entire transmission sequence for this message is `11000000 00000000 01110001 00011011 11100001` (or `C0 00 71 1B E1` in hex).

exercises/practice/intergalactic-transmission/.docs/introduction.md

@@ -0,0 +1,23 @@
+# Introduction
+
+Trillions upon trillions of messages zip between Earth and neighboring galaxies every millisecond.
+But transmitting over such long distances is tricky.
+Pesky solar flares, temporal distortions, stray forces, and even the flap of a space butterfly's wing can cause a random bit to change during transmission.
+
+Now imagine the consequences:
+
+- Crashing the Intergalactic Share Market when "buy low" turns to "sell now".
+- Losing contact with the Kepler Whirl system when "save new worm hole" becomes "cave new worm hole".
+- Or plunging the universe into existential horror by replacing a cowboy emoji 🤠 with a clown emoji 🤡.
+
+Detecting corrupted messages isn't just important — it's critical.
+The receiver _must_ know when something has gone wrong before disaster strikes.
+
+But how?
+Scientists and engineers from across the universe have been battling this problem for eons.
+Entire cosmic AI superclusters churn through the data.
+And then, one day, a legend resurfaces — an ancient, powerful method, whispered in debugging forums, muttered by engineers who've seen too much...
+
+The Parity Bit!
+
+A method so simple, so powerful, that it might just save interstellar communication.

exercises/practice/intergalactic-transmission/.formatter.exs

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

exercises/practice/intergalactic-transmission/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "kahgoh"
+  ],
+  "files": {
+    "solution": [
+      "lib/transmission.ex"
+    ],
+    "test": [
+      "test/transmission_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  },
+  "blurb": "Add parity bits to a message for transmission",
+  "source": "Kah Goh",
+  "source_url": "https://github.com/exercism/problem-specifications/pull/2543"
+}

exercises/practice/intergalactic-transmission/.meta/example.ex

@@ -0,0 +1,77 @@
+defmodule Transmission do
+  import Bitwise
+
+  @doc """
+  Return the transmission sequence for a message.
+  """
+  @spec get_transmit_sequence(binary()) :: binary()
+  def get_transmit_sequence(message) do
+    encode(message)
+  end
+
+  @spec encode(bitstring(), bitstring()) :: binary()
+  defp encode(content, acc \\ <<>>)
+  defp encode(<<>>, acc), do: acc
+
+  defp encode(<<next::7, remaining::bitstring>>, acc) do
+    encode(remaining, <<acc::bitstring, add_parity(<<next::7>>)::bitstring>>)
+  end
+
+  defp encode(<<content::bitstring>>, acc) do
+    <<acc::bitstring, add_parity(<<content::bitstring>>)::bitstring>>
+  end
+
+  @spec add_parity(bitstring()) :: <<_::8>>
+  defp add_parity(data) when bit_size(data) < 8 do
+    <<data::bitstring, 0::size(7 - bit_size(data)), get_parity(data)::bitstring>>
+  end
+
+  @spec get_parity(bitstring()) :: <<_::1>>
+  defp get_parity(data) do
+    bit_count = bit_size(data)
+    <<value::size(bit_count)>> = data
+
+    parity_int =
+      0..(bit_count - 1)
+      |> Enum.map(fn
+        places ->
+          case value &&& 1 <<< places do
+            0 -> 0
+            _other -> 1
+          end
+      end)
+      |> Enum.reduce(0, fn elem, acc -> bxor(elem, acc) end)
+
+    case parity_int do
+      0 -> <<0::1>>
+      _ -> <<1::1>>
+    end
+  end
+
+  @doc """
+  Return the message decoded from the received transmission.
+  """
+  @spec decode_message(binary()) :: {:ok, binary()} | {:error, String.t()}
+  def decode_message(<<>>), do: {:ok, <<>>}
+  def decode_message(received), do: do_decode(received)
+
+  @spec do_decode(bitstring(), bitstring()) :: {:ok, binary()} | {:error, String.t()}
+  defp do_decode(content, acc \\ <<>>)
+
+  defp do_decode(<<data::7, parity::1, remaining::bitstring>>, acc) do
+    if get_parity(<<data::7>>) == <<parity::1>> do
+      case remaining do
+        <<>> -> {:ok, finalize(<<data::7>>, acc)}
+        _ -> do_decode(remaining, <<acc::bitstring, data::7>>)
+      end
+    else
+      {:error, "wrong parity"}
+    end
+  end
+
+  defp finalize(data, acc) when bit_size(data) == 7 do
+    bits_count = 8 - rem(bit_size(acc), 8)
+    <<head::size(bits_count), _::bitstring>> = data
+    <<acc::bitstring, head::size(bits_count)>>
+  end
+end

exercises/practice/intergalactic-transmission/.meta/tests.toml

@@ -0,0 +1,88 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[f99d4046-b429-4582-9324-f0bcac7ab51c]
+description = "calculate transmit sequences -> empty message"
+
+[ee27ea2d-8999-4f23-9275-8f6879545f86]
+description = "calculate transmit sequences -> 0x00 is transmitted as 0x0000"
+
+[97f27f98-8020-402d-be85-f21ba54a6df0]
+description = "calculate transmit sequences -> 0x02 is transmitted as 0x0300"
+
+[24712fb9-0336-4e2f-835e-d2350f29c420]
+description = "calculate transmit sequences -> 0x06 is transmitted as 0x0600"
+
+[7630b5a9-dba1-4178-b2a0-4a376f7414e0]
+description = "calculate transmit sequences -> 0x05 is transmitted as 0x0581"
+
+[ab4fe80b-ef8e-4a99-b4fb-001937af415d]
+description = "calculate transmit sequences -> 0x29 is transmitted as 0x2881"
+
+[4e200d84-593b-4449-b7c0-4de1b6a0955e]
+description = "calculate transmit sequences -> 0xc001c0de is transmitted as 0xc000711be1"
+
+[fbc537e9-6b21-4f4a-8c2b-9cf9b702a9b7]
+description = "calculate transmit sequences -> six byte message"
+
+[d5b75adf-b5fc-4f77-b4ab-77653e30f07c]
+description = "calculate transmit sequences -> seven byte message"
+
+[6d8b297b-da1d-435e-bcd7-55fbb1400e73]
+description = "calculate transmit sequences -> eight byte message"
+
+[54a0642a-d5aa-490c-be89-8e171a0cab6f]
+description = "calculate transmit sequences -> twenty byte message"
+
+[9a8084dd-3336-474c-90cb-8a852524604d]
+description = "decode received messages -> empty message"
+
+[879af739-0094-4736-9127-bd441b1ddbbf]
+description = "decode received messages -> zero message"
+
+[7a89eeef-96c5-4329-a246-ec181a8e959a]
+description = "decode received messages -> 0x0300 is decoded to 0x02"
+
+[3e515af7-8b62-417f-960c-3454bca7f806]
+description = "decode received messages -> 0x0581 is decoded to 0x05"
+
+[a1b4a3f7-9f05-4b7a-b86e-d7c6fc3f16a9]
+description = "decode received messages -> 0x2881 is decoded to 0x29"
+
+[2e99d617-4c91-4ad5-9217-e4b2447d6e4a]
+description = "decode received messages -> first byte has wrong parity"
+
+[507e212d-3dae-42e8-88b4-2223838ff8d2]
+description = "decode received messages -> second byte has wrong parity"
+
+[b985692e-6338-46c7-8cea-bc38996d4dfd]
+description = "decode received messages -> 0xcf4b00 is decoded to 0xce94"
+
+[7a1f4d48-696d-4679-917c-21b7da3ff3fd]
+description = "decode received messages -> 0xe2566500 is decoded to 0xe2ad90"
+
+[467549dc-a558-443b-80c5-ff3d4eb305d4]
+description = "decode received messages -> six byte message"
+
+[1f3be5fb-093a-4661-9951-c1c4781c71ea]
+description = "decode received messages -> seven byte message"
+
+[6065b8b3-9dcd-45c9-918c-b427cfdb28c1]
+description = "decode received messages -> last byte has wrong parity"
+
+[98af97b7-9cca-4c4c-9de3-f70e227a4cb1]
+description = "decode received messages -> eight byte message"
+
+[aa7d4785-2bb9-43a4-a38a-203325c464fb]
+description = "decode received messages -> twenty byte message"
+
+[4c86e034-b066-42ac-8497-48f9bc1723c1]
+description = "decode received messages -> wrong parity on 16th byte"

exercises/practice/intergalactic-transmission/lib/transmission.ex

@@ -0,0 +1,15 @@
+defmodule Transmission do
+  @doc """
+  Return the transmission sequence for a message.
+  """
+  @spec get_transmit_sequence(binary()) :: binary()
+  def get_transmit_sequence(message) do
+  end
+
+  @doc """
+  Return the message decoded from the received transmission.
+  """
+  @spec decode_message(binary()) :: {:ok, binary()} | {:error, String.t()}
+  def decode_message(received_data) do
+  end
+end

exercises/practice/intergalactic-transmission/mix.exs

@@ -0,0 +1,28 @@
+defmodule Transmission.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :transmission,
+      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

exercises/practice/intergalactic-transmission/test/test_helper.exs

@@ -0,0 +1,2 @@
+ExUnit.start()
+ExUnit.configure(exclude: :pending, trace: true)