Add run-length-encoding example

config.json

@@ -11,7 +11,8 @@
      "rna-transcription",
      "hamming",
      "word-count",
-     "bob"
+     "bob",
+     "run-length-encoding"
   ],
   "deprecated": [
 

elm-package.json

@@ -11,7 +11,8 @@
         "./exercises/rna-transcription",
         "./exercises/hamming",
         "./exercises/word-count",
-        "./exercises/bob"
+        "./exercises/bob",
+        "./exercises/run-length-encoding"
     ],
     "exposed-modules": [],
     "dependencies": {

exercises/run-length-encoding/RunLengthEncoding.elm

@@ -0,0 +1 @@
+module RunLengthEncoding (..) where

exercises/run-length-encoding/RunLengthEncoding.example

@@ -0,0 +1,63 @@
+module RunLengthEncoding (..) where
+
+import String exposing (fromChar)
+import List exposing (head, tail)
+import Maybe exposing (withDefault)
+import Regex
+
+
+-- To the unaware: this was written by a very green elmer, so don't consider
+-- it an idiomatic exemplar to follow.
+
+
+encode : String -> String
+encode string =
+  String.toList string
+    |> List.foldr countChars []
+    |> List.map stringifyCounts
+    |> String.join ""
+
+
+countChars : a -> List ( number, a ) -> List ( number, a )
+countChars current counted =
+  case head counted of
+    Just ( count, previous ) ->
+      if previous == current then
+        ( count + 1, current ) :: withDefault [] (tail counted)
+      else
+        ( 1, current ) :: counted
+
+    Nothing ->
+      [ ( 1, current ) ]
+
+
+stringifyCounts : ( number, Char ) -> String
+stringifyCounts ( count, char ) =
+  if count > 1 then
+    toString count ++ fromChar char
+  else
+    fromChar char
+
+
+decode : String -> String
+decode string =
+  string
+    |> Regex.find Regex.All (Regex.regex "(\\d+)|(\\D)")
+    |> List.map .match
+    |> List.foldl expandCounts ( "", Nothing )
+    |> fst
+
+
+expandCounts : String -> ( String, Maybe Int ) -> ( String, Maybe Int )
+expandCounts match ( result, count ) =
+  case count of
+    Just number ->
+      ( result ++ String.repeat number match, Nothing )
+
+    Nothing ->
+      case String.toInt match of
+        Ok number ->
+          ( result, Just number )
+
+        Err _ ->
+          ( result ++ match, Nothing )

exercises/run-length-encoding/RunLengthEncodingTests.elm

@@ -0,0 +1,40 @@
+module Main (..) where
+
+import Task
+import Console
+import ElmTest exposing (..)
+import RunLengthEncoding exposing (decode, encode)
+
+
+tests : Test
+tests =
+  suite
+    "RunLengthEncoding"
+    [ test "encode simple" (assertEqual "2A3B4C" (encode "AABBBCCCC"))
+    , test "decode simple" (assertEqual "AABBBCCCC" (decode "2A3B4C"))
+    , test
+        "encode with single values"
+        (assertEqual
+          "12WB12W3B24WB"
+          (encode "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB")
+        )
+    , test
+        "decode with single values"
+        (assertEqual
+          "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"
+          (decode "12WB12W3B24WB")
+        )
+    , test
+        "decode(encode(...)) combination"
+        (assertEqual
+          "zzz ZZ  zZ"
+          (decode (encode "zzz ZZ  zZ"))
+        )
+    , test "encode unicode" (assertEqual "⏰3⚽2⭐⏰" (encode "⏰⚽⚽⚽⭐⭐⏰"))
+    , test "decode unicode" (assertEqual "⏰⚽⚽⚽⭐⭐⏰" (decode "⏰3⚽2⭐⏰"))
+    ]
+
+
+port runner : Signal (Task.Task x ())
+port runner =
+  Console.run (consoleRunner tests)