Add run-length-encoding exercise

config.json

@@ -563,6 +563,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "run-length-encoding",
+        "name": "Run-Length Encoding",
+        "uuid": "d6b8a979-8ce8-4f46-a6cd-35cd44b19701",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "all-your-base",
         "name": "All Your Base",

exercises/practice/run-length-encoding/.docs/instructions.md

@@ -0,0 +1,20 @@
+# 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.

exercises/practice/run-length-encoding/.meta/Example.roc

@@ -0,0 +1,44 @@
+module [encode, decode]
+
+encode : Str -> Result Str [BadUtf8 _ _]
+encode = \string ->
+    appendCountAndLetter = \state ->
+        if state.count == 0 then
+            []
+        else if state.count == 1 then
+            state.chars |> List.append state.lastChar
+        else
+            digits = state.count |> Num.toStr |> Str.toUtf8
+            state.chars |> List.concat digits |> List.append state.lastChar
+
+    string
+    |> Str.toUtf8
+    |> List.walk { chars: [], lastChar: 0, count: 0 } \state, char ->
+        if state.count == 0 then
+            { chars: [], lastChar: char, count: 1 }
+        else if state.lastChar == char then
+            { state & count: state.count + 1 }
+        else
+            chars = appendCountAndLetter state
+            { chars, lastChar: char, count: 1 }
+    |> \state -> appendCountAndLetter state
+    |> Str.fromUtf8
+
+decode : Str -> Result Str [BadUtf8 _ _, InvalidNumStr]
+decode = \string ->
+    string
+        |> Str.toUtf8
+        |> List.walkTry? { chars: [], digits: [] } \state, char ->
+            if char >= '0' && char <= '9' then
+                digits = state.digits |> List.append char
+                Ok { state & digits }
+            else if state.digits == [] then
+                chars = state.chars |> List.append char
+                Ok { state & chars }
+            else
+                countStr = Str.fromUtf8? state.digits
+                count = Str.toU64? countStr
+                chars = state.chars |> List.concat (List.repeat char count)
+                Ok { chars, digits: [] }
+        |> .chars
+        |> Str.fromUtf8

exercises/practice/run-length-encoding/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ageron"
+  ],
+  "files": {
+    "solution": [
+      "RunLengthEncoding.roc"
+    ],
+    "test": [
+      "run-length-encoding-test.roc"
+    ],
+    "example": [
+      ".meta/Example.roc"
+    ]
+  },
+  "blurb": "Implement run-length encoding and decoding.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Run-length_encoding"
+}

exercises/practice/run-length-encoding/.meta/template.j2

@@ -0,0 +1,27 @@
+{%- import "generator_macros.j2" as macros with context -%}
+{{ macros.canonical_ref() }}
+{{ macros.header() }}
+
+import {{ exercise | to_pascal }} exposing [encode, decode]
+
+{% for supercase in cases %}
+##
+## {{ supercase["description"] }}
+##
+
+{% for case in supercase["cases"] -%}
+# {{ case["description"] }}
+expect
+    string = {{ case["input"]["string"] | to_roc }} 
+    {%- if case["property"] == "consistency" %}
+    result = string |> encode |> Result.try decode
+    result == Ok string
+{%- else %}
+    result = string |> {{ case["property"] | to_camel }}
+    expected = {{ case["expected"] | to_roc }} 
+    result == Ok expected
+{%- endif %}
+
+{% endfor %}
+{% endfor %}
+

exercises/practice/run-length-encoding/.meta/tests.toml

@@ -0,0 +1,49 @@
+# 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.
+
+[ad53b61b-6ffc-422f-81a6-61f7df92a231]
+description = "run-length encode a string -> empty string"
+
+[52012823-b7e6-4277-893c-5b96d42f82de]
+description = "run-length encode a string -> single characters only are encoded without count"
+
+[b7868492-7e3a-415f-8da3-d88f51f80409]
+description = "run-length encode a string -> string with no single characters"
+
+[859b822b-6e9f-44d6-9c46-6091ee6ae358]
+description = "run-length encode a string -> single characters mixed with repeated characters"
+
+[1b34de62-e152-47be-bc88-469746df63b3]
+description = "run-length encode a string -> multiple whitespace mixed in string"
+
+[abf176e2-3fbd-40ad-bb2f-2dd6d4df721a]
+description = "run-length encode a string -> lowercase characters"
+
+[7ec5c390-f03c-4acf-ac29-5f65861cdeb5]
+description = "run-length decode a string -> empty string"
+
+[ad23f455-1ac2-4b0e-87d0-b85b10696098]
+description = "run-length decode a string -> single characters only"
+
+[21e37583-5a20-4a0e-826c-3dee2c375f54]
+description = "run-length decode a string -> string with no single characters"
+
+[1389ad09-c3a8-4813-9324-99363fba429c]
+description = "run-length decode a string -> single characters with repeated characters"
+
+[3f8e3c51-6aca-4670-b86c-a213bf4706b0]
+description = "run-length decode a string -> multiple whitespace mixed in string"
+
+[29f721de-9aad-435f-ba37-7662df4fb551]
+description = "run-length decode a string -> lowercase string"
+
+[2a762efd-8695-4e04-b0d6-9736899fbc16]
+description = "encode and then decode -> encode followed by decode gives original string"

exercises/practice/run-length-encoding/RunLengthEncoding.roc

@@ -0,0 +1,9 @@
+module [encode, decode]
+
+encode : Str -> Result Str _
+encode = \string ->
+    crash "Please implement the 'encode' function"
+
+decode : Str -> Result Str _
+decode = \string ->
+    crash "Please implement the 'decode' function"

exercises/practice/run-length-encoding/run-length-encoding-test.roc

@@ -0,0 +1,114 @@
+# These tests are auto-generated with test data from:
+# https://github.com/exercism/problem-specifications/tree/main/exercises/run-length-encoding/canonical-data.json
+# File last updated on 2024-10-10
+app [main] {
+    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.15.0/SlwdbJ-3GR7uBWQo6zlmYWNYOxnvo8r6YABXD-45UOw.tar.br",
+}
+
+main =
+    Task.ok {}
+
+import RunLengthEncoding exposing [encode, decode]
+
+##
+## run-length encode a string
+##
+
+# empty string
+expect
+    string = ""
+    result = string |> encode
+    expected = ""
+    result == Ok expected
+
+# single characters only are encoded without count
+expect
+    string = "XYZ"
+    result = string |> encode
+    expected = "XYZ"
+    result == Ok expected
+
+# string with no single characters
+expect
+    string = "AABBBCCCC"
+    result = string |> encode
+    expected = "2A3B4C"
+    result == Ok expected
+
+# single characters mixed with repeated characters
+expect
+    string = "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"
+    result = string |> encode
+    expected = "12WB12W3B24WB"
+    result == Ok expected
+
+# multiple whitespace mixed in string
+expect
+    string = "  hsqq qww  "
+    result = string |> encode
+    expected = "2 hs2q q2w2 "
+    result == Ok expected
+
+# lowercase characters
+expect
+    string = "aabbbcccc"
+    result = string |> encode
+    expected = "2a3b4c"
+    result == Ok expected
+
+##
+## run-length decode a string
+##
+
+# empty string
+expect
+    string = ""
+    result = string |> decode
+    expected = ""
+    result == Ok expected
+
+# single characters only
+expect
+    string = "XYZ"
+    result = string |> decode
+    expected = "XYZ"
+    result == Ok expected
+
+# string with no single characters
+expect
+    string = "2A3B4C"
+    result = string |> decode
+    expected = "AABBBCCCC"
+    result == Ok expected
+
+# single characters with repeated characters
+expect
+    string = "12WB12W3B24WB"
+    result = string |> decode
+    expected = "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWB"
+    result == Ok expected
+
+# multiple whitespace mixed in string
+expect
+    string = "2 hs2q q2w2 "
+    result = string |> decode
+    expected = "  hsqq qww  "
+    result == Ok expected
+
+# lowercase string
+expect
+    string = "2a3b4c"
+    result = string |> decode
+    expected = "aabbbcccc"
+    result == Ok expected
+
+##
+## encode and then decode
+##
+
+# encode followed by decode gives original string
+expect
+    string = "zzz ZZ  zZ"
+    result = string |> encode |> Result.try decode
+    result == Ok string
+