Add rail-fence-cipher exercise (#165)

config.json

@@ -683,6 +683,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "d4569bd5-b6f3-4f2e-bdc0-0c6122540800",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "acronym",
         "name": "Acronym",

exercises/practice/rail-fence-cipher/.docs/instructions.md

@@ -0,0 +1,57 @@
+# Instructions
+
+Implement encoding and decoding for the rail fence cipher.
+
+The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded.
+It was already used by the ancient Greeks.
+
+In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag).
+Finally the message is then read off in rows.
+
+For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . A . . . I . . . V . . . D . . . E . . . N . .
+```
+
+Then reads off:
+
+```text
+WECRLTEERDSOEEFEAOCAIVDEN
+```
+
+To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.
+
+```text
+? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+The first row has seven spots that can be filled with "WECRLTE".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Now the 2nd row takes "ERDSOEEFEAOC".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Leaving "AIVDEN" for the last row.
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . A . . . I . . . V . . . D . . . E . . . N . .
+```
+
+If you now read along the zig-zag shape you can read the original message.

exercises/practice/rail-fence-cipher/.meta/Example.roc

@@ -0,0 +1,49 @@
+module [encode, decode]
+
+encodedIndices : U64, U64 -> List U64
+encodedIndices = \len, rails ->
+    indices = List.range { start: At 0, end: Before len }
+    List.range { start: At 0, end: Before rails }
+    |> List.map \rail ->
+        period = 2 * (rails - 1)
+        indices
+        |> List.mapWithIndex \index, originalIndex ->
+            toRail = originalIndex % period
+            if toRail == rail || toRail == period - rail then
+                [index]
+            else
+                []
+        |> List.join
+    |> List.join
+
+decodedIndices : U64, U64 -> List U64
+decodedIndices = \len, rails ->
+    encodedIndices len rails
+    |> List.mapWithIndex \encoded, decoded -> { encoded, decoded }
+    |> List.sortWith \{ encoded: encoded1 }, { encoded: encoded2 } ->
+        Num.compare encoded1 encoded2
+    |> List.map .decoded
+
+encode : Str, U64 -> Result Str [ZeroRails, BadUtf8 _ _]
+encode = \message, rails ->
+    message |> reorderWith encodedIndices rails
+
+decode : Str, U64 -> Result Str [ZeroRails, BadUtf8 _ _]
+decode = \encrypted, rails ->
+    encrypted |> reorderWith decodedIndices rails
+
+reorderWith : Str, (U64, U64 -> List U64), U64 -> Result Str [ZeroRails, BadUtf8 _ _]
+reorderWith = \message, getIndices, rails ->
+    if rails == 0 then
+        Err ZeroRails
+    else if rails == 1 then
+        Ok message
+        else
+
+    chars = message |> Str.toUtf8
+    result =
+        getIndices (List.len chars) rails
+        |> List.mapTry \index -> chars |> List.get index
+    when result is
+        Ok encryptedChars -> encryptedChars |> Str.fromUtf8
+        Err OutOfBounds -> crash "Unreachable: indices cannot be out of bounds here"

exercises/practice/rail-fence-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ageron"
+  ],
+  "files": {
+    "solution": [
+      "RailFenceCipher.roc"
+    ],
+    "test": [
+      "rail-fence-cipher-test.roc"
+    ],
+    "example": [
+      ".meta/Example.roc"
+    ]
+  },
+  "blurb": "Implement encoding and decoding for the rail fence cipher.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher"
+}

exercises/practice/rail-fence-cipher/.meta/template.j2

@@ -0,0 +1,21 @@
+{%- 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
+    message = {{ case["input"]["msg"] | to_roc }}
+    result = message |> {{ case["property"] | to_camel }} {{ case["input"]["rails"] | to_roc }}
+    expected = Ok {{ case["expected"] | to_roc }} 
+    result == expected
+
+{% endfor %}
+{% endfor %}

exercises/practice/rail-fence-cipher/.meta/tests.toml

@@ -0,0 +1,28 @@
+# 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.
+
+[46dc5c50-5538-401d-93a5-41102680d068]
+description = "encode -> encode with two rails"
+
+[25691697-fbd8-4278-8c38-b84068b7bc29]
+description = "encode -> encode with three rails"
+
+[384f0fea-1442-4f1a-a7c4-5cbc2044002c]
+description = "encode -> encode with ending in the middle"
+
+[cd525b17-ec34-45ef-8f0e-4f27c24a7127]
+description = "decode -> decode with three rails"
+
+[dd7b4a98-1a52-4e5c-9499-cbb117833507]
+description = "decode -> decode with five rails"
+
+[93e1ecf4-fac9-45d9-9cd2-591f47d3b8d3]
+description = "decode -> decode with six rails"

exercises/practice/rail-fence-cipher/RailFenceCipher.roc

@@ -0,0 +1,9 @@
+module [encode, decode]
+
+encode : Str, U64 -> Result Str _
+encode = \message, rails ->
+    crash "Please implement the 'encode' function"
+
+decode : Str, U64 -> Result Str _
+decode = \encrypted, rails ->
+    crash "Please implement the 'decode' function"

exercises/practice/rail-fence-cipher/rail-fence-cipher-test.roc

@@ -0,0 +1,62 @@
+# These tests are auto-generated with test data from:
+# https://github.com/exercism/problem-specifications/tree/main/exercises/rail-fence-cipher/canonical-data.json
+# File last updated on 2024-10-23
+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 RailFenceCipher exposing [encode, decode]
+
+##
+## encode
+##
+
+# encode with two rails
+expect
+    message = "XOXOXOXOXOXOXOXOXO"
+    result = message |> encode 2
+    expected = Ok "XXXXXXXXXOOOOOOOOO"
+    result == expected
+
+# encode with three rails
+expect
+    message = "WEAREDISCOVEREDFLEEATONCE"
+    result = message |> encode 3
+    expected = Ok "WECRLTEERDSOEEFEAOCAIVDEN"
+    result == expected
+
+# encode with ending in the middle
+expect
+    message = "EXERCISES"
+    result = message |> encode 4
+    expected = Ok "ESXIEECSR"
+    result == expected
+
+##
+## decode
+##
+
+# decode with three rails
+expect
+    message = "TEITELHDVLSNHDTISEIIEA"
+    result = message |> decode 3
+    expected = Ok "THEDEVILISINTHEDETAILS"
+    result == expected
+
+# decode with five rails
+expect
+    message = "EIEXMSMESAORIWSCE"
+    result = message |> decode 5
+    expected = Ok "EXERCISMISAWESOME"
+    result == expected
+
+# decode with six rails
+expect
+    message = "133714114238148966225439541018335470986172518171757571896261"
+    result = message |> decode 6
+    expected = Ok "112358132134558914423337761098715972584418167651094617711286"
+    result == expected
+