[New Exercise] Add rail fence cipher exercise

config.json

@@ -1181,6 +1181,19 @@
           "transforming"
         ]
       },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "f78800c1-8e89-4aac-83a4-3988e276225f",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5,
+        "topics": [
+          "algorithms",
+          "strings",
+          "arrays"
+        ]
+      },
       {
         "slug": "bowling",
         "name": "Bowling",

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/Sources/RailFenceCipher/RailFenceCipherExample.swift

@@ -0,0 +1,39 @@
+
+func encode(_ message: String, rails: Int) -> String {
+    encodeDecode(message, rails: rails, mode: .encode)
+}
+
+func decode(_ message: String, rails: Int) -> String {
+    encodeDecode(message, rails: rails, mode: .decode)
+}
+
+fileprivate enum Mode {
+    case encode
+    case decode
+}
+
+fileprivate func encodeDecode(_ message: String, rails: Int, mode: Mode) -> String {
+    var output = Array(repeating: Character(" "), count: message.count)
+    let maxStep = 2 * (rails - 1)
+    var posCipher = 0
+    let chars = Array(message)
+
+    for rail in 0..<rails {
+        var step = (rail == 0) ? maxStep : 2 * rail
+        var posText = rail
+
+        while posText < chars.count {
+            if mode == .encode {
+                output[posCipher] = chars[posText]
+            } else {
+                output[posText] = chars[posCipher]
+            }
+
+            posCipher += 1
+            step = (step == maxStep) ? maxStep : maxStep - step
+            posText += step
+        }
+    }
+
+    return String(output)
+}

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

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "Sencudra"
+  ],
+  "files": {
+    "solution": [
+      "Sources/RailFenceCipher/RailFenceCipher.swift"
+    ],
+    "test": [
+      "Tests/RailFenceCipherTests/RailFenceCipherTests.swift"
+    ],
+    "example": [
+      ".meta/Sources/RailFenceCipher/RailFenceCipherExample.swift"
+    ]
+  },
+  "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.swift

@@ -0,0 +1,20 @@
+import Testing
+import Foundation
+@testable import {{exercise | camelCase}}
+
+let RUNALL = Bool(ProcessInfo.processInfo.environment["RUNALL", default: "false"]) ?? false
+
+@Suite struct {{exercise | camelCase}}Tests {
+    {% outer: for case in cases %}
+        {% for subCase in case.cases %}
+        {% if forloop.outer.first and forloop.first %}
+            @Test("{{subCase.description}}")
+        {% else -%}
+            @Test("{{subCase.description}}", .enabled(if: RUNALL))
+        {% endif -%}
+            func test{{subCase.description | camelCase}}() {
+                #expect({{subCase.property}}("{{subCase.input.msg}}", rails: {{subCase.input.rails}}) == "{{subCase.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/Package.swift

@@ -0,0 +1,21 @@
+// swift-tools-version: 6.0
+
+import PackageDescription
+
+let package = Package(
+  name: "RailFenceCipher",
+  products: [
+    .library(
+      name: "RailFenceCipher",
+      targets: ["RailFenceCipher"])
+  ],
+  dependencies: [],
+  targets: [
+    .target(
+      name: "RailFenceCipher",
+      dependencies: []),
+    .testTarget(
+      name: "RailFenceCipherTests",
+      dependencies: ["RailFenceCipher"]),
+  ]
+)

exercises/practice/rail-fence-cipher/Sources/RailFenceCipher/RailFenceCipher.swift

@@ -0,0 +1,2 @@
+
+// Write your solution to the 'RailFenceCipher' exercise in this file.

exercises/practice/rail-fence-cipher/Tests/RailFenceCipherTests/RailFenceCipherTests.swift

@@ -0,0 +1,42 @@
+import Foundation
+import Testing
+
+@testable import RailFenceCipher
+
+let RUNALL = Bool(ProcessInfo.processInfo.environment["RUNALL", default: "false"]) ?? false
+
+@Suite struct RailFenceCipherTests {
+
+  @Test("encode with two rails")
+  func testEncodeWithTwoRails() {
+    #expect(encode("XOXOXOXOXOXOXOXOXO", rails: 2) == "XXXXXXXXXOOOOOOOOO")
+  }
+
+  @Test("encode with three rails", .enabled(if: RUNALL))
+  func testEncodeWithThreeRails() {
+    #expect(encode("WEAREDISCOVEREDFLEEATONCE", rails: 3) == "WECRLTEERDSOEEFEAOCAIVDEN")
+  }
+
+  @Test("encode with ending in the middle", .enabled(if: RUNALL))
+  func testEncodeWithEndingInTheMiddle() {
+    #expect(encode("EXERCISES", rails: 4) == "ESXIEECSR")
+  }
+
+  @Test("decode with three rails", .enabled(if: RUNALL))
+  func testDecodeWithThreeRails() {
+    #expect(decode("TEITELHDVLSNHDTISEIIEA", rails: 3) == "THEDEVILISINTHEDETAILS")
+  }
+
+  @Test("decode with five rails", .enabled(if: RUNALL))
+  func testDecodeWithFiveRails() {
+    #expect(decode("EIEXMSMESAORIWSCE", rails: 5) == "EXERCISMISAWESOME")
+  }
+
+  @Test("decode with six rails", .enabled(if: RUNALL))
+  func testDecodeWithSixRails() {
+    #expect(
+      decode("133714114238148966225439541018335470986172518171757571896261", rails: 6)
+        == "112358132134558914423337761098715972584418167651094617711286")
+  }
+
+}