[New Exercise]: Rail Fence Cipher

config.json

@@ -736,6 +736,13 @@
         ],
         "prerequisites": [],
         "difficulty": 4
+      },{
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "33012310-6494-47ff-a29f-6cc168fe12b4",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
       },
       {
         "slug": "all-your-base",

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/RailFenceCipher.example.ps1

@@ -0,0 +1,101 @@
+#Adapt from the awesome solution:
+#https://exercism.org/tracks/python/exercises/rail-fence-cipher/solutions/tcarobruce
+Function Invoke-Encode() {
+    <#
+    .SYNOPSIS
+    Implement encoding for the rail fence cipher.
+
+    .DESCRIPTION
+    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.
+
+    .PARAMETER Message
+    The message to be encoded.
+
+    .PARAMETER Rails
+    The number of rails to be use to construct the rail fence.
+
+    .EXAMPLE
+    Invoke-Encode -Message "EXERCISM" -Rails 2
+    Returns: "EECSXRIM"
+    #>
+    [CmdletBinding()]
+    Param(
+        [string]$Message,
+        [int]$Rails
+    )
+    $pattern = RailPattern -Text $Message -Number $Rails
+    $zipped  = Zipper -Values $Message.ToCharArray() -Indexes $pattern
+
+    $encoded = $zipped | Sort-Object -Property Index -Stable | ForEach-Object {$_.Value} 
+    -join $encoded
+}
+
+Function Invoke-Decode() {
+    <#
+    .SYNOPSIS
+    Implement decoding for the rail fence cipher.
+
+    .DESCRIPTION
+
+    .PARAMETER CipherText
+    The ciphertext to be decoded.
+
+    .PARAMETER Rails
+    The number of rails to be use to construct the rail fence.
+
+    .EXAMPLE
+    Invoke-Decode -Ciphertext "EECSXRIM" -Rails 2
+    Returns: "EXERCISM"
+    #>
+    [CmdletBinding()]
+    Param(
+        [string]$Ciphertext,
+        [int]$Rails
+    )
+    $pattern = RailPattern -Text $Ciphertext -Number $Rails
+    # sorted index based on pattern
+    $indexes = (Zipper -Values (0..($Ciphertext.Length-1)) -Indexes $pattern) | Sort-Object -Property Index -Stable | ForEach-Object {$_.Value}
+
+    $decode  = @($null) * $Ciphertext.Length
+    for ($i = 0; $i -lt $Ciphertext.Length; $i++) {
+        $decode[$indexes[$i]] = $Ciphertext[$i]
+    }
+    -join $decode
+}
+
+function RailPattern() {
+    <#
+    .DESCRIPTION
+    Helper function to get the pattern of the rail fence. This will match the full length of the input text.
+    For example: 3 rails will yield: 0,1,2,1,0...
+    #>
+    param (
+        [string]$Text,
+        [int]$Number
+    )
+    $pattern = 0..($Number-1) + ($Number-2)..1
+
+    #get the pattern to match the text
+    $fullPattern = $pattern * [math]::Ceiling($Text.Length/$pattern.Count)       
+    $fullPattern[0..($Text.Length-1)]
+}
+
+function Zipper() {
+    <#
+    .DESCRIPTION
+    Helper function act as a zipper for two collections.
+    Return one collection with custom objects
+    #>
+    param (
+        [object[]]$Values,
+        [object[]]$Indexes
+    )
+    $zipped =  0..($Values.Count-1) | ForEach-Object {
+        [PSCustomObject]@{
+            Value = $Values[$_]
+            Index = $Indexes[$_]
+        }
+    }
+    $zipped
+} 

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

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "glaxxie"
+  ],
+  "files": {
+    "solution": [
+      "RailFenceCipher.ps1"
+    ],
+    "test": [
+      "RailFenceCipher.tests.ps1"
+    ],
+    "example": [
+      ".meta/RailFenceCipher.example.ps1"
+    ]
+  },
+  "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/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.ps1

@@ -0,0 +1,53 @@
+Function Invoke-Encode() {
+    <#
+    .SYNOPSIS
+    Implement encoding for the rail fence cipher.
+
+    .DESCRIPTION
+    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.
+
+    .PARAMETER Message
+    The message to be encoded.
+
+    .PARAMETER Rails
+    The number of rails to be use to construct the rail fence.
+
+    .EXAMPLE
+    Invoke-Encode -Message "EXERCISM" -Rails 2
+    Returns: "EECSXRIM"
+    #>
+    [CmdletBinding()]
+    Param(
+        [string]$Message,
+        [int]$Rails
+    )
+
+    Throw "Please implement this function"
+}
+
+Function Invoke-Decode() {
+    <#
+    .SYNOPSIS
+    Implement decoding for the rail fence cipher.
+
+    .DESCRIPTION
+
+    .PARAMETER CipherText
+    The ciphertext to be decoded.
+
+    .PARAMETER Rails
+    The number of rails to be use to construct the rail fence.
+
+    .EXAMPLE
+    Invoke-Decode -Ciphertext "EECSXRIM" -Rails 2
+    Returns: "EXERCISM"
+    #>
+    [CmdletBinding()]
+    Param(
+        [string]$Ciphertext,
+        [int]$Rails
+    )
+
+    Throw "Please implement this function"
+}

exercises/practice/rail-fence-cipher/RailFenceCipher.tests.ps1

@@ -0,0 +1,51 @@
+BeforeAll {
+    . "./RailFenceCipher.ps1"
+}
+
+Describe "RailFenceCipher test cases" {
+    Context "encoding" {
+        It "encode with two rails" {
+            $got  = Invoke-Encode -Message "XOXOXOXOXOXOXOXOXO" -Rails 2
+            $want = "XXXXXXXXXOOOOOOOOO"
+
+            $got | Should -BeExactly $want
+        }
+
+        It "encode with three rails" {
+            $got  = Invoke-Encode -Message "WEAREDISCOVEREDFLEEATONCE" -Rails 3
+            $want = "WECRLTEERDSOEEFEAOCAIVDEN"
+
+            $got | Should -BeExactly $want
+        }
+
+        It "encode with ending in the middle" {
+            $got  = Invoke-Encode -Message "EXERCISES" -Rails 4
+            $want = "ESXIEECSR"
+
+            $got | Should -BeExactly $want
+        }
+    }
+
+    Context "decoding" {
+        It "decode with three rails" {
+            $got  = Invoke-Decode -Ciphertext "TEITELHDVLSNHDTISEIIEA" -Rails 3
+            $want = "THEDEVILISINTHEDETAILS"
+
+            $got | Should -BeExactly $want
+        }
+
+        It "decode with five rails" {
+            $got  = Invoke-Decode -Ciphertext "EIEXMSMESAORIWSCE" -Rails 5
+            $want = "EXERCISMISAWESOME"
+
+            $got | Should -BeExactly $want
+        }
+
+        It "decode with six rails" {
+            $got  = Invoke-Decode -Ciphertext "133714114238148966225439541018335470986172518171757571896261" -Rails 6
+            $want = "112358132134558914423337761098715972584418167651094617711286"
+
+            $got | Should -BeExactly $want
+        }
+    }
+}