Add rail-fence-cipher

config.json

@@ -826,6 +826,14 @@
         "prerequisites": [],
         "difficulty": 6
       },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "0dfe9492-7349-456a-a4fd-afde1383c393",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
+      },
       {
         "slug": "say",
         "name": "Say",

exercises/practice/rail-fence-cipher/.busted

@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}

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/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "rail_fence_cipher.moon"
+    ],
+    "test": [
+      "rail_fence_cipher_spec.moon"
+    ],
+    "example": [
+      ".meta/example.moon"
+    ]
+  },
+  "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/example.moon

@@ -0,0 +1,39 @@
+rail_iter = (n) ->
+  coroutine.wrap ->
+    while true
+      coroutine.yield i for i = 1, n-1
+      coroutine.yield i for i = n, 2, -1
+
+{
+  encode: (n, plaintext) ->
+    rails = [{} for _ = 1, n]
+    next_rail = rail_iter n
+    for char in plaintext\gmatch '.'
+      r = next_rail!
+      table.insert rails[r], char
+
+    table.concat [table.concat rail for rail in *rails]
+
+  decode: (n, ciphertext) ->
+    len = #ciphertext
+
+    -- find the length of each rail
+    cycle_length = 2*(n - 1)
+    cycles = len // cycle_length
+    lengths = [cycles * ((i == 1 or i == n) and 1 or 2) for i = 1, n]
+    -- account for the last partial cycle
+    next_rail = rail_iter n
+    for i = 1, len - cycles * cycle_length
+      lengths[next_rail!] += 1
+
+    -- extract the rails from the ciphertext
+    rails = {}
+    start = 1
+    for r = 1, n
+      chunk = ciphertext\sub start, start + lengths[r] - 1
+      rails[r] = [c for c in chunk\gmatch '.']
+      start += lengths[r]
+
+    next_rail = rail_iter n
+    table.concat [table.remove rails[next_rail!], 1 for _ = 1, len]
+}

exercises/practice/rail-fence-cipher/.meta/spec_generator.moon

@@ -0,0 +1,11 @@
+{
+  module_name: 'RailFenceCipher',
+
+  generate_test: (case, level) ->
+    lines = {
+      "result = RailFenceCipher.#{case.property} #{case.input.rails}, #{quote case.input.msg}",
+      "expected = #{quote case.expected}",
+      "assert.are.equal expected, result"
+    }
+    table.concat [indent line, level for line in *lines], '\n'
+}

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/rail_fence_cipher.moon

@@ -0,0 +1,7 @@
+{
+  encode: (n, plaintext) ->
+    error 'Implement me'
+
+  decode: (n, ciphertext) ->
+    error 'Implement me'
+}

exercises/practice/rail-fence-cipher/rail_fence_cipher_spec.moon

@@ -0,0 +1,34 @@
+RailFenceCipher = require 'rail_fence_cipher'
+
+describe 'rail-fence-cipher', ->
+  describe 'encode', ->
+    it 'encode with two rails', ->
+      result = RailFenceCipher.encode 2, 'XOXOXOXOXOXOXOXOXO'
+      expected = 'XXXXXXXXXOOOOOOOOO'
+      assert.are.equal expected, result
+
+    pending 'encode with three rails', ->
+      result = RailFenceCipher.encode 3, 'WEAREDISCOVEREDFLEEATONCE'
+      expected = 'WECRLTEERDSOEEFEAOCAIVDEN'
+      assert.are.equal expected, result
+
+    pending 'encode with ending in the middle', ->
+      result = RailFenceCipher.encode 4, 'EXERCISES'
+      expected = 'ESXIEECSR'
+      assert.are.equal expected, result
+
+  describe 'decode', ->
+    pending 'decode with three rails', ->
+      result = RailFenceCipher.decode 3, 'TEITELHDVLSNHDTISEIIEA'
+      expected = 'THEDEVILISINTHEDETAILS'
+      assert.are.equal expected, result
+
+    pending 'decode with five rails', ->
+      result = RailFenceCipher.decode 5, 'EIEXMSMESAORIWSCE'
+      expected = 'EXERCISMISAWESOME'
+      assert.are.equal expected, result
+
+    pending 'decode with six rails', ->
+      result = RailFenceCipher.decode 6, '133714114238148966225439541018335470986172518171757571896261'
+      expected = '112358132134558914423337761098715972584418167651094617711286'
+      assert.are.equal expected, result