Add rail-fence-cipher exercise (#59)

Author: keiravillekode

config.json

@@ -413,6 +413,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 7
+      },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "4e80fefa-8209-43c4-9740-cf8e54b9aecd",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 7
       }
     ]
   },

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": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "rail_fence_cipher.fut"
+    ],
+    "test": [
+      "test.fut"
+    ],
+    "example": [
+      ".meta/example.fut"
+    ]
+  },
+  "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.fut

@@ -0,0 +1,33 @@
+def rail_lengths (rails: i64) (n: i64): [rails]i64 =
+  let (lengths, _, _) = loop (lengths, rail, direction) = (replicate rails 0, 0, 1) for _ in 0..<n do
+    let rail2 = rail + direction
+    let direction2 = if rail2 > 0 && rail2 + 1 < rails then direction else -direction
+    in
+      (lengths with [rail] = lengths[rail] + 1, rail2, direction2)
+  in
+    lengths
+
+def rail_offsets (rails: i64) (n: i64): [rails]i64 =
+  let lengths = rail_lengths rails n
+  let (offsets, _) = loop (offsets, acc) = (replicate rails 0, 0) for rail in 0..<rails do
+    (offsets with [rail] = acc, acc + lengths[rail])
+  in
+    offsets
+
+def process [n] (msg: [n]u8) (rails: i64) (is_decode: bool): [n]u8 =
+  let (result, _, _, _) = loop (result, offsets, rail, direction) = (replicate n 0u8, rail_offsets rails n, 0, 1) for i in 0..<n do
+    let rail2 = rail + direction
+    let direction2 = if rail2 > 0 && rail2 + 1 < rails then direction else -direction
+    let offset = offsets[rail]
+    let offsets2 = offsets with [rail] = offset + 1
+    let result2 = if is_decode then result with [i] = msg[offset] else result with [offset] = msg[i]
+    in
+      (result2, offsets2, rail2, direction2)
+  in
+    result
+
+def encode (msg: []u8) (rails: i64): []u8 =
+  process msg rails false
+
+def decode (msg: []u8) (rails: i64): []u8 =
+  process msg rails true

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.fut

@@ -0,0 +1,3 @@
+def encode (msg: []u8) (rails: i64): []u8 = ???
+
+def decode (msg: []u8) (rails: i64): []u8 = ???

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

@@ -0,0 +1,43 @@
+import "rail_fence_cipher"
+
+-- encode with two rails
+-- ==
+-- entry: test_encode
+-- input  { "XOXOXOXOXOXOXOXOXO" 2i64 }
+-- output { "XXXXXXXXXOOOOOOOOO" }
+
+-- encode with three rails
+-- ==
+-- entry: test_encode
+-- input  { "WEAREDISCOVEREDFLEEATONCE" 3i64 }
+-- output { "WECRLTEERDSOEEFEAOCAIVDEN" }
+
+-- encode with ending in the middle
+-- ==
+-- entry: test_encode
+-- input  { "EXERCISES" 4i64 }
+-- output { "ESXIEECSR" }
+
+-- decode with three rails
+-- ==
+-- entry: test_decode
+-- input  { "TEITELHDVLSNHDTISEIIEA" 3i64 }
+-- output { "THEDEVILISINTHEDETAILS" }
+
+-- decode with five rails
+-- ==
+-- entry: test_decode
+-- input  { "EIEXMSMESAORIWSCE" 5i64 }
+-- output { "EXERCISMISAWESOME" }
+
+-- decode with six rails
+-- ==
+-- entry: test_decode
+-- input  { "133714114238148966225439541018335470986172518171757571896261" 6i64 }
+-- output { "112358132134558914423337761098715972584418167651094617711286" }
+
+entry test_encode (msg: []u8) (rails: i64): []u8 =
+  encode msg rails
+
+entry test_decode (msg: []u8) (rails: i64): []u8 =
+  decode msg rails