Add rail-fence-cipher exercise

config.json

@@ -704,6 +704,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 3
+      },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "e4b63b53-d6ca-4a1d-8e02-e295bd2c9c65",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

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": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "rail-fence-cipher.R"
+    ],
+    "test": [
+      "test_rail-fence-cipher.R"
+    ],
+    "example": [
+      ".meta/example.R"
+    ]
+  },
+  "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.R

@@ -0,0 +1,19 @@
+encode <- function(rails, plaintext) {
+  transform(rails, plaintext, \(i, fence) fence[i])
+}
+
+decode <- function(rails, ciphertext) {
+  transform(rails, ciphertext, \(i, fence) match(i, fence))
+}
+
+transform <- function(rails, text, char_index) {
+  chars <- strsplit(text, "") |> unlist()
+  fence <- rail_fence(rails, length(chars))
+  indices <- sapply(seq_along(chars), \(i) char_index(i, fence))
+  chars[indices] |> paste0(collapse = "")
+}
+
+rail_fence <- function(rails, num_chars) {
+  zigzag <- c(seq_len(rails), seq(rails - 1, 2))
+  rep(zigzag, length.out = num_chars) |> order()
+}

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.R

@@ -0,0 +1,7 @@
+encode <- function(rails, plaintext) {
+
+}
+
+decode <- function(rails, ciphertext) {
+
+}

exercises/practice/rail-fence-cipher/test_rail-fence-cipher.R

@@ -0,0 +1,44 @@
+source("./rail-fence-cipher.R")
+library(testthat)
+
+test_that("Encode with two rails", {
+  rails <- 2
+  plaintext <- "XOXOXOXOXOXOXOXOXO"
+  expected <- "XXXXXXXXXOOOOOOOOO"
+  expect_equal(encode(rails, plaintext), expected)
+})
+
+test_that("Encode with three rails", {
+  rails <- 3
+  plaintext <- "WEAREDISCOVEREDFLEEATONCE"
+  expected <- "WECRLTEERDSOEEFEAOCAIVDEN"
+  expect_equal(encode(rails, plaintext), expected)
+})
+
+test_that("Encode with ending in the middle", {
+  rails <- 4
+  plaintext <- "EXERCISES"
+  expected <- "ESXIEECSR"
+  expect_equal(encode(rails, plaintext), expected)
+})
+
+test_that("Decode with three rails", {
+  rails <- 3
+  ciphertext <- "TEITELHDVLSNHDTISEIIEA"
+  expected <- "THEDEVILISINTHEDETAILS"
+  expect_equal(decode(rails, ciphertext), expected)
+})
+
+test_that("Decode with five rails", {
+  rails <- 5
+  ciphertext <- "EIEXMSMESAORIWSCE"
+  expected <- "EXERCISMISAWESOME"
+  expect_equal(decode(rails, ciphertext), expected)
+})
+
+test_that("Decode with six rails", {
+  rails <- 6
+  ciphertext <- "133714114238148966225439541018335470986172518171757571896261"
+  expected <- "112358132134558914423337761098715972584418167651094617711286"
+  expect_equal(decode(rails, ciphertext), expected)
+})