Add atbash-cipher exercise (#356)

config.json

@@ -705,6 +705,14 @@
         "prerequisites": [],
         "difficulty": 3
       },
+      {
+        "slug": "atbash-cipher",
+        "name": "Atbash Cipher",
+        "uuid": "81133e3d-4feb-4985-8852-3354e8f72847",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3
+      },
       {
         "slug": "rail-fence-cipher",
         "name": "Rail Fence Cipher",

exercises/practice/atbash-cipher/.docs/instructions.md

@@ -0,0 +1,27 @@
+# Instructions
+
+Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
+
+The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards.
+The first letter is replaced with the last letter, the second with the second-last, and so on.
+
+An Atbash cipher for the Latin alphabet would be as follows:
+
+```text
+Plain:  abcdefghijklmnopqrstuvwxyz
+Cipher: zyxwvutsrqponmlkjihgfedcba
+```
+
+It is a very weak cipher because it only has one possible key, and it is a simple mono-alphabetic substitution cipher.
+However, this may not have been an issue in the cipher's time.
+
+Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, leaving numbers unchanged, and punctuation is excluded.
+This is to make it harder to guess things based on word boundaries.
+All text will be encoded as lowercase letters.
+
+## Examples
+
+- Encoding `test` gives `gvhg`
+- Encoding `x123 yes` gives `c123b vh`
+- Decoding `gvhg` gives `test`
+- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`

exercises/practice/atbash-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "atbash-cipher.R"
+    ],
+    "test": [
+      "test_atbash-cipher.R"
+    ],
+    "example": [
+      ".meta/example.R"
+    ]
+  },
+  "blurb": "Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Atbash"
+}

exercises/practice/atbash-cipher/.meta/example.R

@@ -0,0 +1,12 @@
+mapping <- setNames(c(letters, 0:9), c(rev(letters), 0:9))
+
+encode <- function(plaintext) {
+  chars <- plaintext |> tolower() |> strsplit("") |> unlist()
+  mapped <- lapply(chars, \(c) mapping[c]) |> unlist() |> na.omit()
+  grouped <- split(mapped, ceiling(seq_along(mapped) / 5))
+  paste0(sapply(grouped, paste0, collapse = ""), collapse = " ")
+}
+
+decode <- function(ciphertext) {
+  encode(ciphertext) |> gsub(pattern = " ", replacement = "")
+}

exercises/practice/atbash-cipher/.meta/tests.toml

@@ -0,0 +1,52 @@
+# 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.
+
+[2f47ebe1-eab9-4d6b-b3c6-627562a31c77]
+description = "encode -> encode yes"
+
+[b4ffe781-ea81-4b74-b268-cc58ba21c739]
+description = "encode -> encode no"
+
+[10e48927-24ab-4c4d-9d3f-3067724ace00]
+description = "encode -> encode OMG"
+
+[d59b8bc3-509a-4a9a-834c-6f501b98750b]
+description = "encode -> encode spaces"
+
+[31d44b11-81b7-4a94-8b43-4af6a2449429]
+description = "encode -> encode mindblowingly"
+
+[d503361a-1433-48c0-aae0-d41b5baa33ff]
+description = "encode -> encode numbers"
+
+[79c8a2d5-0772-42d4-b41b-531d0b5da926]
+description = "encode -> encode deep thought"
+
+[9ca13d23-d32a-4967-a1fd-6100b8742bab]
+description = "encode -> encode all the letters"
+
+[bb50e087-7fdf-48e7-9223-284fe7e69851]
+description = "decode -> decode exercism"
+
+[ac021097-cd5d-4717-8907-b0814b9e292c]
+description = "decode -> decode a sentence"
+
+[18729de3-de74-49b8-b68c-025eaf77f851]
+description = "decode -> decode numbers"
+
+[0f30325f-f53b-415d-ad3e-a7a4f63de034]
+description = "decode -> decode all the letters"
+
+[39640287-30c6-4c8c-9bac-9d613d1a5674]
+description = "decode -> decode with too many spaces"
+
+[b34edf13-34c0-49b5-aa21-0768928000d5]
+description = "decode -> decode with no spaces"

exercises/practice/atbash-cipher/atbash-cipher.R

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

exercises/practice/atbash-cipher/test_atbash-cipher.R

@@ -0,0 +1,86 @@
+source("./atbash-cipher.R")
+library(testthat)
+
+test_that("Encode yes", {
+  plaintext <- "yes"
+  expected <- "bvh"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode no", {
+  plaintext <- "no"
+  expected <- "ml"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode OMG", {
+  plaintext <- "OMG"
+  expected <- "lnt"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode spaces", {
+  plaintext <- "O M G"
+  expected <- "lnt"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode mindblowingly", {
+  plaintext <- "mindblowingly"
+  expected <- "nrmwy oldrm tob"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode numbers", {
+  plaintext <- "Testing,1 2 3, testing."
+  expected <- "gvhgr mt123 gvhgr mt"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode deep thought", {
+  plaintext <- "Truth is fiction."
+  expected <- "gifgs rhurx grlm"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Encode all the letters", {
+  plaintext <- "The quick brown fox jumps over the lazy dog."
+  expected <- "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt"
+  expect_equal(encode(plaintext), expected)
+})
+
+test_that("Decode exercism", {
+  ciphertext <- "vcvix rhn"
+  expected <- "exercism"
+  expect_equal(decode(ciphertext), expected)
+})
+
+test_that("Decode a sentence", {
+  ciphertext <- "zmlyh gzxov rhlug vmzhg vkkrm thglm v"
+  expected <- "anobstacleisoftenasteppingstone"
+  expect_equal(decode(ciphertext), expected)
+})
+
+test_that("Decode numbers", {
+  ciphertext <- "gvhgr mt123 gvhgr mt"
+  expected <- "testing123testing"
+  expect_equal(decode(ciphertext), expected)
+})
+
+test_that("Decode all the letters", {
+  ciphertext <- "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt"
+  expected <- "thequickbrownfoxjumpsoverthelazydog"
+  expect_equal(decode(ciphertext), expected)
+})
+
+test_that("Decode with too many spaces", {
+  ciphertext <- "vc vix    r hn"
+  expected <- "exercism"
+  expect_equal(decode(ciphertext), expected)
+})
+
+test_that("Decode with no spaces", {
+  ciphertext <- "zmlyhgzxovrhlugvmzhgvkkrmthglmv"
+  expected <- "anobstacleisoftenasteppingstone"
+  expect_equal(decode(ciphertext), expected)
+})