Add atbash-cipher exercise (#251)

config.json

@@ -462,6 +462,14 @@
           "string_manipulation"
         ]
       },
+      {
+        "slug": "atbash-cipher",
+        "name": "Atbash Cipher",
+        "uuid": "c845c319-60f8-409d-88a6-84a7c529eaf7",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "circular-buffer",
         "name": "Circular Buffer",

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": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "source/atbash_cipher.d"
+    ],
+    "test": [
+      "source/atbash_cipher.d"
+    ],
+    "example": [
+      "example/atbash_cipher.d"
+    ]
+  },
+  "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/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/dub.sdl

@@ -0,0 +1,2 @@
+name "atbash-cipher"
+buildRequirements "disallowDeprecations"

exercises/practice/atbash-cipher/example/atbash_cipher.d

@@ -0,0 +1,38 @@
+module atbash_cipher;
+
+import std.ascii : isAlphaNum, isDigit, toLower;
+
+pure string encode(immutable string phrase)
+{
+    size_t nextSpace = 5;
+    string result;
+    foreach (dchar c; phrase) {
+        if (!isAlphaNum(c)) {
+            continue;
+        }
+        if (result.length == nextSpace) {
+            result ~= ' ';
+            nextSpace += 6;
+        }
+        if (!isDigit(c)) {
+            c = 'a' + 'z' - toLower(c);
+        }
+        result ~= c;
+    }
+    return result;
+}
+
+pure string decode(immutable string phrase)
+{
+    string result;
+    foreach (dchar c; phrase) {
+        if (c == ' ') {
+            continue;
+        }
+        if (!isDigit(c)) {
+            c = 'a' + 'z' - c;
+        }
+        result ~= c;
+    }
+    return result;
+}

exercises/practice/atbash-cipher/source/atbash_cipher.d

@@ -0,0 +1,61 @@
+module atbash_cipher;
+
+pure string encode(immutable string phrase)
+{
+    // implement this function
+}
+
+pure string decode(immutable string phrase)
+{
+    // implement this function
+}
+
+unittest
+{
+    immutable int allTestsEnabled = 0;
+
+    // Encode yes
+    assert(encode("yes") == "bvh");
+
+    static if (allTestsEnabled)
+    {
+        // Encode no
+        assert(encode("no") == "ml");
+
+        // Encode OMG
+        assert(encode("OMG") == "lnt");
+
+        // Encode spaces
+        assert(encode("O M G") == "lnt");
+
+        // Encode mindblowingly
+        assert(encode("mindblowingly") == "nrmwy oldrm tob");
+
+        // Encode numbers
+        assert(encode("Testing,1 2 3, testing.") == "gvhgr mt123 gvhgr mt");
+
+        // Encode deep thought
+        assert(encode("Truth is fiction.") == "gifgs rhurx grlm");
+
+        // Encode all the letters
+        assert(encode("The quick brown fox jumps over the lazy dog.") == "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt");
+
+        // Decode exercism
+        assert(decode("vcvix rhn") == "exercism");
+
+        // Decode a sentence
+        assert(decode("zmlyh gzxov rhlug vmzhg vkkrm thglm v") == "anobstacleisoftenasteppingstone");
+
+        // Decode numbers
+        assert(decode("gvhgr mt123 gvhgr mt") == "testing123testing");
+
+        // Decode all the letters
+        assert(decode("gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt") == "thequickbrownfoxjumpsoverthelazydog");
+
+        // Decode with too many spaces
+        assert(decode("vc vix    r hn") == "exercism");
+
+        // Decode with no spaces
+        assert(decode("zmlyhgzxovrhlugvmzhgvkkrmthglmv") == "anobstacleisoftenasteppingstone");
+    }
+}