Simple cipher

config.json

@@ -929,6 +929,14 @@
           "nested_classes",
           "reactive_programming"
         ]
+      },
+      {
+        "slug": "simple-cipher",
+        "name": "Simple Cipher",
+        "uuid": "3ce7b91b-330f-4fbb-8e9a-3638bd90281b",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

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

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+# Instructions
+
+Create an implementation of the [Vigenère cipher][wiki].
+The Vigenère cipher is a simple substitution cipher.
+
+## Cipher terminology
+
+A cipher is an algorithm used to encrypt, or encode, a string.
+The unencrypted string is called the _plaintext_ and the encrypted string is called the _ciphertext_.
+Converting plaintext to ciphertext is called _encoding_ while the reverse is called _decoding_.
+
+In a _substitution cipher_, each plaintext letter is replaced with a ciphertext letter which is computed with the help of a _key_.
+(Note, it is possible for replacement letter to be the same as the original letter.)
+
+## Encoding details
+
+In this cipher, the key is a series of lowercase letters, such as `"abcd"`.
+Each letter of the plaintext is _shifted_ or _rotated_ by a distance based on a corresponding letter in the key.
+An `"a"` in the key means a shift of 0 (that is, no shift).
+A `"b"` in the key means a shift of 1.
+A `"c"` in the key means a shift of 2, and so on.
+
+The first letter of the plaintext uses the first letter of the key, the second letter of the plaintext uses the second letter of the key and so on.
+If you run out of letters in the key before you run out of letters in the plaintext, start over from the start of the key again.
+
+If the key only contains one letter, such as `"dddddd"`, then all letters of the plaintext are shifted by the same amount (three in this example), which would make this the same as a rotational cipher or shift cipher (sometimes called a Caesar cipher).
+For example, the plaintext `"iamapandabear"` would become `"ldpdsdqgdehdu"`.
+
+If the key only contains the letter `"a"` (one or more times), the shift distance is zero and the ciphertext is the same as the plaintext.
+
+Usually the key is more complicated than that, though!
+If the key is `"abcd"` then letters of the plaintext would be shifted by a distance of 0, 1, 2, and 3.
+If the plaintext is `"hello"`, we need 5 shifts so the key would wrap around, giving shift distances of 0, 1, 2, 3, and 0.
+Applying those shifts to the letters of `"hello"` we get `"hfnoo"`.
+
+## Random keys
+
+If no key is provided, generate a key which consists of at least 100 random lowercase letters from the Latin alphabet.
+
+[wiki]: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

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

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "Narkunan"
+  ],
+  "files": {
+    "solution": [
+      "source/simple_cipher.d"
+    ],
+    "test": [
+      "source/simple_cipher.d"
+    ],
+    "example": [
+      "example/simple_cipher.d"
+    ]
+  },
+  "blurb": "Implement the Vigenère cipher, a simple substitution cipher.",
+  "source": "Substitution Cipher at Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Substitution_cipher"
+}

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

@@ -0,0 +1,46 @@
+# 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.
+
+[b8bdfbe1-bea3-41bb-a999-b41403f2b15d]
+description = "Random key cipher -> Can encode"
+
+[3dff7f36-75db-46b4-ab70-644b3f38b81c]
+description = "Random key cipher -> Can decode"
+
+[8143c684-6df6-46ba-bd1f-dea8fcb5d265]
+description = "Random key cipher -> Is reversible. I.e., if you apply decode in a encoded result, you must see the same plaintext encode parameter as a result of the decode method"
+
+[defc0050-e87d-4840-85e4-51a1ab9dd6aa]
+description = "Random key cipher -> Key is made only of lowercase letters"
+
+[565e5158-5b3b-41dd-b99d-33b9f413c39f]
+description = "Substitution cipher -> Can encode"
+
+[d44e4f6a-b8af-4e90-9d08-fd407e31e67b]
+description = "Substitution cipher -> Can decode"
+
+[70a16473-7339-43df-902d-93408c69e9d1]
+description = "Substitution cipher -> Is reversible. I.e., if you apply decode in a encoded result, you must see the same plaintext encode parameter as a result of the decode method"
+
+[69a1458b-92a6-433a-a02d-7beac3ea91f9]
+description = "Substitution cipher -> Can double shift encode"
+
+[21d207c1-98de-40aa-994f-86197ae230fb]
+description = "Substitution cipher -> Can wrap on encode"
+
+[a3d7a4d7-24a9-4de6-bdc4-a6614ced0cb3]
+description = "Substitution cipher -> Can wrap on decode"
+
+[e31c9b8c-8eb6-45c9-a4b5-8344a36b9641]
+description = "Substitution cipher -> Can encode messages longer than the key"
+
+[93cfaae0-17da-4627-9a04-d6d1e1be52e3]
+description = "Substitution cipher -> Can decode messages longer than the key"

exercises/practice/simple-cipher/dub.sdl

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+name "simple-cipher"
+buildRequirements "disallowDeprecations"

exercises/practice/simple-cipher/example/simple_cipher.d

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+import std.stdio;
+import std.random;
+import std.string;
+import std.algorithm;
+import std.array;
+import std.range;
+import std.exception;
+
+class VigenereCipher {
+    private string key;
+
+    this(string key = "") {
+            this.key = key;
+    }
+
+    // Encode plaintext
+    string encode(string plaintext) {
+        return transformText(plaintext, true);
+    }
+
+    // Decode ciphertext
+    string decode(string ciphertext) {
+        return transformText(ciphertext, false);
+    }
+
+    // Core logic
+    private string transformText(string text, bool encodeMode) {
+        string result;
+        size_t keyIndex = 0;
+
+        foreach (char c; text) {
+            if (c >= 'a' && c <= 'z') {
+                int shift = key[keyIndex % key.length] - 'a';
+
+                if (!encodeMode) {
+                    shift = -shift;
+                }
+
+                int newChar = (c - 'a' + shift) % 26;
+                if (newChar < 0) newChar += 26;
+
+                result ~= cast(char)('a' + newChar);
+                keyIndex++;
+            } else {
+                // Keep non-lowercase chars unchanged
+                result ~= c;
+            }
+        }
+
+        return result;
+    }
+
+}

exercises/practice/simple-cipher/source/simple_cipher.d

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+import std.stdio;
+import std.random;
+import std.string;
+import std.algorithm;
+import std.array;
+import std.range;
+import std.exception;
+
+class VigenereCipher {
+    private string key;
+
+    this(string key = "") {
+        this.key = key;
+    }
+
+    // Encode plaintext
+    string encode(string plaintext) {
+        // implement this function
+    }
+
+    // Decode ciphertext
+    string decode(string ciphertext) {
+        // implement this function
+    }
+
+}
+
+unittest {
+    // Basic encoding test
+    auto cipher = new VigenereCipher("abcd");
+    assert(cipher.encode("hello") == "hfnoo");
+}
+
+unittest {
+    // Decode should reverse encode
+    auto cipher = new VigenereCipher("abcd");
+    string original = "hello";
+    auto encoded = cipher.encode(original);
+    auto decoded = cipher.decode(encoded);
+
+    assert(decoded == original);
+}
+
+unittest {
+    // Single character key (Caesar cipher behavior)
+    auto cipher = new VigenereCipher("d"); // shift = 3
+    assert(cipher.encode("abc") == "def");
+    assert(cipher.decode("def") == "abc");
+}
+
+unittest {
+    // Key = "a" should not change text
+    auto cipher = new VigenereCipher("a");
+    assert(cipher.encode("hello") == "hello");
+    assert(cipher.decode("hello") == "hello");
+}
+
+unittest {
+    // Key wrapping
+    auto cipher = new VigenereCipher("ab");
+    // shifts: 0,1,0,1,...
+    assert(cipher.encode("aaaa") == "abab");
+}
+
+unittest {
+    // Non-lowercase characters should remain unchanged
+    auto cipher = new VigenereCipher("abc");
+    assert(cipher.encode("hello world!") == "hfnlp yosnd!");
+}
+