Add ROT13 implementation (#27)

Co-authored-by: CuriousCorrelation <[email protected]>
Co-authored-by: Alexander González <[email protected]>

README.md

@@ -83,6 +83,7 @@ cargo test
 
 - [x] Transposition
 - [x] Caesar cipher
+- [x] ROT13
 
 ### Bit Manipulation
 

src/ciphers/README.md

@@ -27,4 +27,21 @@ top to bottom and left to right.
 
 ###### Source: [Wikipedia](https://en.wikipedia.org/wiki/Transposition_cipher)
 
+### [ROT13](./rot13.rs)
+![alt text][rot13]
+
+**ROT13** or **"rotate by 13 places"**, sometimes hyphenated **ROT-13** is a simple letter substitution cipher that replaces a letter with the 13th letter after it in the alphabet.<br>
+Because there are 26 letters (2×13) in the basic Latin alphabet, ROT13 is its own inverse; that is, to undo ROT13, the same algorithm is applied, so the same action can be used for encoding and decoding.<br>
+The algorithm provides virtually no cryptographic security, and is often cited as a canonical example of weak encryption.
+
+The transformation can be done using a lookup table, such as the following:
+
+```
+| Input  | ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz |
+| Output | NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm |
+```
+
+###### Source: [Wikipedia](https://en.wikipedia.org/wiki/ROT13)
+
 [caesar]: https://upload.wikimedia.org/wikipedia/commons/4/4a/Caesar_cipher_left_shift_of_3.svg
+[rot13]: https://upload.wikimedia.org/wikipedia/commons/3/33/ROT13_table_with_example.svg

src/ciphers/mod.rs

@@ -1,5 +1,7 @@
 mod caesar;
+mod rot13;
 mod transposition;
 
 pub use self::caesar::caesar;
+pub use self::rot13::rot13;
 pub use self::transposition::transposition;

src/ciphers/rot13.rs

@@ -0,0 +1,68 @@
+//! Rot13 or "rotate by 13 places"
+//!
+//! # Algorithm
+
+/// Encrypts a given [`&str`] using ROT13 cipher.
+///
+/// See [ROT13](https://en.wikipedia.org/wiki/ROT13) for more information.
+///
+/// Replaces each character with the 13th letter after it in the alphabet.
+/// Rot13 is a special case of [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher)
+///
+/// The most basic example is ROT 13, which rotates 'a' to 'n'.
+/// This implementation does not rotate unicode characters.
+///
+/// # Arguments
+///
+/// `text` - String to transform.
+///
+/// # Returns
+///
+/// An owned [`String`]
+///
+/// # Panic
+///
+/// This function won't panic.
+///
+/// # Examples
+/// ```
+/// # use rust_algorithms::ciphers::rot13;
+/// let encoded = rot13("hello world");
+/// assert_eq!(encoded, "URYYB JBEYQ");
+/// ```
+pub fn rot13(text: &str) -> String {
+    let to_enc = text.to_uppercase();
+    to_enc
+        .chars()
+        .map(|c| match c {
+            'A'..='M' => ((c as u8) + 13) as char,
+            'N'..='Z' => ((c as u8) - 13) as char,
+            _ => c,
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod test {
+    use super::rot13;
+
+    #[test]
+    fn test_single_letter() {
+        assert_eq!("N", rot13("A"));
+    }
+
+    #[test]
+    fn test_bunch_of_letters() {
+        assert_eq!("NOP", rot13("ABC"));
+    }
+
+    #[test]
+    fn test_non_ascii() {
+        assert_eq!("😀NO", rot13("😀AB"));
+    }
+
+    #[test]
+    fn test_twice() {
+        assert_eq!("ABCD", rot13(&rot13("ABCD")));
+    }
+}