feat: add Gronsfeld cipher implementation

[qsrbit]

Description

This PR adds a Gronsfeld Cipher implementation to the ciphers module.

About Gronsfeld Cipher

The Gronsfeld cipher, a Vigenère variant where the key is a digit sequence (0-9). Includes 20 tests covering key validation, case preservation, alphabet wrap-around, and round-trip correctness.

Implementation Details

The implementation provides two public functions:

1. gronsfeld_encrypt(text: &str, key: &str) -> Result<String, &'static str>

   • Encrypts plaintext by shifting each letter forward by the corresponding key digit
   • Validates the key: must be non-empty and contain only digits (0–9)
   • Preserves the case of alphabetic characters
   • Passes non-alphabetic characters (spaces, punctuation, Unicode) through unchanged
   • Cycles the key if it is shorter than the plaintext

2. gronsfeld_decrypt(text: &str, key: &str) -> Result<String, &'static str>

   • Decrypts ciphertext by shifting each letter backward by the corresponding key digit
   • Same key validation and passthrough rules as encryption
   • Inverse of gronsfeld_encrypt: decrypt(encrypt(text, key), key) == text

Algorithm Details

• Character Set: Works with all ASCII alphabetic characters (A–Z, a–z)
• Key Format: Digit string, e.g. "31415" or "9876543210"
• Key Cycling: The key repeats cyclically when shorter than the text; only alphabetic characters consume a key digit
• Shift Range: Each digit shifts by 0–9 positions (a strict subset of Vigenère's 0–25)
• Case Preservation: Uppercase input stays uppercase, lowercase stays lowercase
• Time Complexity: O(n) where n is the length of the input text
• Space Complexity: O(n) for storing the result

Testing

All tests pass and the codebase is clean:

cargo test gronsfeld
cargo fmt --check
cargo clippy -- -D warnings

Test Coverage

• ✅ Empty key returns error
• ✅ Non-digit key returns error
• ✅ Encrypt empty text
• ✅ Encrypt basic: "abc" + "123" = "bdf"
• ✅ Encrypt preserves uppercase
• ✅ Encrypt mixed case
• ✅ Encrypt passthrough non-alpha: spaces and punctuation unchanged
• ✅ Encrypt key wraps cyclically
• ✅ Encrypt zero shift: text unchanged
• ✅ Encrypt wraps around alphabet: 'z' + 1 = 'a'
• ✅ Encrypt with single-digit key: "Hello, World!" + "5" = "Mjqqt, Btwqi!"
• ✅ Decrypt empty text
• ✅ Decrypt basic: "bdf" + "123" = "abc"
• ✅ Decrypt preserves case
• ✅ Decrypt passthrough non-alpha
• ✅ Decrypt zero shift
• ✅ Decrypt wraps around alphabet: 'a' - 1 = 'z'
• ✅ Round-trip basic
• ✅ Round-trip long text (pangram)
• ✅ Round-trip with Unicode passthrough

Examples

use the_algorithms_rust::ciphers::{gronsfeld_encrypt, gronsfeld_decrypt};

// Basic encryption
let ciphertext = gronsfeld_encrypt("Hello, World!", "31415").unwrap();
assert_eq!(ciphertext, "Kfnop, Yosnd!");

// Basic decryption
let plaintext = gronsfeld_decrypt("Kfnop, Yosnd!", "31415").unwrap();
assert_eq!(plaintext, "Hello, World!");

// Zero-shift key leaves text unchanged
let ciphertext = gronsfeld_encrypt("hello", "0").unwrap();
assert_eq!(ciphertext, "hello");

// Alphabet wrap-around
let ciphertext = gronsfeld_encrypt("z", "1").unwrap();
assert_eq!(ciphertext, "a");

// Invalid key returns Err
assert!(gronsfeld_encrypt("hello", "").is_err());
assert!(gronsfeld_encrypt("hello", "12a3").is_err());

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my implementation is correct
• New and existing unit tests pass locally with my changes
• I have checked my code with cargo fmt
• I have checked my code with cargo clippy
• Any dependent changes have been merged and published

[qsrbit]

@siriak Hi, can you take a look at this PR in your convenience time. Thank you

[Copilot]

Pull request overview

Adds a new Gronsfeld cipher implementation to the ciphers module, exposing encrypt/decrypt APIs and accompanying unit tests.

Changes:

• Introduce gronsfeld_encrypt / gronsfeld_decrypt with key validation and ASCII letter shifting.
• Add unit tests for key validation, case preservation, wrap-around, and round-trip behavior.
• Wire the new module into src/ciphers/mod.rs for compilation and public re-export.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/ciphers/mod.rs	Registers the new gronsfeld module and re-exports its public functions.
src/ciphers/gronsfeld.rs	Implements Gronsfeld encrypt/decrypt logic plus unit tests.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.84%. Comparing base (7789289) to head (10c0431).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1045      +/-   ##
==========================================
+ Coverage   95.82%   95.84%   +0.01%     
==========================================
  Files         393      394       +1     
  Lines       29971    30081     +110     
==========================================
+ Hits        28721    28830     +109     
- Misses       1250     1251       +1     

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[siriak]

Please review Copilot's comments, everything else looks good