fix: RailFenceCipher silently drops newline characters

[SEPURI-SAI-KRISHNA]

Problem

RailFenceCipher.encrypt builds a rails x length matrix and fills every cell with '\n' to mark it as unused. When reading the matrix back it skips every cell that still holds '\n':

Arrays.fill(strRail[i], '\n');
...
if (ch != '\n') {
    encryptedString.append(ch);
}

The placeholder is indistinguishable from a real newline in the plaintext, so any '\n' in the input is silently discarded. The ciphertext comes out shorter than the plaintext and the round trip is corrupted:

RailFenceCipher cipher = new RailFenceCipher();
String message = "ab\ncdef";              // 7 characters

cipher.encrypt(message, 3);               // "adbcef"  -> 6 characters, the newline is gone
cipher.decrypt(cipher.encrypt(message, 3), 3);  // "abfcde" -> not the original message

A transposition cipher must be a permutation of its input, so dropping characters is a correctness bug rather than a formatting quirk.

Fix

encrypt now collects each rail into its own StringBuilder instead of writing into a matrix that needs a sentinel value. Every character of the input is appended exactly once and read back in rail order, so no character can be mistaken for a blank cell. This also drops the memory use from O(rails * length) to O(length).

Additionally, encrypt and decrypt now reject null input and non-positive rail counts with IllegalArgumentException. Previously encrypt("abc", 0) threw ArrayIndexOutOfBoundsException and encrypt("abc", -1) threw NegativeArraySizeException, both leaking internal implementation details.

decrypt is unchanged: its '*' marker is written and consumed positionally, so a '*' in the ciphertext was never at risk.

Tests

The class had no test class at all. RailFenceCipherTest is added, covering:

• the classic WEAREDISCOVEREDFLEEATONCE / 3 rails vector, in both directions

• round trips over several messages and rail counts

• control characters (\n, \r, \t) surviving encryption for 2..5 rails — this fails on the old code

• degenerate rail counts (rails == 1, rails >= length, empty input)

• the new validation for non-positive rail counts and null input

• I have read CONTRIBUTING.md.

• This pull request is all my own work -- I have not plagiarized it.

• All filenames are in PascalCase.

• All functions and variable names follow Java naming conventions.

• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.

• All new algorithms include a corresponding test class that validates their functionality.

• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.43%. Comparing base (346f591) to head (75728a4).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7572   +/-   ##
=========================================
  Coverage     80.42%   80.43%           
- Complexity     7460     7461    +1     
=========================================
  Files           815      815           
  Lines         24056    24057    +1     
  Branches       4733     4734    +1     
=========================================
+ Hits          19348    19351    +3     
+ Misses         3945     3944    -1     
+ Partials        763      762    -1     

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[DenizAltunkapan]

@SEPURI-SAI-KRISHNA thanks