Add Baby-Step Giant-Step (BSGS) Discrete Logarithm Algorithm

[rajucreate]

Summary

This PR adds the Baby-Step Giant-Step (BSGS) algorithm for solving the discrete logarithm problem:

Find x such that:
a^x ≡ b (mod m)

This algorithm is widely used in number theory and cryptography and runs in O(√m) time.

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What’s Included

i) Added full implementation:

• src/main/java/com/thealgorithms/maths/DiscreteLogarithmBSGS.java

ii) Added JUnit tests:

• src/test/java/com/thealgorithms/maths/DiscreteLogarithmBSGSTest.java

iii) Added entry in DIRECTORY.md under the maths section

iv) Includes detailed Javadoc and follows repository coding conventions

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Algorithm Overview

• Time Complexity: O(√m)
• Space Complexity: O(√m)
• Uses the Baby-Step Giant-Step technique to compute discrete logarithms efficiently
• Includes modular exponentiation helper using fast power

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.53%. Comparing base (a14e1e3) to head (29e7dbe).

Files with missing lines	Patch %	Lines
...com/thealgorithms/maths/DiscreteLogarithmBSGS.java	92.59%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7118      +/-   ##
============================================
+ Coverage     78.51%   78.53%   +0.01%     
- Complexity     6752     6760       +8     
============================================
  Files           759      760       +1     
  Lines         22402    22429      +27     
  Branches       4400     4406       +6     
============================================
+ Hits          17589    17614      +25     
- Misses         4108     4110       +2     
  Partials        705      705              

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[DenizAltunkapan]

Could you please add a test case with m being not prime. E.g. a=2 b=8 m=9 => x=3 ? @rajucreate