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Add Unit Test for Negative Hash Codes in HashTable and Note on TimSort Test Limitations #466

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Kalkwst
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@Kalkwst Kalkwst commented Aug 24, 2024

Description

This PR introduces a new unit test to ensure that our custom HashTable implementation correctly handles keys with negative hash codes. Additionally, it includes a discussion on the challenges and limitations of testing certain aspects of the TimSort algorithm due to their deep integration within the private sections of the codebase.

Key Changes

  1. Unit Test for Negative Hash Codes:

    • New Test: Test_NegativeHashKey_ReturnsCorrectValue
      • Purpose: Validates that the HashTable can handle keys that generate negative hash codes, ensuring that the data structure remains robust under such conditions.
      • Implementation: The test creates a NegativeHashKey class that generates negative hash codes, adds a key-value pair to the HashTable, and verifies that the value can be retrieved correctly using the same key.
  2. Discussion on TimSort Test Limitations:

    • Private Method: FinalizeMerge(TimChunk<T> left, TimChunk<T> right, int dest)

      • The condition left.Remaining == 0 should theoretically never occur if the TimSort algorithm is functioning correctly. This would imply that the left chunk has been entirely consumed before this method is called, indicating a potential bug in the merge logic or an error in the comparison method that leads to an incorrect merge sequence.
      • Writing tests to cover this scenario, as well as other deep parts of the TimSort implementation, is challenging because these methods are private and tightly coupled with the internal logic of the algorithm. To effectively test these edge cases, significant refactoring would be required to expose these methods or alter the structure of the code, which is disproportionate to the value gained from these minor tests.
    • Note on Coverage: Other uncovered methods in TimSort are also deeply embedded within the call chain, making it difficult to create the conditions necessary to trigger them in a controlled testing environment. Given the complexity and potential for unintended side effects, comprehensive testing of these methods would likely require a broader refactoring effort that extends beyond the scope of this PR.

Future Considerations

  • Refactoring for Testability: While it is not addressed in this PR, it may be worth considering a future refactor of the TimSort implementation to make it more testable. This could involve exposing key methods or breaking down complex functions into more testable components.

  • I have performed a self-review of my code
  • My code follows the style guidelines of this project
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • Comments in areas I changed are up to date
  • I have added comments to hard-to-understand areas of my code
  • I have made corresponding changes to the README.md

…plementation

This commit introduces a new unit test and a supporting class to validate the handling of negative hash codes within our custom HashTable implementation.

Changes:

Unit Test: Test_NegativeHashKey_ReturnsCorrectValue

Purpose: The test ensures that the HashTable correctly handles keys with negative hash codes. This scenario is important for robustness, as real-world use cases might involve hash codes that are negative, especially when custom GetHashCode implementations are involved.
Implementation:
A new HashTable is instantiated with a small initial capacity (4) to ensure hash collisions and proper management of entries.
The test adds a key-value pair to the HashTable where the key (NegativeHashKey) intentionally generates a negative hash code.
The test then asserts that the value can be correctly retrieved using a key that generates the same negative hash code, verifying the integrity of the HashTable under these conditions.
Supporting Class: NegativeHashKey

Purpose: The NegativeHashKey class is designed to simulate keys that produce negative hash codes, which is essential for triggering the edge case being tested.
Implementation:
The class contains an integer id used to generate a negative hash code by returning the negation of id in the GetHashCode method.
The Equals method is overridden to ensure correct key comparison based on the id field, allowing the HashTable to manage and compare instances of NegativeHashKey accurately.
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codecov bot commented Aug 24, 2024

Codecov Report

Attention: Patch coverage is 98.80952% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.95%. Comparing base (5eb0254) to head (d7034a8).
Report is 1 commits behind head on master.

Files with missing lines Patch % Lines
Algorithms/Sorters/Comparison/TimSorter.cs 93.75% 0 Missing and 1 partial ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
- Coverage   95.04%   94.95%   -0.10%     
==========================================
  Files         241      242       +1     
  Lines       10213    10228      +15     
  Branches     1450     1453       +3     
==========================================
+ Hits         9707     9712       +5     
- Misses        389      398       +9     
- Partials      117      118       +1     

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@Kalkwst Kalkwst marked this pull request as ready for review August 24, 2024 11:30
@Kalkwst Kalkwst requested a review from siriak as a code owner August 24, 2024 11:30
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siriak commented Aug 26, 2024

Could you then remove the check for left.Remaining == 0 in FinalizeMerge? If it's impossible based on the algorithm, we shouldn't validate it because it's a private method. We can guarantee that it won't be called with 0.

To find test cases for other methods, you can do fuzz testing with random arrays and set a breakpoint in the place you want to test. When the breakpoint is hit, you can see arguments that were passed to tim sort and add a test case with these arguments.

Anyway, this PR looks good and I can merge it as-is. Let me know if I need to merge it in the current state or you want to make Tim sort changes here.

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Kalkwst commented Sep 21, 2024

Let's keep this PR for the time, I am working on a solution but it is not ready yet

Refactored TimSorter to introduce a TimSorterSettings class, which encapsulates configuration parameters like minMerge and minGallop. This change separates configuration concerns from the sorting logic, improving code readability, maintainability, and testability.

- Introduced TimSorterSettings class with minMerge and minGallop parameters.
- Updated TimSorter constructor to accept a settings object for configuration.
- Enhanced testability by allowing customizable settings for different test scenarios.
- Simplified TimSorter’s constructor and reduced parameter clutter.
- Facilitated future scalability by allowing easy extension of configuration options.

This change adheres to the Single Responsibility Principle (SRP) and improves flexibility in sorting behavior across different contexts.
@Kalkwst Kalkwst force-pushed the bug/issue-465-CoverageIssueforHashTableandTimSorterReducingOverallCodeCoverage branch 5 times, most recently from d530658 to d634a74 Compare September 22, 2024 11:49
- Moved galloping logic (GallopLeft, GallopRight, LeftRun, RightRun, FinalOffset) from TimSorter to a new GallopingStrategy static class.
- Simplified the code by removing the interface and making all methods static since there's no need for instance-specific behavior.
- The refactored GallopingStrategy class now encapsulates galloping functionality, improving modularity and testability.
- Updated TimSorter to use GallopingStrategy for gallop operations, enhancing code clarity and separation of concerns.
@Kalkwst Kalkwst force-pushed the bug/issue-465-CoverageIssueforHashTableandTimSorterReducingOverallCodeCoverage branch from d634a74 to d97a99f Compare September 22, 2024 11:54
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Kalkwst commented Sep 22, 2024

I’ve done my best to improve the testability of the TimSorter class by addressing the internal state management and making parts of the code more modular and testable. This PR resolves some or most of the issues around flakiness and testability.

However, I have to note that the overall complexity of the class remains high, making it difficult to understand—especially considering that this repository is intended to be educational. The current implementation, while functional, is still not very accessible for learners or contributors who may want to understand and learn from the code.

Given that, while this PR improves things, we should seriously consider rewriting or refactoring the entire TimSort algorithm to make it clearer, more maintainable, and educationally valuable. I suggest addressing this after Hacktoberfest, as this PR achieves its short-term goals, but a more extensive rewrite would be better suited for a later effort.

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siriak commented Sep 22, 2024

It's much better now indeed. If you know how to improve TimSorter, feel free to do that.

@siriak siriak enabled auto-merge (squash) September 22, 2024 12:35
@siriak siriak merged commit ab2b5cc into TheAlgorithms:master Sep 22, 2024
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