bugfix(partition): Fix inconsistent mine collision behaviour

[Stubbjax]

Fixes #24 from the patch repository

This change fixes inconsistent mine collision detection by replacing the partition manager's circle fill algorithm with a more precise algorithm. The new logic is placed behind a new RETAIL_COMPATIBLE_CIRCLE_FILL_ALGORITHM preprocessor directive to help isolate the change as it is potentially slightly less performant.

The retail circle fill logic - an implementation of the midpoint circle algorithm - does not accurately cover the cells occupied by spherical or cylindrical geometry. This leads to various collision inaccuracies throughout the game, one of which is the infamous mine behaviour described in #24. Various collision logic is only processed within partition cells that are considered occupied - the partition cells act as a 'broad phase' to determine which geometry then needs to be checked for overlaps.

It's particularly important to note that objects with a geometry radius >= 20 and < 40 (the most common range) only cover a single partition cell, for example Tunnel Networks (radius: 25) and Stinger Sites (radius: 30). This can be seen in the video below. A blue/green square indicates an occupied cell.

SINGLE_CELL.mp4

Mines happen to fit right within this range with a radius of 30, and therefore only end up covering a single cell. This is a problem as most units happen to be smaller than a single partition cell, and can end up bypassing adjacent occupied cells altogether. Below is an example of this happening.

DODGER.mp4

Below is a demonstration of the difference in cell coverage in retail (left, blue) vs this change (right, yellow) at different radii. It should be apparent that retail cell coverage becomes more inaccurate as geometry radii increases towards each multiple of the partition cell size (40, 80, 120, etc.), whereas the new algorithm is always perfectly accurate. Note that radii < 20 uses a separate dedicated 'small geometry' algorithm which is 100% accurate.

GRID_COMP.mp4

And below is a direct comparison between retail mine cell coverage and the new mine cell coverage.

Before

Mine cell coverage using the retail algorithm

After

Mine cell coverage using the new algorithm

And some real gameplay demonstrations for good measure.

Before

The Technical skips over several mines, allowing the terrorists to destroy the War Factory

BAD_MINES.mp4

After

The Technical and terrorists explode on the mines as expected

GOOD_MINES.mp4

[greptile-apps]

Greptile Summary

This PR fixes inconsistent mine collision detection by replacing the retail circle fill algorithm with a precise circle-rectangle overlap test. The new algorithm eliminates gaps in partition cell coverage for objects with radii between 20-40 units (like mines with radius 30), ensuring units can no longer bypass adjacent occupied cells.

• Implemented doCircleFillPrecise() using standard AABB collision detection (finds closest point on rectangle to circle center, checks if within radius)
• Added RETAIL_COMPATIBLE_CIRCLE_FILL_ALGORITHM preprocessor directive (defaults to enabled) to maintain backward compatibility
• Applied changes to both Generals and GeneralsMD codebases to fix vanilla and Zero Hour expansion
• The implementation correctly handles edge cases (null cells, boundary conditions, negative coordinates)
• Trade-off: slightly less performant for large radii (O(r²) vs O(r)), but accuracy improvement is significant for common object sizes

Confidence Score: 5/5

• This PR is safe to merge with high confidence - well-tested bugfix with backward compatibility controls
• The implementation uses a standard, proven algorithm for circle-rectangle collision detection. Code is well-structured with appropriate null checks and boundary handling. The change is properly isolated behind a preprocessor directive allowing fallback to retail behavior if needed. Extensive testing evidenced by videos and visual demonstrations in the PR description confirm the fix works as intended.
• No files require special attention

Important Files Changed

Filename	Overview
Core/GameEngine/Include/Common/GameDefines.h	Added RETAIL_COMPATIBLE_CIRCLE_FILL_ALGORITHM preprocessor directive to control algorithm selection
Generals/Code/GameEngine/Source/GameLogic/Object/PartitionManager.cpp	Implemented precise circle-cell overlap detection using standard AABB algorithm; conditionally enabled via preprocessor directive
GeneralsMD/Code/GameEngine/Source/GameLogic/Object/PartitionManager.cpp	Implemented precise circle-cell overlap detection (Zero Hour version)

_{Last reviewed commit: 10ec115}

[greptile-apps]

_{3 files reviewed, 2 comments}

_{Edit Code Review Agent Settings | Greptile}

[xezon]

I did not look at the code yet but the PRESENTATION and DILIGENCE is WORLD CLASS! Massive APPLAUSE 👏👏👏

[xezon]

Implementation looks very good too.

Could be refactored a little.

Needs rebase.