llt-find-for-changes

llt-find-for-changes: Change-Based Test Discovery

Version: 1.0.0 Category: LowLevelTests Discovery

Shared Reference: See llt-common/SKILL.md for response format, data structures, validation rules, and logging instructions. Purpose: Analyze modified source files and identify relevant test coverage

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Overview

The llt-find-for-changes skill analyzes modified source files and identifies relevant test coverage through:

1. Module Discovery: Walks directory tree upward from each modified file to find parent module (nearest .Build.cs)
2. Direct Test Discovery: Checks for test files in the same module's Tests/ subdirectory
3. Dependency Analysis: Uses module dependency graph to find test modules that depend on modified modules
4. Transitive Inclusion: Optionally includes tests that transitively depend on modified modules

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Input Parameters

Parameter	Required	Description
project_root	Yes	Path to Unreal Engine project root directory
files	Yes	List of modified file paths (relative to project root or absolute)
include_transitive	No	Include transitive test modules (default: true)

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Output Schema

Returns standard LLT JSON envelope (see llt-common/SKILL.md). The data field contains:

{
  "summary": {
    "modified_files": 1,
    "modules_affected": 1,
    "direct_test_files": 2,
    "transitive_test_modules": 3,
    "unmapped_files": 0
  },
  "file_to_module": {
    "Engine/Source/Runtime/Core/Private/Misc/LruCache.cpp": "Core"
  },
  "direct_tests": {
    "Engine/Source/Runtime/Core/Private/Misc/LruCache.cpp": [
      "Engine/Source/Runtime/Core/Tests/Misc/LruCacheTest.cpp"
    ]
  },
  "transitive_test_modules": {
    "Core": ["CoreTests", "OnlineServicesMcpTests"]
  },
  "unmapped_files": [],
  "recommendations": [
    "Run 2 direct test file(s) in modified modules",
    "Build and run 3 test module(s) that depend on modified modules"
  ]
}

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Algorithm

Step 1: Module Discovery (for each modified file)

1. Skip test infrastructure files -- check if the file path contains /Catch2/, /OnlineTestsCore/, or /LowLevelTestsRunner/. If so, skip with a warning.
2. Walk directory tree upward from the file's parent directory toward the project root.
3. At each directory, glob for *.Build.cs files.
4. When found, extract the module name from the .Build.cs filename (e.g., Core.Build.cs -> Core).
5. The directory containing the .Build.cs is the module root.
6. Check for a Tests/ subdirectory in the module root.
7. If no .Build.cs is found before reaching the project root, the file is unmapped (add to unmapped_files with a MODULE_NOT_FOUND warning).

Step 2: Direct Test Discovery (for each module)

1. If the module has a Tests/ subdirectory, recursively find all *.cpp files in it.
2. These are the direct test files for that module.

Step 3: Dependency Graph Analysis

1. Glob all .Build.cs files in the project using these patterns:
   • Engine/Source/**/*.Build.cs
   • Engine/Plugins/**/*.Build.cs
   • FortniteGame/Source/**/*.Build.cs
   • FortniteGame/Plugins/**/*.Build.cs
2. Parse each .Build.cs to extract PublicDependencyModuleNames and PrivateDependencyModuleNames using regex patterns like:
   • PublicDependencyModuleNames\.Add\("(\w+)"\)
   • PublicDependencyModuleNames\.AddRange\(new string\[\] \{([^}]+)\}\)
   • Same patterns for PrivateDependencyModuleNames
3. Build a directed dependency graph where edges point from module A to module B if A depends on B.
4. Identify test modules -- modules whose names end with Tests or contain test-related patterns.

Step 4: Test Module Discovery (for each modified module)

1. For each test module in the graph, check if it directly depends on the modified module.
2. If include_transitive is true, also compute transitive dependencies (BFS/DFS from the test module) and check if the modified module is reachable.
3. Exclude test infrastructure modules: Catch2, OnlineTestsCore, LowLevelTestsRunner.
4. Return the sorted list of test module names.

Step 5: Build Recommendations

• If direct test files were found: "Run N direct test file(s) in modified modules"
• If transitive test modules were found: "Build and run N test module(s) that depend on modified modules"
• If neither found: "No tests found for modified files"

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Error Handling

Code	Severity	Description
INVALID_PROJECT_ROOT	error	Project root path does not exist
MODULE_NOT_FOUND	warning	Could not find parent module for file
TEST_INFRASTRUCTURE_FILE	warning	File is part of test infrastructure (skipped)
TEST_DISCOVERY_FAILED	error	Internal error during test discovery

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Limitations

• Module-level granularity: Identifies test modules, not individual test cases
• Static analysis only: Does not consider runtime behavior or reflection-based tests
• Requires .Build.cs: Files without parent .Build.cs cannot be mapped to modules
• UE module system: Only works within Unreal Engine module structure