for-file-optimization.md

Re-architect codeowners-rs

The CLI behaviors live in src/cli.rs.

What this tool does

• Generate CODEOWNERS: Builds .github/CODEOWNERS from multiple ownership sources. See mappers in src/ownership/mapper/.
• Answer per-file ownership: The for-file command returns the owner of a given file even if the checked-in CODEOWNERS is not up to date.

Current implementations

• Fast but potentially inaccurate: Runner::for_file_from_codeowners (in src/runner.rs) parses the existing CODEOWNERS file to find a file’s owner. It’s very fast, but can be wrong if CODEOWNERS is stale.
• Accurate but slow: Runner::for_file is correct but can take up to ~4 seconds on large repositories because it effectively determines ownership for many files and then returns the single result needed.

Ownership resolution is complex because definitions often live in other files (packages, team configs, globs, etc.).

Assignment

Implement a faster for_file that remains accurate.

Requirements

• Performance: Under 1 second on large codebases (e.g., $HOME/workspace/large).
• Correctness: All existing tests must continue to pass.
• Compatibility: No changes to the external behavior of the for-file CLI command.