"""Scope tree-measuring instruments to the files that ship with the repo.

The censuses and structural guards measure ``backend/app`` by walking the
filesystem — but a dev box's ``app/plugins/`` also holds *installed* extension
copies that are not tracked in git (only the builtin extensions are). A walk
that counts those measures the machine, not the product: the raw-subprocess
ceiling picked up 4 call sites from locally-installed extensions that CI's
clean checkout can never see, which is exactly the silent-slack failure mode
the ceilings exist to prevent.

``tracked_py_files()`` returns the set of git-tracked ``.py`` paths under a
root, so an instrument can skip everything else. When git is unavailable it
returns ``None`` and callers fall back to walking everything — measuring too
much is recoverable; measuring nothing is not.
"""

import os
import subprocess


def tracked_py_files(root):
    """Absolute, ``os.path.normcase``-normalised paths of git-tracked ``.py``
    files under *root*, or ``None`` when git cannot answer."""
    try:
        proc = subprocess.run(
            ['git', 'ls-files', '-z', '--', '.'],
            cwd=root, capture_output=True, text=True, timeout=30,
        )
    except (OSError, subprocess.TimeoutExpired):
        return None
    if proc.returncode != 0:
        return None
    return {
        os.path.normcase(os.path.abspath(os.path.join(root, rel)))
        for rel in proc.stdout.split('\0')
        if rel.endswith('.py')
    }


def is_tracked(path, tracked):
    """True when *path* is in *tracked*, or when there is no filter."""
    if tracked is None:
        return True
    return os.path.normcase(os.path.abspath(path)) in tracked