"""Read the diagnostic kernel translation unit before test-only injection.

Only component module C includes are expanded. Header includes and the
amalgamation linkage switch remain for the ordinary C preprocessor to handle.
An older, unsplit kernels.c is returned unchanged.
"""
from pathlib import Path
import re


NATIVE = Path(__file__).resolve().parents[1] / "native"
_SOURCE_INCLUDE = re.compile(r'^[ \t]*#[ \t]*include[ \t]+"([^"\n]+\.c)"[ \t]*$', re.MULTILINE)


def native_kernel_source(native: Path = NATIVE) -> str:
    entry = native / "components/kernels.c"
    modules = (native / "components/modules").resolve()

    def expand(match: re.Match[str]) -> str:
        source = (entry.parent / match[1]).resolve()
        if not source.is_relative_to(modules):
            return match[0]
        return source.read_text(encoding="utf-8")

    return _SOURCE_INCLUDE.sub(expand, entry.read_text(encoding="utf-8"))