#!/usr/bin/env python3
"""Render .github/CODEOWNERS from .github/codeowners-roles.yml.

The yml is the source of truth — editing CODEOWNERS directly is
rejected by CI (see .github/workflows/codeowners.yml). This script
expands the role-based yml into the flat path→owners format GitHub
expects.

Usage:
    python3 .github/scripts/render-codeowners.py

Exits non-zero on:
    - Missing PyYAML.
    - Unknown role referenced in `paths` or `default`.
    - Role with no members (a role must always resolve to at least
      one owner; otherwise CODEOWNERS would assign nobody and GitHub
      would silently fall back to "no required reviewer", which
      defeats the purpose).
"""

from __future__ import annotations

import sys
from pathlib import Path

try:
    import yaml
except ImportError:
    sys.exit(
        "error: PyYAML is required. Install with `pip install pyyaml` "
        "or `python3 -m pip install pyyaml`."
    )

REPO_ROOT = Path(__file__).resolve().parents[2]
SOURCE = REPO_ROOT / ".github" / "codeowners-roles.yml"
OUTPUT = REPO_ROOT / ".github" / "CODEOWNERS"

BANNER = """\
# AUTOGENERATED from .github/codeowners-roles.yml. Do not edit by hand.
#
# To change role membership or path assignments:
#   1. Edit .github/codeowners-roles.yml
#   2. Run `python3 .github/scripts/render-codeowners.py`
#   3. Commit both files together
#
# CI fails if this file drifts from its source, and rejects PRs that
# edit this file directly without also editing the yml.
"""


def resolve(role_name: str, roles: dict) -> list[str]:
    role = roles.get(role_name)
    if role is None:
        sys.exit(
            f"error: unknown role '{role_name}'. "
            f"Known roles: {sorted(roles.keys())}"
        )
    members = role.get("members") or []
    if not members:
        sys.exit(
            f"error: role '{role_name}' has no members. "
            f"A role must resolve to at least one owner."
        )
    return members


def owners_for(role_names: list[str], roles: dict) -> list[str]:
    """Return @-prefixed GitHub handles, deduped, preserving order."""
    seen: list[str] = []
    for role_name in role_names:
        for member in resolve(role_name, roles):
            handle = f"@{member}"
            if handle not in seen:
                seen.append(handle)
    return seen


def main() -> int:
    if not SOURCE.exists():
        sys.exit(f"error: source file not found: {SOURCE}")
    spec = yaml.safe_load(SOURCE.read_text())

    roles = spec.get("roles") or {}
    if not roles:
        sys.exit("error: codeowners-roles.yml declares no roles")

    paths = spec.get("paths") or {}
    if not paths:
        sys.exit("error: codeowners-roles.yml declares no paths")

    lines: list[str] = [BANNER]

    # Pad the path column for alignment. Width is the longest pattern
    # plus a small margin.
    width = max(len(p) for p in paths) + 2

    # GitHub CODEOWNERS uses "last match wins" semantics. Emit the
    # default catch-all `*` FIRST so specific patterns below override
    # it for the paths they cover. If we emitted `*` last, every file
    # would resolve to the default owners regardless of more-specific
    # rules — which would silently nullify any role distinction.
    if "default" in spec:
        default_owners = owners_for(spec["default"], roles)
        lines.append(f"{'*':<{width}} {' '.join(default_owners)}")
        lines.append("")

    for pattern, role_names in paths.items():
        owners = owners_for(role_names, roles)
        lines.append(f"{pattern:<{width}} {' '.join(owners)}")

    lines.append("")  # trailing newline so the file ends cleanly

    rendered = "\n".join(lines)

    # Regression check: the catch-all `*` line (if any) must precede
    # every specific-path line. Failure here means the generator is
    # silently nullifying specific rules.
    if "default" in spec:
        non_comment = [ln for ln in rendered.splitlines() if ln and not ln.startswith("#")]
        first_pattern = non_comment[0].split()[0] if non_comment else None
        if first_pattern != "*":
            sys.exit(
                f"error: generator invariant violated — first emitted pattern is "
                f"{first_pattern!r}, expected '*'. CODEOWNERS uses last-match-wins; "
                f"the catch-all must come first."
            )

    OUTPUT.write_text(rendered)
    print(f"wrote {OUTPUT.relative_to(REPO_ROOT)}")
    return 0


if __name__ == "__main__":
    sys.exit(main())