"""The one `.mam.env` parser (Issue #26).

Rules: blank lines and `#` comments are skipped; `KEY=VALUE` with the key and
the value stripped; one matching pair of surrounding quotes is removed; no
shell interpretation (`$(...)`, `${...}` and backticks stay literal). A key
must match ``^[A-Za-z_][A-Za-z0-9_]*$``; a line that does not, or that has no
`=`, is skipped with a warning naming the line (never the value — the file
holds secrets). Variables already in the environment win over the file.

Stdlib only: the delegate-job wrapper runs this file by path
(``envfile.py --shell <path>``) and evals the ``export`` lines it prints, so
values travel over a pipe, never on a command line.
"""
import os
import re
import shlex
import sys
from typing import List, MutableMapping, Tuple

_KEY = re.compile(r"[A-Za-z_][A-Za-z0-9_]*")


def parse(path: str) -> Tuple[List[Tuple[str, str]], List[str]]:
    """Return ([(key, value)], [warning]) for the file at ``path``."""
    pairs: List[Tuple[str, str]] = []
    warnings: List[str] = []
    try:
        with open(path, "r", encoding="utf-8") as f:
            lines = f.read().splitlines()
    except (OSError, UnicodeDecodeError) as e:
        return pairs, [f"{path}: cannot read ({type(e).__name__})"]
    for n, raw in enumerate(lines, 1):
        line = raw.strip()
        if not line or line.startswith("#"):
            continue
        key, sep, val = line.partition("=")
        key = key.strip()
        if not sep:
            warnings.append(f"{path}:{n}: skipped (no '=')")
            continue
        if not _KEY.fullmatch(key):
            warnings.append(f"{path}:{n}: skipped (invalid key)")
            continue
        val = val.strip()
        if len(val) >= 2 and val[0] == val[-1] and val[0] in "\"'":
            val = val[1:-1]
        pairs.append((key, val))
    return pairs, warnings


def apply(path: str, environ: MutableMapping[str, str]) -> Tuple[List[Tuple[str, str]], List[str]]:
    """Set each pair not already in ``environ``; return (the pairs set, warnings)."""
    pairs, warnings = parse(path)
    applied = []
    for key, val in pairs:
        if key not in environ:
            environ[key] = val
            applied.append((key, val))
    return applied, warnings


def main(argv: List[str]) -> int:
    if len(argv) != 2 or argv[0] != "--shell":
        print("usage: envfile.py --shell <path>", file=sys.stderr)
        return 2
    path = argv[1]
    if not os.path.isfile(path):
        return 0
    applied, warnings = apply(path, dict(os.environ))
    for w in warnings:
        print(f"WARNING: {w}", file=sys.stderr)
    for key, val in applied:
        print(f"export {key}={shlex.quote(val)}")
    return 0


if __name__ == "__main__":
    sys.exit(main(sys.argv[1:]))