"""Read local environment configuration without evaluating its contents."""

from __future__ import annotations

import os
import re
import sys
from pathlib import Path


_ASSIGNMENT = re.compile(r"(?:export[ \t]+)?([A-Za-z_][A-Za-z0-9_]*)=(.*)")


def load_environment(path: Path, current: dict[str, str]) -> dict[str, str]:
    """Merge a local .env file with the current environment, which takes priority.

    Values are literal: shell commands, variable references, backslashes and
    inline ``#`` characters are never interpreted. Optional surrounding quotes
    are removed, without interpreting escapes. Keep the project's .env values
    unquoted (and omit ``export``) for compatibility with Docker ``--env-file``.
    """
    try:
        contents = path.read_bytes()
    except FileNotFoundError:
        return dict(current)

    values: dict[str, str] = {}
    for line_number, raw_line in enumerate(contents.split(b"\n"), start=1):
        # Accept both LF and CRLF, but not embedded line breaks in a value.
        raw_line = raw_line.removesuffix(b"\r")
        try:
            line = raw_line.decode("utf-8")
        except UnicodeDecodeError:
            raise ValueError(f"Invalid .env syntax at line {line_number}") from None

        if "\x00" in line or "\r" in line:
            raise ValueError(f"Invalid .env syntax at line {line_number}")
        line = line.lstrip(" \t")
        if not line or line.startswith("#"):
            continue
        match = _ASSIGNMENT.fullmatch(line)
        if match is None:
            raise ValueError(f"Invalid .env syntax at line {line_number}")

        key, value = match.groups()
        if value.startswith(("'", '"')):
            quoted_value = value.rstrip(" \t")
            if len(quoted_value) < 2 or quoted_value[-1] != quoted_value[0]:
                raise ValueError(f"Invalid .env syntax at line {line_number}")
            value = quoted_value[1:-1]
        values[key] = value

    values.update(current)
    return values


def main(argv: list[str] | None = None) -> int:
    """Run a command with the project's .env, retaining the caller's directory."""
    command = sys.argv[1:] if argv is None else argv
    if not command or command[0] in ("-h", "--help"):
        print(
            "Usage: python3 scripts/env_config.py COMMAND [ARGS...]\n"
            "Run COMMAND with the project .env; existing environment variables take priority.\n"
            "The working directory is preserved. From backend, for example:\n"
            "  python3 ../scripts/env_config.py ./mvnw test",
            file=sys.stderr if not command else sys.stdout,
        )
        return 2 if not command else 0

    try:
        environment = load_environment(Path(__file__).resolve().parents[1] / ".env", dict(os.environ))
    except ValueError as error:
        message = str(error)
        if re.fullmatch(r"Invalid \.env syntax at line [0-9]+", message) is None:
            message = "Invalid .env syntax."
        print(message, file=sys.stderr)
        return 1
    except OSError:
        print("Unable to read .env.", file=sys.stderr)
        return 1

    try:
        os.execvpe(command[0], command, environment)
    except OSError:
        print("Unable to start command.", file=sys.stderr)
        return 1
    return 0


if __name__ == "__main__":
    raise SystemExit(main())