"""Local backend process skeleton (U-007): pure stdlib `http.server`, no
third-party deps (05 §5.7) -- this process is also where the Graph Engine
scheduler will live later (05 §5.4.1), but that's a different Unit.

MVP scope here: process start/stop, and `c_pick_folder` (03-C §3.-1) as the
one real route -- macOS only for now (`osascript`); Windows/Linux native
dialogs are PH3 (U-022).
"""

import json
import platform
import subprocess
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer


class DialogUnavailableError(Exception):
    """DIALOG_UNAVAILABLE -- 03-C §3.-1: no native-dialog capability here."""


def pick_folder():
    """macOS implementation of c_pick_folder. Returns {"selected_path": str}
    on success, {"selected_path": None} on user cancel (03-C: not an error).
    Raises DialogUnavailableError off macOS or if osascript itself fails for
    a reason other than the user cancelling."""
    if platform.system() != "Darwin":
        raise DialogUnavailableError("native folder picker only implemented for macOS so far (PH3 adds Windows/Linux)")

    script = 'POSIX path of (choose folder with prompt "Select a graphlemon project folder")'
    result = subprocess.run(["osascript", "-e", script], capture_output=True, text=True)
    if result.returncode != 0:
        if "User canceled" in result.stderr or "(-128)" in result.stderr:
            return {"selected_path": None}
        raise DialogUnavailableError(result.stderr.strip())
    return {"selected_path": result.stdout.strip()}


class _Handler(BaseHTTPRequestHandler):
    def do_POST(self):
        if self.path == "/api/system/pick-folder":
            self._handle_pick_folder()
        else:
            self._send_json(404, {"error": "NOT_FOUND"})

    def _handle_pick_folder(self):
        try:
            self._send_json(200, pick_folder())
        except DialogUnavailableError as e:
            self._send_json(422, {"error": "DIALOG_UNAVAILABLE", "detail": str(e)})

    def _send_json(self, status, obj):
        body = json.dumps(obj).encode("utf-8")
        self.send_response(status)
        self.send_header("Content-Type", "application/json")
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)

    def log_message(self, format, *args):
        pass  # quiets test output; real request logging is a later Unit's concern


def create_server(host="127.0.0.1", port=0):
    """port=0 lets the OS assign a free port -- avoids collisions in tests
    and lets multiple graphlemon instances run side by side if ever needed."""
    return ThreadingHTTPServer((host, port), _Handler)