"""Stable, anonymous per-machine identifier for telemetry.

Produces an irreversible device hash so the backend can count how many distinct
machines use a single activation key, without ever learning the machine's real
identity. Measurement only: no enforcement. The hash is a one-way SHA256 digest,
so the raw machine id never leaves the user's computer.
"""
from __future__ import annotations

import hashlib
import uuid

from qgis.core import QgsSettings
from qgis.PyQt.QtCore import QSysInfo

# Random per-machine fallback seed, persisted when the OS machine id is unavailable.
_SETTINGS_KEY = "AIEdit/device_seed"
# 16 hex chars (64 bits): collision-safe across our user base, well under the
# server's 32-char cap, and stays lowercase hex as the route handler expects.
_HASH_LEN = 16

_cached: str | None = None


def _machine_seed(settings) -> bytes:
    """Return a stable per-machine seed.

    Prefers the OS machine id (QSysInfo). Falls back to a random UUID persisted
    in QSettings when the OS id is unavailable (some containers, locked-down hosts).
    """
    try:
        raw = bytes(QSysInfo.machineUniqueId())
        if raw:
            return raw
    except Exception:  # nosec B110
        pass

    seed = settings.value(_SETTINGS_KEY, "", type=str)
    if not seed:
        seed = uuid.uuid4().hex
        settings.setValue(_SETTINGS_KEY, seed)
    return seed.encode("utf-8")


def get_device_hash(settings=None) -> str:
    """Return a 16-char lowercase-hex hash identifying this machine.

    Irreversible (SHA256). Cached for the process lifetime.
    """
    global _cached
    if _cached is not None:
        return _cached

    s = settings or QgsSettings()
    digest = hashlib.sha256(_machine_seed(s)).hexdigest()
    _cached = digest[:_HASH_LEN]
    return _cached
