"""platform/session — stateless signed session tokens (SD-0001 §6.2).

Sessions are signed HTTP-only cookies; there is no session table. A token is
`base64url(payload_json).base64url(hmac_sha256(payload))` keyed by the configured session
secret (INV-8). `verify` checks the MAC in constant time and returns the payload dict, or
None for any malformed/tampered/wrong-key token. No server-side state to revoke — a leaked
secret is rotated, invalidating every session at once (§6.2, accepted at MVP scale).
"""
from __future__ import annotations

import base64
import hashlib
import hmac
import json
from typing import Any


def _b64e(raw: bytes) -> str:
    return base64.urlsafe_b64encode(raw).rstrip(b"=").decode("ascii")


def _b64d(s: str) -> bytes:
    pad = "=" * (-len(s) % 4)
    return base64.urlsafe_b64decode(s + pad)


def _mac(payload_b64: str, secret: str) -> str:
    digest = hmac.new(secret.encode("utf-8"), payload_b64.encode("ascii"), hashlib.sha256).digest()
    return _b64e(digest)


def sign(payload: dict[str, Any], secret: str) -> str:
    """Serialize and sign a session payload into a cookie-safe token."""
    payload_b64 = _b64e(json.dumps(payload, separators=(",", ":"), sort_keys=True).encode("utf-8"))
    return f"{payload_b64}.{_mac(payload_b64, secret)}"


def verify(token: str, secret: str) -> dict[str, Any] | None:
    """Return the payload if the token's signature is valid, else None."""
    if not token or "." not in token:
        return None
    payload_b64, _, sig = token.partition(".")
    expected = _mac(payload_b64, secret)
    if not hmac.compare_digest(sig, expected):
        return None
    try:
        return json.loads(_b64d(payload_b64))
    except (ValueError, json.JSONDecodeError):
        return None