"""Author RAG Chatbot SaaS — Subscription Token Cryptography.

Generates and verifies HMAC-SHA256 signed subscription tokens.
Token format: base64url(payload_json).base64url(hmac_signature)
"""

import base64
import hashlib
import hmac
import json
from datetime import datetime, timezone


def generate_subscription_token(
    author_id: str,
    plan: str,
    expires_at: str,
    secret: str,
) -> str:
    """Generate a signed subscription token.

    Args:
        author_id: UUID of the author.
        plan: Plan name ('monthly', 'quarterly', etc.).
        expires_at: ISO 8601 expiry datetime string.
        secret: HMAC secret key.

    Returns:
        Signed token string (payload.signature, both base64url-encoded).
    """
    payload = json.dumps({
        "author_id": author_id,
        "plan": plan,
        "expires_at": expires_at,
    }, separators=(",", ":"))

    encoded_payload = base64.urlsafe_b64encode(payload.encode()).decode().rstrip("=")
    signature = _sign(encoded_payload, secret)
    return f"{encoded_payload}.{signature}"


def verify_subscription_token(token: str, secret: str) -> dict:
    """Verify and decode a subscription token.

    Args:
        token: Token string from generate_subscription_token.
        secret: HMAC secret key.

    Returns:
        Decoded payload dict with author_id, plan, expires_at.

    Raises:
        ValueError: If signature is invalid or token is malformed.
        ValueError: If token has expired (message contains 'expired').
    """
    try:
        parts = token.split(".")
        if len(parts) != 2:
            raise ValueError("Malformed token: expected 2 parts")

        encoded_payload, provided_sig = parts

        # Verify signature
        expected_sig = _sign(encoded_payload, secret)
        if not hmac.compare_digest(expected_sig, provided_sig):
            raise ValueError("Token signature is invalid")

        # Decode payload
        padded = encoded_payload + "=" * (4 - len(encoded_payload) % 4)
        payload = json.loads(base64.urlsafe_b64decode(padded).decode())

        # Check expiry
        expires_at = datetime.fromisoformat(payload["expires_at"])
        if expires_at.tzinfo is None:
            expires_at = expires_at.replace(tzinfo=timezone.utc)
        if expires_at < datetime.now(timezone.utc):
            raise ValueError(f"Token has expired at {payload['expires_at']}")

        return payload

    except ValueError:
        raise
    except Exception as e:
        raise ValueError(f"Token verification failed: {e}") from e


def _sign(payload: str, secret: str) -> str:
    """Compute HMAC-SHA256 signature of payload.

    Args:
        payload: String to sign.
        secret: HMAC secret key.

    Returns:
        Base64url-encoded signature (no padding).
    """
    sig = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).digest()
    return base64.urlsafe_b64encode(sig).decode().rstrip("=")