from __future__ import annotations

import base64
import hashlib
import hmac
import os
from datetime import datetime, timedelta, timezone
from typing import Any

import jwt
from fastapi import Depends, HTTPException, Request, status
from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
from sqlalchemy import select
from sqlalchemy.orm import Session

from app.core.config import get_settings
from app.core.database import get_db
from app.models.user import User


bearer_scheme = HTTPBearer(auto_error=False)
MEDIA_AUTH_COOKIE = "docdoe_media_token"


def get_current_user_optional(
    request: Request,
    credentials: HTTPAuthorizationCredentials | None = Depends(bearer_scheme),
    db: Session = Depends(get_db),
) -> User | None:
    settings = get_settings()
    if not settings.auth_enabled:
        if not (
            settings.environment == "development"
            and settings.allow_insecure_dev_auth
        ):
            raise HTTPException(
                status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
                detail="Authentication is not configured for this deployment.",
            )
        return get_or_create_dev_user(db)

    token = (
        credentials.credentials
        if credentials is not None and credentials.scheme.lower() == "bearer"
        else request.cookies.get(MEDIA_AUTH_COOKIE)
    )
    if not token:
        return None
    auth_provider = (settings.auth_provider or "jwt").strip().lower()
    if auth_provider == "supabase":
        return _user_from_supabase_token(db, token)
    if auth_provider == "jwt":
        return _user_from_local_jwt(db, token)
    return None


def get_current_user(
    user: User | None = Depends(get_current_user_optional),
) -> User | None:
    return user


def require_user(user: User | None = Depends(get_current_user_optional)) -> User:
    if user is None:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Authentication required",
            headers={"WWW-Authenticate": "Bearer"},
        )
    return user


def get_or_create_dev_user(db: Session) -> User:
    user = db.get(User, "usr_demo_student")
    if user is not None:
        if user.name != "Asnan" or user.email != "asnan@example.com":
            user.name = "Asnan"
            user.email = "asnan@example.com"
            db.commit()
            db.refresh(user)
        return user

    user = User(
        id="usr_demo_student",
        name="Asnan",
        email="asnan@example.com",
        role="student",
        class_level="Plus Two",
        syllabus="Kerala HSE",
        preferred_language="English",
    )
    db.add(user)
    db.commit()
    db.refresh(user)
    return user


def hash_password(password: str) -> str:
    salt = os.urandom(16)
    digest = hashlib.pbkdf2_hmac("sha256", password.encode("utf-8"), salt, 390_000)
    return "pbkdf2_sha256$390000${}${}".format(
        base64.b64encode(salt).decode("ascii"),
        base64.b64encode(digest).decode("ascii"),
    )


def verify_password(password: str, password_hash: str | None) -> bool:
    if not password_hash:
        return False
    try:
        algorithm, iterations_text, salt_text, digest_text = password_hash.split("$", 3)
        if algorithm != "pbkdf2_sha256":
            return False
        salt = base64.b64decode(salt_text.encode("ascii"))
        expected_digest = base64.b64decode(digest_text.encode("ascii"))
        actual_digest = hashlib.pbkdf2_hmac(
            "sha256",
            password.encode("utf-8"),
            salt,
            int(iterations_text),
        )
        return hmac.compare_digest(actual_digest, expected_digest)
    except Exception:
        return False


def create_access_token(user: User) -> tuple[str, int]:
    settings = get_settings()
    expires_in_seconds = settings.access_token_expire_minutes * 60
    expires_at = datetime.now(timezone.utc) + timedelta(seconds=expires_in_seconds)
    payload: dict[str, Any] = {
        "sub": user.id,
        "email": user.email,
        "role": user.role,
        "ver": user.auth_version,
        "exp": expires_at,
        "iat": datetime.now(timezone.utc),
    }
    token = jwt.encode(payload, settings.jwt_secret_key, algorithm=settings.jwt_algorithm)
    return token, expires_in_seconds


def get_verified_auth_subject(token: str) -> str:
    """Return the verified identity-provider subject for an access token.

    Most application routes only need the hydrated local ``User``. Destructive
    identity operations also need the original provider subject so a legacy
    email-matched local row cannot accidentally be used as a Supabase user ID.
    """
    settings = get_settings()
    auth_provider = (settings.auth_provider or "jwt").strip().lower()
    if auth_provider == "supabase":
        payload = _decode_supabase_jwt(token)
    elif auth_provider == "jwt":
        try:
            payload = jwt.decode(
                token,
                settings.jwt_secret_key,
                algorithms=[settings.jwt_algorithm],
                options={"verify_aud": False},
            )
        except jwt.PyJWTError as exc:
            raise _auth_error() from exc
    else:
        raise _auth_error()

    subject = str(payload.get("sub") or "")
    if not subject:
        raise _auth_error()
    return subject


def _user_from_local_jwt(db: Session, token: str) -> User | None:
    settings = get_settings()
    try:
        payload = jwt.decode(
            token,
            settings.jwt_secret_key,
            algorithms=[settings.jwt_algorithm],
            options={"verify_aud": False},
        )
    except jwt.PyJWTError as exc:
        raise _auth_error() from exc

    user_id = str(payload.get("sub") or "")
    if not user_id:
        raise _auth_error()
    user = db.get(User, user_id)
    if user is None:
        raise _auth_error()
    try:
        token_version = int(payload.get("ver", 1))
    except (TypeError, ValueError) as exc:
        raise _auth_error() from exc
    if token_version != user.auth_version:
        raise _auth_error()
    return user


def _user_from_supabase_token(db: Session, token: str) -> User | None:
    payload = _decode_supabase_jwt(token)

    user_id = str(payload.get("sub") or "")
    email = str(payload.get("email") or "").strip().lower()
    if not user_id:
        raise _auth_error()

    user = db.get(User, user_id)
    if user is not None:
        return user
    if email:
        # A verified Supabase email is not proof that this is the same account
        # as a legacy local-JWT row. Returning that row would merge identities
        # and can expose its private state to a different provider subject.
        existing_email_user = db.scalar(select(User).where(User.email == email))
        if existing_email_user is not None:
            raise _auth_error()

    user = User(
        id=user_id,
        name=email.split("@")[0] if email else "Student",
        email=email or f"{user_id}@supabase.local",
        role="student",
        preferred_language="English",
    )
    db.add(user)
    db.commit()
    db.refresh(user)
    return user


def _decode_supabase_jwt(token: str) -> dict[str, Any]:
    """Verify the exact Supabase issuer and audience before trusting ``sub``.

    DocDoe's current Supabase integration uses the configured legacy HS256 JWT
    secret. A project using asymmetric signing must add JWKS verification before
    changing the Supabase signing-key configuration.
    """
    settings = get_settings()
    if not settings.supabase_jwt_secret or not settings.supabase_url:
        raise HTTPException(
            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
            detail="Supabase auth is enabled but SUPABASE_URL or SUPABASE_JWT_SECRET is not configured.",
        )
    try:
        payload = jwt.decode(
            token,
            settings.supabase_jwt_secret,
            algorithms=["HS256"],
            audience="authenticated",
            issuer=f"{settings.supabase_url.rstrip('/')}/auth/v1",
        )
    except jwt.PyJWTError as exc:
        raise _auth_error() from exc
    return payload


def _auth_error() -> HTTPException:
    return HTTPException(
        status_code=status.HTTP_401_UNAUTHORIZED,
        detail="Invalid or expired authentication token",
        headers={"WWW-Authenticate": "Bearer"},
    )