"""
Field-level encryption for sensitive data using API keys.

This module provides encryption/decryption for API keys stored in the database.
Fernet uses AES-128-CBC with HMAC-SHA256 for authenticated encryption.

OPEN_NOTEBOOK_ENCRYPTION_KEY accepts **any string**. A Fernet key is derived
from it via SHA-256, so users can set a simple passphrase like
``OPEN_NOTEBOOK_ENCRYPTION_KEY=my-secret`` and it will work.

Usage:
    # Encrypt before storing
    encrypted = encrypt_value(api_key)

    # Decrypt when reading
    decrypted = decrypt_value(encrypted)
"""

import base64
import hashlib
import os
from pathlib import Path
from typing import Optional

from cryptography.fernet import Fernet, InvalidToken
from loguru import logger


def get_secret_from_env(var_name: str) -> Optional[str]:
    """
    Get a secret from environment, supporting Docker secrets pattern.

    Checks for VAR_FILE first (Docker secrets), then falls back to VAR.

    Args:
        var_name: Base name of the environment variable (e.g., "OPEN_NOTEBOOK_ENCRYPTION_KEY")

    Returns:
        The secret value, or None if not configured.
    """
    # Check for _FILE variant first (Docker secrets)
    file_path = os.environ.get(f"{var_name}_FILE")
    if file_path:
        try:
            path = Path(file_path)
            if path.exists() and path.is_file():
                secret = path.read_text().strip()
                if secret:
                    logger.debug(f"Loaded {var_name} from file: {file_path}")
                    return secret
                else:
                    logger.warning(f"{var_name}_FILE points to empty file: {file_path}")
            else:
                logger.warning(f"{var_name}_FILE path does not exist: {file_path}")
        except Exception as e:
            logger.error(f"Failed to read {var_name} from file {file_path}: {e}")

    # Fall back to direct environment variable
    return os.environ.get(var_name)


def _get_or_create_encryption_key() -> str:
    """
    Get encryption key from environment, requires explicit configuration.

    Priority:
    1. OPEN_NOTEBOOK_ENCRYPTION_KEY_FILE (Docker secrets)
    2. OPEN_NOTEBOOK_ENCRYPTION_KEY (environment variable)

    For production deployments, you MUST set OPEN_NOTEBOOK_ENCRYPTION_KEY explicitly!

    Returns:
        Encryption key string.

    Raises:
        ValueError: If no encryption key is configured.
    """
    # First check environment/Docker secrets
    key = get_secret_from_env("OPEN_NOTEBOOK_ENCRYPTION_KEY")
    if key:
        return key

    raise ValueError(
        "OPEN_NOTEBOOK_ENCRYPTION_KEY is not set. "
        "Set this environment variable to any secret string to enable "
        "encrypted storage of API keys in the database."
    )


# Lazy-loaded encryption key: initialized on first use, not at import time.
# This prevents the entire app from crashing if the key is not yet configured
# when other modules import from this file.
_ENCRYPTION_KEY: Optional[str] = None


def _get_encryption_key() -> str:
    """Get the encryption key, initializing lazily on first call."""
    global _ENCRYPTION_KEY
    if _ENCRYPTION_KEY is None:
        _ENCRYPTION_KEY = _get_or_create_encryption_key()
    return _ENCRYPTION_KEY


def _ensure_fernet_key(key: str) -> str:
    """
    Derive a valid Fernet key from an arbitrary string via SHA-256.

    Any string is accepted as input. The key is derived by hashing it with
    SHA-256 and encoding the result as URL-safe base64.
    """
    derived = hashlib.sha256(key.encode()).digest()
    return base64.urlsafe_b64encode(derived).decode()


def get_fernet() -> Fernet:
    """
    Get Fernet instance with the configured encryption key.

    Returns:
        Fernet instance.

    Raises:
        ValueError: If encryption key is not configured.
    """
    return Fernet(_ensure_fernet_key(_get_encryption_key()).encode())


def encrypt_value(value: str) -> str:
    """
    Encrypt a string value using Fernet symmetric encryption.

    Args:
        value: The plain text string to encrypt.

    Returns:
        Base64-encoded encrypted string.

    Raises:
        ValueError: If encryption is not configured.
    """
    fernet = get_fernet()
    return fernet.encrypt(value.encode()).decode()


def looks_like_fernet_token(s: str) -> bool:
    """
    Check if string looks like a Fernet encrypted token.

    Fernet tokens are versioned (1 byte) + timestamp (8 bytes) + IV (16 bytes)
    + ciphertext (variable, multiple of 16 with PKCS7 padding) + HMAC (32 bytes).
    Minimum decoded size is 73 bytes (1+8+16+16+32) for the smallest payload.
    """
    if len(s) < 100:  # Base64 of 73 bytes = ~100 chars minimum
        return False
    try:
        decoded = base64.urlsafe_b64decode(s)
        # Fernet: version(1) + timestamp(8) + IV(16) + ciphertext(>=16) + HMAC(32)
        # Minimum 73 bytes, ciphertext must be multiple of 16 (AES block size)
        if len(decoded) < 73:
            return False
        ciphertext_len = len(decoded) - 1 - 8 - 16 - 32
        return ciphertext_len > 0 and ciphertext_len % 16 == 0
    except Exception:
        return False


def decrypt_value(value: str) -> str:
    """
    Decrypt a Fernet-encrypted string value.

    Handles graceful fallback for legacy unencrypted data.

    Args:
        value: The encrypted string (or plain text for legacy data).

    Returns:
        Decrypted plain text string, or original value if not encrypted.

    Raises:
        ValueError: If encryption is not configured or if decryption fails
            for what appears to be encrypted data (wrong key).
    """
    fernet = get_fernet()

    try:
        return fernet.decrypt(value.encode()).decode()
    except InvalidToken:
        if looks_like_fernet_token(value):
            # Looks like encrypted data but failed to decrypt - likely wrong key
            raise ValueError(
                "Decryption failed: data appears to be encrypted but key is incorrect. "
                "Check OPEN_NOTEBOOK_ENCRYPTION_KEY configuration."
            )
        # Not a valid token - treat as legacy plaintext
        return value
    except Exception as e:
        logger.error(f"Decryption failed: {e}")
        raise ValueError(f"Decryption failed: {str(e)}")