eidetica 0.2.0

//!
//! Provides the main database structures (`Instance` and `Database`).
//!
//! `Instance` manages multiple `Database` instances and interacts with the storage `Database`.
//! `Database` represents a single, independent history of data entries, analogous to a table or branch.

use std::{
    collections::HashMap,
    sync::{Arc, Mutex, Weak},
};

use ed25519_dalek::VerifyingKey;
use handle_trait::Handle;

use crate::{
    Database, Entry, Result, auth::crypto::format_public_key, backend::BackendImpl, entry::ID,
    sync::Sync, user::User,
};

pub mod backend;
pub mod errors;
pub mod legacy_ops;
pub mod settings_merge;

// Re-export main types for easier access
use backend::Backend;
pub use errors::InstanceError;
pub use legacy_ops::LegacyInstanceOps;

/// Private constants for device identity management
const DEVICE_KEY_NAME: &str = "_device_key";

/// Indicates whether an entry write originated locally or from a remote source (e.g., sync).
///
/// This distinction allows different callbacks to be triggered based on the write source,
/// enabling behaviors like "only trigger sync for local writes" or "only update UI for remote writes".
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum WriteSource {
    /// Write originated from a local transaction commit
    Local,
    /// Write originated from a remote source (e.g., sync, replication)
    Remote,
}

/// Callback function trait for write operations.
///
/// Receives the entry that was written, the database it was written to, and the instance.
/// Used for both local and remote write callbacks.
///
/// This trait alias can be used both as a trait bound (e.g., `F: WriteCallback`) and as a
/// trait object type for storage (e.g., `Arc<dyn WriteCallback>`).
pub trait WriteCallback:
    Fn(&Entry, &Database, &Instance) -> Result<()> + Send + std::marker::Sync
{
}

// Blanket implementation: any type that satisfies the bounds automatically implements WriteCallback
impl<T> WriteCallback for T where
    T: Fn(&Entry, &Database, &Instance) -> Result<()> + Send + std::marker::Sync
{
}

/// Type alias for a collection of write callbacks
type CallbackVec = Vec<Arc<dyn WriteCallback>>;

/// Type alias for the per-database callback map key
type CallbackKey = (WriteSource, ID);

/// Internal state for Instance
///
/// This structure holds the actual implementation data for Instance.
/// Instance itself is just a cheap-to-clone handle wrapping Arc<InstanceInternal>.
pub(crate) struct InstanceInternal {
    /// The database storage backend
    backend: Backend,
    /// Synchronization module for this database instance
    /// TODO: Overengineered, Sync can be created by default but disabled
    sync: std::sync::OnceLock<Arc<Sync>>,
    /// Root ID of the _users system database
    users_db_id: ID,
    /// Root ID of the _databases system database
    databases_db_id: ID,
    /// Per-database callbacks keyed by (WriteSource, tree_id)
    write_callbacks: Mutex<HashMap<CallbackKey, CallbackVec>>,
    /// Global callbacks keyed by WriteSource (triggered regardless of database)
    global_write_callbacks: Mutex<HashMap<WriteSource, CallbackVec>>,
}

impl std::fmt::Debug for InstanceInternal {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("InstanceInternal")
            .field("backend", &"<BackendDB>")
            .field("sync", &self.sync)
            .field("users_db_id", &self.users_db_id)
            .field("databases_db_id", &self.databases_db_id)
            .field(
                "write_callbacks",
                &format!(
                    "<{} per-db callbacks>",
                    self.write_callbacks.lock().unwrap().len()
                ),
            )
            .field(
                "global_write_callbacks",
                &format!(
                    "<{} global callbacks>",
                    self.global_write_callbacks.lock().unwrap().len()
                ),
            )
            .finish()
    }
}
/// Database implementation on top of the storage backend.
///
/// Instance manages infrastructure only:
/// - Backend storage and device identity (_device_key)
/// - System databases (_users, _databases, _sync)
/// - User account management (create, login, list)
///
/// All database creation and key operations happen through User after login.
///
/// Instance is a cheap-to-clone handle around `Arc<InstanceInternal>`.
///
/// ## Example
///
/// ```
/// # use eidetica::{backend::database::InMemory, Instance, crdt::Doc};
/// let instance = Instance::open(Box::new(InMemory::new()))?;
///
/// // Create passwordless user
/// instance.create_user("alice", None)?;
/// let mut user = instance.login_user("alice", None)?;
///
/// // Use User API for operations
/// let mut settings = Doc::new();
/// settings.set_string("name", "my_database");
/// let default_key = user.get_default_key()?;
/// let db = user.create_database(settings, &default_key)?;
/// # Ok::<(), eidetica::Error>(())
/// ```
#[derive(Clone, Debug, Handle)]
pub struct Instance {
    inner: Arc<InstanceInternal>,
}

/// Weak reference to an Instance.
///
/// This is a weak handle that does not prevent the Instance from being dropped.
/// Dependent objects (Database, Sync, BackgroundSync) hold weak references to avoid
/// circular reference cycles that would leak memory.
///
/// Use `upgrade()` to convert to a strong `Instance` reference.
#[derive(Clone, Debug, Handle)]
pub struct WeakInstance {
    inner: Weak<InstanceInternal>,
}

impl Instance {
    /// Load an existing Instance or create a new one (recommended).
    ///
    /// This is the recommended method for initializing an Instance. It automatically detects
    /// whether the backend contains existing system state (device key and system databases)
    /// and loads them, or creates new ones if starting fresh.
    ///
    /// Instance manages infrastructure only:
    /// - Backend storage and device identity (_device_key)
    /// - System databases (_users, _databases, _sync)
    /// - User account management (create, login, list)
    ///
    /// All database creation and key operations require explicit User login.
    ///
    /// # Arguments
    /// * `backend` - The storage backend to use
    ///
    /// # Returns
    /// A Result containing the configured Instance
    ///
    /// # Example
    /// ```
    /// # use eidetica::{backend::database::InMemory, Instance, crdt::Doc};
    /// let backend = InMemory::new();
    /// let instance = Instance::open(Box::new(backend))?;
    ///
    /// // Create and login user explicitly
    /// instance.create_user("alice", None)?;
    /// let mut user = instance.login_user("alice", None)?;
    ///
    /// // Use User API for operations
    /// let mut settings = Doc::new();
    /// settings.set_string("name", "my_database");
    /// let default_key = user.get_default_key()?;
    /// let db = user.create_database(settings, &default_key)?;
    /// # Ok::<(), eidetica::Error>(())
    /// ```
    pub fn open(backend: Box<dyn BackendImpl>) -> Result<Self> {
        use crate::constants::{DATABASES, USERS};

        let backend: Arc<dyn BackendImpl> = Arc::from(backend);

        // Load device_key first
        let _device_key = match backend.get_private_key(DEVICE_KEY_NAME)? {
            Some(key) => key,
            None => {
                // New backend: initialize like create()
                return Self::create_internal(backend);
            }
        };

        // Existing backend: load system databases
        let all_roots = backend.all_roots()?;

        // Find system databases by name
        let mut users_db_root = None;
        let mut databases_db_root = None;

        for root_id in all_roots {
            // FIXME(security): handle the security and loading of these databases in a better way
            // Use open_readonly temporarily to check name without setting up auth
            // Note: We can't use self.clone() here because self doesn't exist yet during construction
            // So we create a temporary Instance just for this lookup
            //
            // SAFETY: The temporary instance has empty users_db_id and databases_db_id placeholders.
            // This is safe because:
            // 1. We only use it for Database::open_readonly() which doesn't access these fields
            // 2. The Database only calls get_name() which reads from the settings store
            // 3. The temporary instance is dropped immediately after name lookup
            // 4. No other code paths will access the invalid system database IDs
            let temp_instance = Self {
                inner: Arc::new(InstanceInternal {
                    backend: Backend::new(Arc::clone(&backend)),
                    sync: std::sync::OnceLock::new(),
                    users_db_id: ID::from(""), // Placeholder - not accessed during name lookup
                    databases_db_id: ID::from(""), // Placeholder - not accessed during name lookup
                    write_callbacks: Mutex::new(HashMap::new()),
                    global_write_callbacks: Mutex::new(HashMap::new()),
                }),
            };
            let temp_db = Database::open_readonly(root_id.clone(), &temp_instance)?;
            if let Ok(name) = temp_db.get_name() {
                match name.as_str() {
                    USERS => {
                        if users_db_root.is_some() {
                            panic!(
                                "CRITICAL SECURITY ERROR: Multiple {USERS} databases found in backend. \
                                     This indicates database corruption or a potential security breach. \
                                     Backend integrity compromised."
                            );
                        }
                        users_db_root = Some(root_id);
                    }
                    DATABASES => {
                        if databases_db_root.is_some() {
                            panic!(
                                "CRITICAL SECURITY ERROR: Multiple {DATABASES} databases found in backend. \
                                     This indicates database corruption or a potential security breach. \
                                     Backend integrity compromised."
                            );
                        }
                        databases_db_root = Some(root_id);
                    }
                    _ => {} // Ignore other databases
                }
            }

            // Stop searching if we found both
            if users_db_root.is_some() && databases_db_root.is_some() {
                break;
            }
        }

        // Verify we found both system databases
        let users_db_root = users_db_root.ok_or(InstanceError::SystemDatabaseNotFound {
            database_name: USERS.to_string(),
        })?;
        let databases_db_root = databases_db_root.ok_or(InstanceError::SystemDatabaseNotFound {
            database_name: DATABASES.to_string(),
        })?;

        let inner = Arc::new(InstanceInternal {
            backend: Backend::new(backend),
            sync: std::sync::OnceLock::new(),
            users_db_id: users_db_root,
            databases_db_id: databases_db_root,
            write_callbacks: Mutex::new(HashMap::new()),
            global_write_callbacks: Mutex::new(HashMap::new()),
        });

        Ok(Self { inner })
    }

    /// Create a new Instance on a fresh backend (strict creation).
    ///
    /// This method creates a new Instance and fails if the backend is already initialized
    /// (contains a device key and system databases). Use this when you want to ensure
    /// you're creating a fresh instance.
    ///
    /// Instance manages infrastructure only:
    /// - Backend storage and device identity (_device_key)
    /// - System databases (_users, _databases, _sync)
    /// - User account management (create, login, list)
    ///
    /// All database creation and key operations require explicit User login.
    ///
    /// For most use cases, prefer `Instance::open()` which automatically handles both
    /// new and existing backends.
    ///
    /// # Arguments
    /// * `backend` - The storage backend to use (must be uninitialized)
    ///
    /// # Returns
    /// A Result containing the configured Instance, or InstanceAlreadyExists error
    /// if the backend is already initialized.
    ///
    /// # Example
    /// ```
    /// # use eidetica::{backend::database::InMemory, Instance, crdt::Doc};
    /// let backend = InMemory::new();
    /// let instance = Instance::create(Box::new(backend))?;
    ///
    /// // Create and login user explicitly
    /// instance.create_user("alice", None)?;
    /// let mut user = instance.login_user("alice", None)?;
    ///
    /// // Use User API for operations
    /// let mut settings = Doc::new();
    /// settings.set_string("name", "my_database");
    /// let default_key = user.get_default_key()?;
    /// let db = user.create_database(settings, &default_key)?;
    /// # Ok::<(), eidetica::Error>(())
    /// ```
    pub fn create(backend: Box<dyn BackendImpl>) -> Result<Self> {
        let backend: Arc<dyn BackendImpl> = Arc::from(backend);

        // Check if already initialized
        if backend.get_private_key(DEVICE_KEY_NAME)?.is_some() {
            return Err(InstanceError::InstanceAlreadyExists.into());
        }

        // Create new instance
        Self::create_internal(backend)
    }

    /// Internal implementation of new that works with Arc<dyn BackendImpl>
    pub(crate) fn create_internal(backend: Arc<dyn BackendImpl>) -> Result<Self> {
        use crate::{
            auth::crypto::{format_public_key, generate_keypair},
            user::system_databases::{create_databases_tracking, create_users_database},
        };

        // 1. Generate and store instance device key (_device_key)
        let (device_key, device_pubkey) = generate_keypair();
        let device_pubkey_str = format_public_key(&device_pubkey);
        backend.store_private_key(DEVICE_KEY_NAME, device_key.clone())?;

        // 2. Create system databases with device_key passed directly
        // Create a temporary Instance for database creation (databases will store full IDs later)
        //
        // SAFETY: The temporary instance has empty users_db_id and databases_db_id placeholders.
        // This is safe because:
        // 1. We only use it to create new system databases via Database::create()
        // 2. Database::create() doesn't access the instance's system database IDs
        // 3. The system databases don't exist yet, so their IDs can't be referenced
        // 4. The temporary instance is only used during initial setup and discarded
        // 5. The real instance is constructed afterward with the correct database IDs
        let temp_instance = Self {
            inner: Arc::new(InstanceInternal {
                backend: Backend::new(Arc::clone(&backend)),
                sync: std::sync::OnceLock::new(),
                users_db_id: ID::from(""), // Placeholder - system DBs don't exist yet
                databases_db_id: ID::from(""), // Placeholder - system DBs don't exist yet
                write_callbacks: Mutex::new(HashMap::new()),
                global_write_callbacks: Mutex::new(HashMap::new()),
            }),
        };
        let users_db = create_users_database(&temp_instance, &device_key, &device_pubkey_str)?;
        let databases_db =
            create_databases_tracking(&temp_instance, &device_key, &device_pubkey_str)?;

        // 3. Store root IDs and return instance
        let inner = Arc::new(InstanceInternal {
            backend: Backend::new(backend),
            sync: std::sync::OnceLock::new(),
            users_db_id: users_db.root_id().clone(),
            databases_db_id: databases_db.root_id().clone(),
            write_callbacks: Mutex::new(HashMap::new()),
            global_write_callbacks: Mutex::new(HashMap::new()),
        });

        Ok(Self { inner })
    }

    /// Get a reference to the backend
    pub fn backend(&self) -> &Backend {
        &self.inner.backend
    }

    // === Backend pass-through methods (pub(crate) for internal use) ===

    /// Get an entry from the backend
    pub(crate) fn get(&self, id: &crate::entry::ID) -> Result<crate::entry::Entry> {
        self.inner.backend.get(id)
    }

    /// Put an entry into the backend
    pub(crate) fn put(
        &self,
        verification_status: crate::backend::VerificationStatus,
        entry: crate::entry::Entry,
    ) -> Result<()> {
        self.inner.backend.put(verification_status, entry)
    }

    /// Get tips for a tree
    pub(crate) fn get_tips(&self, tree: &crate::entry::ID) -> Result<Vec<crate::entry::ID>> {
        self.inner.backend.get_tips(tree)
    }

    // === System database accessors ===

    /// Get the _users database
    ///
    /// This constructs a Database instance on-the-fly to avoid circular references.
    pub(crate) fn users_db(&self) -> Result<Database> {
        let device_key = self
            .inner
            .backend
            .get_private_key(DEVICE_KEY_NAME)?
            .ok_or(InstanceError::DeviceKeyNotFound)?;

        Database::open(
            self.clone(),
            &self.inner.users_db_id,
            device_key,
            "_device_key".to_string(),
        )
    }

    // === User Management ===

    /// Create a new user account with flexible password handling.
    ///
    /// Creates a user with or without password protection. Passwordless users are appropriate
    /// for embedded applications where filesystem access = database access.
    ///
    /// # Arguments
    /// * `user_id` - Unique user identifier (username)
    /// * `password` - Optional password. If None, user is passwordless (instant login, no encryption)
    ///
    /// # Returns
    /// A Result containing the user's UUID (stable internal identifier)
    pub fn create_user(&self, user_id: &str, password: Option<&str>) -> Result<String> {
        use crate::user::system_databases::create_user;

        let users_db = self.users_db()?;
        let (user_uuid, _user_info) = create_user(&users_db, self, user_id, password)?;
        Ok(user_uuid)
    }

    /// Login a user with flexible password handling.
    ///
    /// Returns a User session object that provides access to user operations.
    /// For password-protected users, provide the password. For passwordless users, pass None.
    ///
    /// # Arguments
    /// * `user_id` - User identifier (username)
    /// * `password` - Optional password. None for passwordless users.
    ///
    /// # Returns
    /// A Result containing the User session
    pub fn login_user(&self, user_id: &str, password: Option<&str>) -> Result<User> {
        use crate::user::system_databases::login_user;

        let users_db = self.users_db()?;
        login_user(&users_db, self, user_id, password)
    }

    /// List all user IDs.
    ///
    /// # Returns
    /// A Result containing a vector of user IDs
    pub fn list_users(&self) -> Result<Vec<String>> {
        use crate::user::system_databases::list_users;

        let users_db = self.users_db()?;
        list_users(&users_db)
    }

    // === User-Sync Integration ===

    // === Device Identity Management ===
    //
    // The Instance's device identity (_device_key) is stored in the backend.

    /// Get the device ID (public key).
    ///
    /// The device key (_device_key) is stored in the backend.
    ///
    /// # Returns
    /// A `Result` containing the device's public key (device ID).
    pub fn device_id(&self) -> Result<VerifyingKey> {
        let device_key = self
            .inner
            .backend
            .get_private_key(DEVICE_KEY_NAME)?
            .ok_or_else(|| crate::Error::from(InstanceError::DeviceKeyNotFound))?;
        Ok(device_key.verifying_key())
    }

    /// Get the device ID as a formatted string.
    ///
    /// This is a convenience method that returns the device ID (public key)
    /// in a standard formatted string representation.
    ///
    /// # Returns
    /// A `Result` containing the formatted device ID string.
    pub fn device_id_string(&self) -> Result<String> {
        let device_key = self.device_id()?;
        Ok(format_public_key(&device_key))
    }

    /// Load an existing database from the backend by its root ID.
    ///
    /// # Arguments
    /// * `root_id` - The content-addressable ID of the root `Entry` of the database to load.
    ///
    /// # Returns
    /// A `Result` containing the loaded `Database` or an error if the root ID is not found.
    pub fn load_database(&self, root_id: &ID) -> Result<Database> {
        // First validate the root_id exists in the backend
        // Make sure the entry exists
        self.inner.backend.get(root_id)?;

        // Create a database object with the given root_id
        let database = Database::open_readonly(root_id.clone(), self)?;
        Ok(database)
    }

    /// Load all databases stored in the backend.
    ///
    /// This retrieves all known root entry IDs from the backend and constructs
    /// `Database` instances for each.
    ///
    /// # Returns
    /// A `Result` containing a vector of all `Database` instances or an error.
    pub fn all_databases(&self) -> Result<Vec<Database>> {
        let root_ids = self.inner.backend.all_roots()?;
        let mut databases = Vec::new();

        for root_id in root_ids {
            let database = Database::open_readonly(root_id.clone(), self)?;
            databases.push(database);
        }

        Ok(databases)
    }

    /// Find databases by their assigned name.
    ///
    /// Searches through all databases in the backend and returns those whose "name"
    /// setting matches the provided name.
    ///
    /// # Arguments
    /// * `name` - The name to search for.
    ///
    /// # Returns
    /// A `Result` containing a vector of `Database` instances whose name matches,
    /// or an error.
    ///
    /// # Errors
    /// Returns `InstanceError::DatabaseNotFound` if no databases with the specified name are found.
    pub fn find_database(&self, name: impl AsRef<str>) -> Result<Vec<Database>> {
        let name = name.as_ref();
        let all_databases = self.all_databases()?;
        let mut matching_databases = Vec::new();

        for database in all_databases {
            // Attempt to get the name from the database's settings
            if let Ok(database_name) = database.get_name()
                && database_name == name
            {
                matching_databases.push(database);
            }
            // Ignore databases where getting the name fails or doesn't match
        }

        if matching_databases.is_empty() {
            Err(InstanceError::DatabaseNotFound {
                name: name.to_string(),
            }
            .into())
        } else {
            Ok(matching_databases)
        }
    }

    // === Authentication Key Management ===

    /// List all private key IDs.
    ///
    /// # Returns
    /// A `Result` containing a vector of key IDs or an error.
    pub fn list_private_keys(&self) -> Result<Vec<String>> {
        // List keys from backend storage
        self.inner.backend.list_private_keys()
    }

    // === Synchronization Management ===
    //
    // These methods provide access to the Sync module for managing synchronization
    // settings and state for this database instance.

    /// Initializes the Sync module for this instance.
    ///
    /// Enables synchronization operations for this instance. This method is idempotent;
    /// calling it multiple times has no effect.
    ///
    /// # Errors
    /// Returns an error if the sync settings database cannot be created or if device key
    /// generation/storage fails.
    pub fn enable_sync(&self) -> Result<()> {
        // Check if there is an existing Sync database already configured
        if self.inner.sync.get().is_some() {
            return Ok(());
        }
        let sync = Sync::new(self.clone())?;
        let sync_arc = Arc::new(sync);

        // Register global callback for automatic sync on local writes
        let sync_for_callback = Arc::clone(&sync_arc);
        self.register_global_write_callback(
            WriteSource::Local,
            move |entry, database, instance| {
                sync_for_callback.on_local_write(entry, database, instance)
            },
        )?;

        let _ = self.inner.sync.set(sync_arc);
        Ok(())
    }

    /// Get a reference to the Sync module.
    ///
    /// Returns a cheap-to-clone Arc handle to the Sync module. The Sync module
    /// uses interior mutability (AtomicBool and OnceLock) so &self methods are sufficient.
    ///
    /// # Returns
    /// An `Option` containing an `Arc<Sync>` if the Sync module is initialized.
    pub fn sync(&self) -> Option<Arc<Sync>> {
        self.inner.sync.get().map(Arc::clone)
    }

    // === Entry Write Coordination ===
    //
    // All entry writes go through Instance::put_entry() which handles backend storage
    // and callback dispatch. This centralizes write coordination and ensures hooks fire.

    /// Register a callback to be invoked when entries are written to a database.
    ///
    /// The callback receives the entry, database, and instance as parameters.
    ///
    /// # Arguments
    /// * `source` - The write source to monitor (Local or Remote)
    /// * `tree_id` - The root ID of the database tree to monitor
    /// * `callback` - Function to invoke on writes
    ///
    /// # Returns
    /// A Result indicating success or failure
    pub(crate) fn register_write_callback<F>(
        &self,
        source: WriteSource,
        tree_id: ID,
        callback: F,
    ) -> Result<()>
    where
        F: Fn(&Entry, &Database, &Instance) -> Result<()> + Send + std::marker::Sync + 'static,
    {
        let mut callbacks = self.inner.write_callbacks.lock().unwrap();
        callbacks
            .entry((source, tree_id))
            .or_default()
            .push(Arc::new(callback));
        Ok(())
    }

    /// Register a global callback to be invoked on all writes of a specific source.
    ///
    /// Global callbacks are invoked for all writes of the specified source across all databases.
    /// This is useful for system-wide operations like synchronization that need to track
    /// changes across all databases.
    ///
    /// # Arguments
    /// * `source` - The write source to monitor (Local or Remote)
    /// * `callback` - Function to invoke on all writes
    ///
    /// # Returns
    /// A Result indicating success or failure
    pub(crate) fn register_global_write_callback<F>(
        &self,
        source: WriteSource,
        callback: F,
    ) -> Result<()>
    where
        F: Fn(&Entry, &Database, &Instance) -> Result<()> + Send + std::marker::Sync + 'static,
    {
        let mut callbacks = self.inner.global_write_callbacks.lock().unwrap();
        callbacks
            .entry(source)
            .or_default()
            .push(Arc::new(callback));
        Ok(())
    }

    /// Write an entry to the backend and dispatch callbacks.
    ///
    /// This is the central coordination point for all entry writes in the system.
    /// All writes must go through this method to ensure:
    /// - Entries are persisted to the backend
    /// - Appropriate callbacks are triggered based on write source
    /// - Hooks have full context (entry, database, instance)
    ///
    /// # Arguments
    /// * `tree_id` - The root ID of the database being written to
    /// * `verification` - Authentication verification status of the entry
    /// * `entry` - The entry to write
    /// * `source` - Whether this is a local or remote write
    ///
    /// # Returns
    /// A Result indicating success or failure
    pub fn put_entry(
        &self,
        tree_id: &ID,
        verification: crate::backend::VerificationStatus,
        entry: Entry,
        source: WriteSource,
    ) -> Result<()> {
        // 1. Persist to backend storage
        self.backend().put(verification, entry.clone())?;

        // 2. Look up and execute callbacks based on write source
        // Clone the callbacks to avoid holding the lock while executing callbacks.
        let per_db_callbacks = self
            .inner
            .write_callbacks
            .lock()
            .unwrap()
            .get(&(source, tree_id.clone()))
            .cloned();

        let global_callbacks = self
            .inner
            .global_write_callbacks
            .lock()
            .unwrap()
            .get(&source)
            .cloned();

        // 3. Execute callbacks if any are registered
        let has_callbacks = per_db_callbacks.is_some() || global_callbacks.is_some();
        if has_callbacks {
            // Create a Database handle for the callbacks
            // Use open_readonly since we only need it for callback context
            let database = Database::open_readonly(tree_id.clone(), self)?;

            // Execute per-database callbacks
            if let Some(callbacks) = per_db_callbacks {
                for callback in callbacks {
                    if let Err(e) = callback(&entry, &database, self) {
                        tracing::error!(
                            tree_id = %tree_id,
                            entry_id = %entry.id(),
                            source = ?source,
                            "Per-database callback failed: {}", e
                        );
                        // Continue executing other callbacks even if one fails
                    }
                }
            }

            // Execute global callbacks
            if let Some(callbacks) = global_callbacks {
                for callback in callbacks {
                    if let Err(e) = callback(&entry, &database, self) {
                        tracing::error!(
                            tree_id = %tree_id,
                            entry_id = %entry.id(),
                            source = ?source,
                            "Global callback failed: {}", e
                        );
                        // Continue executing other callbacks even if one fails
                    }
                }
            }
        }

        Ok(())
    }

    /// Downgrade to a weak reference.
    ///
    /// Creates a weak reference that does not prevent the Instance from being dropped.
    /// This is useful for preventing circular reference cycles in dependent objects.
    ///
    /// # Returns
    /// A `WeakInstance` that can be upgraded back to a strong reference.
    pub fn downgrade(&self) -> WeakInstance {
        WeakInstance {
            inner: Arc::downgrade(&self.inner),
        }
    }
}

impl WeakInstance {
    /// Upgrade to a strong reference.
    ///
    /// Attempts to upgrade this weak reference to a strong `Instance` reference.
    /// Returns `None` if the Instance has already been dropped.
    ///
    /// # Returns
    /// `Some(Instance)` if the Instance still exists, `None` otherwise.
    ///
    /// # Example
    /// ```
    /// # use eidetica::{backend::database::InMemory, Instance};
    /// let instance = Instance::open(Box::new(InMemory::new()))?;
    /// let weak = instance.downgrade();
    ///
    /// // Upgrade works while instance exists
    /// assert!(weak.upgrade().is_some());
    ///
    /// drop(instance);
    /// // Upgrade fails after instance is dropped
    /// assert!(weak.upgrade().is_none());
    /// # Ok::<(), eidetica::Error>(())
    /// ```
    pub fn upgrade(&self) -> Option<Instance> {
        self.inner.upgrade().map(|inner| Instance { inner })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{Error, backend::database::InMemory, crdt::Doc, instance::LegacyInstanceOps};

    #[test]
    fn test_create_user() -> Result<(), Error> {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend))?;

        // Create user with password
        let user_uuid = instance.create_user("alice", Some("password123")).unwrap();

        assert!(!user_uuid.is_empty());

        // Verify user appears in list
        let users = instance.list_users().unwrap();
        assert_eq!(users.len(), 1);
        assert_eq!(users[0], "alice");
        Ok(())
    }

    #[test]
    fn test_login_user() -> Result<(), Error> {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend))?;

        // Create user
        instance.create_user("alice", Some("password123")).unwrap();

        // Login user
        let user = instance.login_user("alice", Some("password123")).unwrap();
        assert_eq!(user.username(), "alice");

        // Invalid password should fail
        let result = instance.login_user("alice", Some("wrong_password"));
        assert!(result.is_err());
        Ok(())
    }

    #[test]
    fn test_new_database() {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend)).expect("Failed to create test instance");

        // Create database with deprecated API
        let mut settings = Doc::new();
        settings.set_string("name", "test_db");

        let database = instance.new_database(settings, "_device_key").unwrap();
        assert_eq!(database.get_name().unwrap(), "test_db");
    }

    #[test]
    fn test_new_database_default() {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend)).expect("Failed to create test instance");

        // Create database with default settings
        let database = instance.new_database_default("_device_key").unwrap();
        let settings = database.get_settings().unwrap();

        // Should have auto-generated database_id
        assert!(settings.get_string("database_id").is_ok());
    }

    #[test]
    fn test_new_database_without_key_fails() -> Result<(), Error> {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend))?;

        // Create database requires a signing key
        let mut settings = Doc::new();
        settings.set_string("name", "test_db");

        // This will succeed if a valid key is provided, but we're testing without a valid key
        let result = instance.new_database(settings, "nonexistent_key");
        assert!(result.is_err());
        Ok(())
    }

    #[test]
    fn test_load_database() {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend)).expect("Failed to create test instance");

        // Create a database
        let mut settings = Doc::new();
        settings.set_string("name", "test_db");
        let database = instance.new_database(settings, "_device_key").unwrap();
        let root_id = database.root_id().clone();

        // Load the database
        let loaded_database = instance.load_database(&root_id).unwrap();
        assert_eq!(loaded_database.get_name().unwrap(), "test_db");
    }

    #[test]
    fn test_all_databases() {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend)).expect("Failed to create test instance");

        // Create multiple databases
        let mut settings1 = Doc::new();
        settings1.set_string("name", "db1");
        instance.new_database(settings1, "_device_key").unwrap();

        let mut settings2 = Doc::new();
        settings2.set_string("name", "db2");
        instance.new_database(settings2, "_device_key").unwrap();

        // Get all databases (should include system databases + user databases)
        let databases = instance.all_databases().unwrap();
        assert!(databases.len() >= 2); // At least our 2 databases + system databases
    }

    #[test]
    fn test_find_database() {
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend)).expect("Failed to create test instance");

        // Create database with name
        let mut settings = Doc::new();
        settings.set_string("name", "my_special_db");
        instance.new_database(settings, "_device_key").unwrap();

        // Find by name
        let found = instance.find_database("my_special_db").unwrap();
        assert_eq!(found.len(), 1);
        assert_eq!(found[0].get_name().unwrap(), "my_special_db");

        // Not found
        let result = instance.find_database("nonexistent");
        assert!(result.is_err());
    }

    #[test]
    fn test_instance_load_new_backend() -> Result<(), Error> {
        // Test that Instance::load() creates new system state for empty backend
        let backend = InMemory::new();
        let instance = Instance::open(Box::new(backend))?;

        // Verify device key was created
        assert!(instance.device_id().is_ok());

        // Verify we can create and login a user
        instance.create_user("alice", None)?;
        let user = instance.login_user("alice", None)?;
        assert_eq!(user.username(), "alice");

        Ok(())
    }

    #[test]
    fn test_instance_load_existing_backend() -> Result<(), Error> {
        // Use a temporary file path for testing
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_instance_load.json");

        // Create an instance and user, then save the backend
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;
        instance1.create_user("bob", None)?;
        let mut user1 = instance1.login_user("bob", None)?;

        // Get the default key (earliest created key)
        let default_key = user1.get_default_key()?;

        // Create a user database to verify it persists
        let mut settings = Doc::new();
        settings.set_string("name", "bob_database");
        user1.create_database(settings, &default_key)?;

        // Save the backend to file
        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }

        // Drop the first instance
        drop(instance1);
        drop(user1);

        // Load a new backend from the saved file
        let backend2 = InMemory::load_from_file(&path)?;
        let instance2 = Instance::open(Box::new(backend2))?;

        // Verify the user still exists
        let users = instance2.list_users()?;
        assert_eq!(users.len(), 1);
        assert_eq!(users[0], "bob");

        // Verify we can login the existing user
        let user2 = instance2.login_user("bob", None)?;
        assert_eq!(user2.username(), "bob");

        // Clean up the temporary file
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_load_device_id_persistence() -> Result<(), Error> {
        // Test that device_id remains the same across reloads
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_device_id.json");

        // Create instance and get device_id
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;
        let device_id1 = instance1.device_id_string()?;

        // Save backend
        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Load backend and verify device_id is the same
        let backend2 = InMemory::load_from_file(&path)?;
        let instance2 = Instance::open(Box::new(backend2))?;
        let device_id2 = instance2.device_id_string()?;

        assert_eq!(
            device_id1, device_id2,
            "Device ID should persist across reloads"
        );

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_load_with_password_protected_users() -> Result<(), Error> {
        // Test that password-protected users work correctly after reload
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_password_users.json");

        // Create instance with password-protected user
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;
        instance1.create_user("secure_alice", Some("secret123"))?;
        let user1 = instance1.login_user("secure_alice", Some("secret123"))?;
        assert_eq!(user1.username(), "secure_alice");
        drop(user1);

        // Save backend
        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Reload and verify password still works
        let backend2 = InMemory::load_from_file(&path)?;
        let instance2 = Instance::open(Box::new(backend2))?;

        // Correct password should work
        let user2 = instance2.login_user("secure_alice", Some("secret123"))?;
        assert_eq!(user2.username(), "secure_alice");

        // Wrong password should fail
        let result = instance2.login_user("secure_alice", Some("wrong_password"));
        assert!(result.is_err(), "Login with wrong password should fail");

        // No password should fail
        let result = instance2.login_user("secure_alice", None);
        assert!(
            result.is_err(),
            "Login without password should fail for password-protected user"
        );

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_load_multiple_users() -> Result<(), Error> {
        // Test that multiple users persist correctly
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_multiple_users.json");

        // Create instance with multiple users (mix of passwordless and password-protected)
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;

        instance1.create_user("alice", None)?;
        instance1.create_user("bob", Some("bobpass"))?;
        instance1.create_user("charlie", None)?;
        instance1.create_user("diana", Some("dianapass"))?;

        // Verify all users can login
        instance1.login_user("alice", None)?;
        instance1.login_user("bob", Some("bobpass"))?;
        instance1.login_user("charlie", None)?;
        instance1.login_user("diana", Some("dianapass"))?;

        // Save backend
        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Reload and verify all users still exist and can login
        let backend2 = InMemory::load_from_file(&path)?;
        let instance2 = Instance::open(Box::new(backend2))?;

        let users = instance2.list_users()?;
        assert_eq!(users.len(), 4, "All 4 users should be present after reload");
        assert!(users.contains(&"alice".to_string()));
        assert!(users.contains(&"bob".to_string()));
        assert!(users.contains(&"charlie".to_string()));
        assert!(users.contains(&"diana".to_string()));

        // Verify login still works for all users
        instance2.login_user("alice", None)?;
        instance2.login_user("bob", Some("bobpass"))?;
        instance2.login_user("charlie", None)?;
        instance2.login_user("diana", Some("dianapass"))?;

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_load_user_databases_persist() -> Result<(), Error> {
        // Test that user-created databases persist across reloads
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_user_dbs.json");

        // Create instance, user, and multiple databases
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;
        instance1.create_user("eve", None)?;
        let mut user1 = instance1.login_user("eve", None)?;

        // Get the default key (earliest created key)
        let default_key = user1.get_default_key()?;

        // Create multiple databases
        let mut settings1 = Doc::new();
        settings1.set_string("name", "database_one");
        settings1.set_string("purpose", "testing");
        let db1 = user1.create_database(settings1, &default_key)?;
        let db1_root = db1.root_id().clone();

        let mut settings2 = Doc::new();
        settings2.set_string("name", "database_two");
        settings2.set_string("purpose", "production");
        let db2 = user1.create_database(settings2, &default_key)?;
        let db2_root = db2.root_id().clone();

        drop(db1);
        drop(db2);
        drop(user1);

        // Save backend
        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Reload and verify databases still exist
        let backend2 = InMemory::load_from_file(&path)?;
        let instance2 = Instance::open(Box::new(backend2))?;
        let _user2 = instance2.login_user("eve", None)?;

        // Load databases by root_id and verify their settings
        let loaded_db1 = instance2.load_database(&db1_root)?;
        assert_eq!(loaded_db1.get_name()?, "database_one");
        let settings1_doc = loaded_db1.get_settings()?;
        assert_eq!(settings1_doc.get_string("purpose")?, "testing");

        let loaded_db2 = instance2.load_database(&db2_root)?;
        assert_eq!(loaded_db2.get_name()?, "database_two");
        let settings2_doc = loaded_db2.get_settings()?;
        assert_eq!(settings2_doc.get_string("purpose")?, "production");

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_load_idempotency() -> Result<(), Error> {
        // Test that loading the same backend multiple times gives consistent results
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_idempotency.json");

        // Create and save initial state
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;
        instance1.create_user("frank", None)?;
        let device_id1 = instance1.device_id_string()?;

        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Load the same backend multiple times and verify consistency
        for i in 0..3 {
            let backend = InMemory::load_from_file(&path)?;
            let instance = Instance::open(Box::new(backend))?;

            // Device ID should be the same every time
            let device_id = instance.device_id_string()?;
            assert_eq!(
                device_id, device_id1,
                "Device ID should be consistent on reload {i}"
            );

            // User list should be the same
            let users = instance.list_users()?;
            assert_eq!(users.len(), 1);
            assert_eq!(users[0], "frank");

            // Should be able to login
            let user = instance.login_user("frank", None)?;
            assert_eq!(user.username(), "frank");

            drop(user);
            drop(instance);
        }

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_load_new_vs_existing() -> Result<(), Error> {
        // Test the difference between loading new and existing backends
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_new_vs_existing.json");

        // Create first instance (new backend)
        let backend1 = InMemory::new();
        let instance1 = Instance::open(Box::new(backend1))?;
        let device_id1 = instance1.device_id_string()?;
        instance1.create_user("grace", None)?;

        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Load existing backend
        let backend2 = InMemory::load_from_file(&path)?;
        let instance2 = Instance::open(Box::new(backend2))?;
        let device_id2 = instance2.device_id_string()?;

        // Device ID should match (existing backend)
        assert_eq!(device_id1, device_id2);

        // User should exist (existing backend)
        let users = instance2.list_users()?;
        assert_eq!(users.len(), 1);
        assert_eq!(users[0], "grace");
        drop(instance2);

        // Create completely new instance (different backend)
        let backend3 = InMemory::new();
        let instance3 = Instance::open(Box::new(backend3))?;
        let device_id3 = instance3.device_id_string()?;

        // Device ID should be different (new backend)
        assert_ne!(device_id1, device_id3);

        // No users should exist (new backend)
        let users = instance3.list_users()?;
        assert_eq!(users.len(), 0);

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_create_strict_fails_on_existing() -> Result<(), Error> {
        // Test that Instance::create() fails on already-initialized backend
        let temp_dir = std::env::temp_dir();
        let path = temp_dir.join("eidetica_test_create_strict.json");

        // Create first instance
        let backend1 = InMemory::new();
        let instance1 = Instance::create(Box::new(backend1))?;
        instance1.create_user("alice", None)?;

        // Save backend
        let backend_guard = instance1.backend();
        if let Some(in_memory) = backend_guard.as_any().downcast_ref::<InMemory>() {
            in_memory.save_to_file(&path)?;
        }
        drop(instance1);

        // Try to create() on the existing backend - should fail
        let backend2 = InMemory::load_from_file(&path)?;
        let result = Instance::create(Box::new(backend2));
        assert!(result.is_err(), "create() should fail on existing backend");

        // Verify error type
        if let Err(err) = result {
            if let crate::Error::Instance(instance_err) = err {
                assert!(
                    instance_err.is_already_exists(),
                    "Error should be InstanceAlreadyExists"
                );
            } else {
                panic!("Expected Instance error");
            }
        }

        // Verify open() still works
        let backend3 = InMemory::load_from_file(&path)?;
        let instance3 = Instance::open(Box::new(backend3))?;
        let users = instance3.list_users()?;
        assert_eq!(users.len(), 1);
        assert_eq!(users[0], "alice");

        // Clean up
        if path.exists() {
            std::fs::remove_file(&path).ok();
        }

        Ok(())
    }

    #[test]
    fn test_instance_create_on_fresh_backend() -> Result<(), Error> {
        // Test that Instance::create() succeeds on fresh backend
        let backend = InMemory::new();
        let instance = Instance::create(Box::new(backend))?;

        // Verify instance is properly initialized
        assert!(instance.device_id().is_ok());

        // Verify we can create users
        instance.create_user("bob", None)?;
        let user = instance.login_user("bob", None)?;
        assert_eq!(user.username(), "bob");

        Ok(())
    }
}