// SPDX-FileCopyrightText: Copyright (c) 2024-2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use std::sync::Arc;

use super::block::registry::RegistrationHandle;

/// The [EventManager] is not responsible for managing the history of the blocks, nor what
/// events have been published.
///
/// The [EventManager] is only responsible for issuing events on state changes. In this case,
/// there are two states:
///
/// - Store: a dynamo event plane message will be published which defines the registration/storing
///   of the block. Details include, but are not limited to, the sequence/prefix hash, the local block
///   hash, the sequence position of the block, the block size, and the storage location/class which
///   the block is stored in.
///
/// - Remove: a dynamo event plane message will be published which defines the removal of the block
///   from the cache. This messasge will include enough information to identify the block within a
///   storage hierarchy; minmally, the sequence hash and the storage location/class.
///
/// The [RegistrationHandle] associated from [EventManager::block_register] call is an RAII object
/// which will trigger a `Remove` event on being dropped.
pub trait EventManager: EventPublisher + EventReleaseManager + Send + Sync {
    // fn register_block(&self, token_block: &TokenBlock) -> PublishHandle;
    // fn publisher(&self) -> Publisher;
}

pub trait EventPublisher: Send + Sync {
    fn publish(&self, handles: Vec<Arc<RegistrationHandle>>);
}

pub trait EventReleaseManager: Send + Sync {
    fn block_release(&self, registration_handle: &RegistrationHandle);
}

/// A handle to a registered block.
///
/// Ensures that the register event published before the release event by
/// holding an [Arc] to the [RegistrationHandle], which by extension holds
/// issues the release event when dropped.
///
/// Ownership of the [PublishHandle] transferred to a [Publisher] object
/// which is responsible for coordinating the publication of multiple
/// registration events.
pub struct PublishHandle {
    handle: Arc<RegistrationHandle>,
    publisher: Option<Arc<dyn EventPublisher>>,
}

impl PublishHandle {
    pub fn new(handle: RegistrationHandle, publisher: Arc<dyn EventPublisher>) -> Self {
        let handle = Arc::new(handle);
        let publisher = Some(publisher);
        Self { handle, publisher }
    }

    pub fn remove_handle(&self) -> Arc<RegistrationHandle> {
        self.handle.clone()
    }

    fn disarm(&mut self) {
        self.publisher = None;
    }
}

impl Drop for PublishHandle {
    fn drop(&mut self) {
        if let Some(publisher) = self.publisher.take() {
            publisher.publish(vec![self.handle.clone()]);
        }
    }
}

/// Responsible for publishing multiple registration events.
///
/// Because [EventPublisher::publish] takes a list of shared [RegistrationHandles][RegistrationHandle]
/// this allows the [EventPublisher] logic to optimize the number of events published
/// by consoldiate multiple registration events with additional sequence logic.
///
/// The behavior of the [EventPublisher] is left entirely up to the the implementor.
#[derive(Clone)]
pub struct Publisher {
    handles: Vec<Arc<RegistrationHandle>>,
    publisher: Arc<dyn EventPublisher>,
}

impl Publisher {
    pub fn new(publisher: Arc<dyn EventPublisher>) -> Self {
        Self {
            handles: Vec::new(),
            publisher,
        }
    }

    pub fn take_handle(&mut self, publish_handle: PublishHandle) -> Arc<RegistrationHandle> {
        let handle = publish_handle.remove_handle();
        self.handles.push(handle.clone());
        let mut publish_handle = publish_handle;
        publish_handle.disarm();
        handle
    }

    pub fn publish(&mut self) {
        let handles = std::mem::take(&mut self.handles);
        if !handles.is_empty() {
            self.publisher.publish(handles);
        }
    }
}

impl Drop for Publisher {
    fn drop(&mut self) {
        self.publish();
    }
}

// Implementation notes:
//
// - Removable events are per blocks. I think we will want to leverage a task to collect drop/remove
//   events so that we can batch them together.
//
// - Registration events are can be batched by the nature of the [EventManager::register_blocks] call.

pub struct NullEventManager;

impl NullEventManager {
    pub fn new() -> Arc<Self> {
        Arc::new(Self {})
    }
}

impl EventManager for NullEventManager {}

impl EventPublisher for NullEventManager {
    fn publish(&self, _handles: Vec<Arc<RegistrationHandle>>) {}
}

impl EventReleaseManager for NullEventManager {
    fn block_release(&self, _registration_handle: &RegistrationHandle) {}
}

#[cfg(test)]
pub mod tests {
    use crate::tokens::SequenceHash;

    use super::*;

    #[derive(Debug, PartialEq, Eq)]
    pub enum EventType {
        Register(SequenceHash),
        Remove(SequenceHash),
    }

    pub struct MockEventManager {
        tx: tokio::sync::mpsc::UnboundedSender<Vec<EventType>>,
    }

    impl MockEventManager {
        pub fn new() -> (
            Arc<Self>,
            tokio::sync::mpsc::UnboundedReceiver<Vec<EventType>>,
        ) {
            let (tx, rx) = tokio::sync::mpsc::unbounded_channel();
            (Arc::new(Self { tx }), rx)
        }

        pub fn publisher(self: &Arc<Self>) -> Publisher {
            Publisher::new(self.clone())
        }
    }

    impl EventManager for MockEventManager {}

    impl EventPublisher for MockEventManager {
        fn publish(&self, handles: Vec<Arc<RegistrationHandle>>) {
            let events = handles
                .iter()
                .map(|handle| EventType::Register(handle.sequence_hash()))
                .collect::<Vec<_>>();
            self.tx.send(events).unwrap();
        }
    }

    impl EventReleaseManager for MockEventManager {
        fn block_release(&self, registration_handle: &RegistrationHandle) {
            let events = vec![EventType::Remove(registration_handle.sequence_hash())];
            self.tx.send(events).unwrap();
        }
    }
}