// 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 crate::pipeline::{
    network::egress::push::{AddressedPushRouter, AddressedRequest, PushRouter},
    AsyncEngine, Data, ManyOut, SingleIn,
};
use rand::Rng;
use std::collections::HashMap;
use std::sync::{
    atomic::{AtomicU64, Ordering},
    Arc,
};
use tokio::{net::unix::pipe::Receiver, sync::Mutex};

use crate::{pipeline::async_trait, transports::etcd::WatchEvent, Error};

use super::*;

/// Each state will be have a nonce associated with it
/// The state will be emitted in a watch channel, so we can observe the
/// critical state transitions.
enum MapState {
    /// The map is empty; value = nonce
    Empty(u64),

    /// The map is not-empty; values are (nonce, count)
    NonEmpty(u64, u64),

    /// The watcher has finished, no more events will be emitted
    Finished,
}

enum EndpointEvent {
    Put(String, i64),
    Delete(String),
}

#[derive(Clone)]
pub struct Client<T: Data, U: Data> {
    endpoint: Endpoint,
    router: PushRouter<T, U>,
    watch_rx: tokio::sync::watch::Receiver<Vec<i64>>,
    counter: Arc<AtomicU64>,
}

impl<T, U> Client<T, U>
where
    T: Data + Serialize,
    U: Data + for<'de> Deserialize<'de>,
{
    pub(crate) async fn new(endpoint: Endpoint) -> Result<Self> {
        let router = AddressedPushRouter::new(
            endpoint.component.drt.nats_client.client().clone(),
            endpoint.component.drt.tcp_server().await?,
        )?;

        // create live endpoint watcher
        let prefix_watcher = endpoint
            .component
            .drt
            .etcd_client
            .kv_get_and_watch_prefix(endpoint.etcd_path())
            .await?;

        let (prefix, _watcher, mut kv_event_rx) = prefix_watcher.dissolve();

        let (watch_tx, watch_rx) = tokio::sync::watch::channel(vec![]);

        let secondary = endpoint.component.drt.runtime.secondary().clone();

        // this task should be included in the registry
        // currently this is created once per client, but this object/task should only be instantiated
        // once per worker/instance
        secondary.spawn(async move {
            tracing::debug!("Starting endpoint watcher for prefix: {}", prefix);
            let mut map = HashMap::new();

            loop {
                let kv_event = tokio::select! {
                    _ = watch_tx.closed() => {
                        tracing::debug!("all watchers have closed; shutting down endpoint watcher for prefix: {}", prefix);
                        break;
                    }
                    kv_event = kv_event_rx.recv() => {
                        match kv_event {
                            Some(kv_event) => kv_event,
                            None => {
                                tracing::debug!("watch stream has closed; shutting down endpoint watcher for prefix: {}", prefix);
                                break;
                            }
                        }
                    }
                };

                match kv_event {
                    WatchEvent::Put(kv) => {
                        let key = String::from_utf8(kv.key().to_vec());
                        let val = serde_json::from_slice::<ComponentEndpointInfo>(kv.value());
                        if let (Ok(key), Ok(val)) = (key, val) {
                            map.insert(key.clone(), val.lease_id);
                        } else {
                            tracing::error!("Unable to parse put endpoint event; shutting down endpoint watcher for prefix: {}", prefix);
                            break;
                        }
                    }
                    WatchEvent::Delete(kv) => {
                        match String::from_utf8(kv.key().to_vec()) {
                            Ok(key) => { map.remove(&key); }
                            Err(_) => {
                                tracing::error!("Unable to parse delete endpoint event; shutting down endpoint watcher for prefix: {}", prefix);
                                break;
                            }
                        }
                    }
                }

                let endpoint_ids: Vec<i64> = map.values().cloned().collect();

                if watch_tx.send(endpoint_ids).is_err() {
                    tracing::debug!("Unable to send watch updates; shutting down endpoint watcher for prefix: {}", prefix);
                    break;
                }

            }

            tracing::debug!("Completed endpoint watcher for prefix: {}", prefix);
            let _ = watch_tx.send(vec![]);
        });

        Ok(Client {
            endpoint,
            router,
            watch_rx,
            counter: Arc::new(AtomicU64::new(0)),
        })
    }

    /// String identifying `<namespace>/<component>/<endpoint>`
    pub fn path(&self) -> String {
        self.endpoint.path()
    }

    /// String identifying `<namespace>/component/<component>/<endpoint>`
    pub fn etcd_path(&self) -> String {
        self.endpoint.etcd_path()
    }

    pub fn endpoint_ids(&self) -> &tokio::sync::watch::Receiver<Vec<i64>> {
        &self.watch_rx
    }

    /// Wait for at least one [`Endpoint`] to be available
    pub async fn wait_for_endpoints(&self) -> Result<()> {
        let mut rx = self.watch_rx.clone();
        // wait for there to be 1 or more endpoints
        loop {
            if rx.borrow_and_update().is_empty() {
                rx.changed().await?;
            } else {
                break;
            }
        }

        Ok(())
    }

    /// Issue a request to the next available endpoint in a round-robin fashion
    pub async fn round_robin(&self, request: SingleIn<T>) -> Result<ManyOut<U>> {
        let counter = self.counter.fetch_add(1, Ordering::Relaxed);

        let endpoint_id = {
            let endpoints = self.watch_rx.borrow();
            let count = endpoints.len();
            if count == 0 {
                return Err(error!(
                    "no endpoints found for endpoint {:?}",
                    self.endpoint.etcd_path()
                ));
            }
            let offset = counter % count as u64;
            endpoints[offset as usize]
        };

        let subject = self.endpoint.subject_to(endpoint_id);
        let request = request.map(|req| AddressedRequest::new(req, subject));

        self.router.generate(request).await
    }

    /// Issue a request to a random endpoint
    pub async fn random(&self, request: SingleIn<T>) -> Result<ManyOut<U>> {
        let endpoint_id = {
            let endpoints = self.watch_rx.borrow();
            let count = endpoints.len();
            if count == 0 {
                return Err(error!(
                    "no endpoints found for endpoint {:?}",
                    self.endpoint.etcd_path()
                ));
            }
            let counter = rand::rng().random::<u64>();
            let offset = counter % count as u64;
            endpoints[offset as usize]
        };

        let subject = self.endpoint.subject_to(endpoint_id);
        let request = request.map(|req| AddressedRequest::new(req, subject));

        self.router.generate(request).await
    }

    /// Issue a request to a specific endpoint
    pub async fn direct(&self, request: SingleIn<T>, endpoint_id: i64) -> Result<ManyOut<U>> {
        let found = {
            let endpoints = self.watch_rx.borrow();
            endpoints.contains(&endpoint_id)
        };

        if !found {
            return Err(error!(
                "endpoint_id={} not found for endpoint {:?}",
                endpoint_id,
                self.endpoint.etcd_path()
            ));
        }

        let subject = self.endpoint.subject_to(endpoint_id);
        let request = request.map(|req| AddressedRequest::new(req, subject));

        self.router.generate(request).await
    }
}

#[async_trait]
impl<T, U> AsyncEngine<SingleIn<T>, ManyOut<U>, Error> for Client<T, U>
where
    T: Data + Serialize,
    U: Data + for<'de> Deserialize<'de>,
{
    async fn generate(&self, request: SingleIn<T>) -> Result<ManyOut<U>, Error> {
        self.random(request).await
    }
}
