// SPDX-FileCopyrightText: Copyright (c) 2024-2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

//! Background processes for the KV Router including event consumption and snapshot uploads.

use std::{collections::HashSet, time::Duration};

use anyhow::Result;
use dynamo_runtime::{
    component::Component,
    prelude::*,
    traits::events::EventPublisher,
    transports::{
        etcd::{Client as EtcdClient, DistributedRWLock, WatchEvent},
        nats::{NatsQueue, Slug},
    },
};
use tokio::sync::{mpsc, oneshot};
use tokio_util::sync::CancellationToken;

use crate::{
    discovery::KV_ROUTERS_ROOT_PATH,
    kv_router::{
        KV_EVENT_SUBJECT, RADIX_STATE_BUCKET, RADIX_STATE_FILE, ROUTER_CLEANUP_LOCK,
        ROUTER_SNAPSHOT_LOCK,
        indexer::{DumpRequest, GetWorkersRequest, RouterEvent},
        protocols::WorkerId,
    },
};

/// Resources required for snapshot operations
#[derive(Clone)]
struct SnapshotResources {
    nats_client: dynamo_runtime::transports::nats::Client,
    bucket_name: String,
    rwlock: DistributedRWLock,
    instances_rx: tokio::sync::watch::Receiver<Vec<dynamo_runtime::component::Instance>>,
    get_workers_tx: mpsc::Sender<GetWorkersRequest>,
    snapshot_tx: mpsc::Sender<DumpRequest>,
}

impl SnapshotResources {
    /// Perform snapshot upload and purge operations with write lock
    async fn purge_then_snapshot(
        &self,
        etcd_client: &EtcdClient,
        nats_queue: &mut NatsQueue,
        remove_worker_tx: &mpsc::Sender<WorkerId>,
    ) -> anyhow::Result<()> {
        // Try to acquire write lock (non-blocking)
        let Some(_write_guard) = self.rwlock.try_write_lock(etcd_client).await else {
            tracing::debug!(
                "Could not acquire write lock for snapshot (readers active or lock held)"
            );
            anyhow::bail!("Write lock unavailable");
        };
        // Purge before snapshot ensures new/warm-restarted routers won't replay already-acknowledged messages.
        // Since KV events are idempotent, this ordering reduces unnecessary reprocessing while maintaining
        // at-least-once delivery guarantees. The snapshot will capture the clean state after purge.
        tracing::info!("Purging acknowledged messages and performing snapshot of radix tree");
        let start_time = std::time::Instant::now();

        // Clean up stale workers before snapshot
        // Get current worker IDs from instances_rx
        let current_instances = self.instances_rx.borrow().clone();
        let current_worker_ids: std::collections::HashSet<u64> = current_instances
            .iter()
            .map(|instance| instance.instance_id)
            .collect();

        // Get worker IDs from the indexer
        let (resp_tx, resp_rx) = tokio::sync::oneshot::channel();
        let get_workers_req = GetWorkersRequest { resp: resp_tx };

        if let Err(e) = self.get_workers_tx.send(get_workers_req).await {
            tracing::warn!("Failed to send get_workers request during snapshot: {e:?}");
        } else {
            match resp_rx.await {
                Ok(indexer_worker_ids) => {
                    // Find workers in indexer but not in current instances
                    for worker_id in indexer_worker_ids {
                        if !current_worker_ids.contains(&worker_id) {
                            tracing::info!(
                                "Removing stale worker {worker_id} from indexer during snapshot"
                            );
                            if let Err(e) = remove_worker_tx.send(worker_id).await {
                                tracing::warn!(
                                    "Failed to send remove_worker for stale worker {worker_id}: {e:?}"
                                );
                            }
                        }
                    }
                }
                Err(e) => {
                    tracing::warn!("Failed to receive worker IDs from indexer: {e:?}");
                }
            }
        }

        // First, purge acknowledged messages from the stream
        nats_queue.purge_acknowledged().await?;

        // Now request a snapshot from the indexer (which reflects the post-purge state)
        let (resp_tx, resp_rx) = oneshot::channel();
        let dump_req = DumpRequest { resp: resp_tx };

        self.snapshot_tx
            .send(dump_req)
            .await
            .map_err(|e| anyhow::anyhow!("Failed to send dump request: {e:?}"))?;

        // Wait for the dump response
        let events = resp_rx
            .await
            .map_err(|e| anyhow::anyhow!("Failed to receive dump response: {e:?}"))?;

        // Upload the snapshot to NATS object store
        let url = url::Url::parse(&format!(
            "nats://{}/{}/{RADIX_STATE_FILE}",
            self.nats_client.addr(),
            self.bucket_name
        ))?;

        self.nats_client
            .object_store_upload_data(&events, &url)
            .await
            .map_err(|e| anyhow::anyhow!("Failed to upload snapshot: {e:?}"))?;

        tracing::info!(
            "Successfully performed snapshot of radix tree with {} events to bucket {} in {}ms",
            events.len(),
            self.bucket_name,
            start_time.elapsed().as_millis()
        );

        Ok(())
    }
}

/// Start a unified background task for event consumption and optional snapshot management
#[allow(clippy::too_many_arguments)]
pub async fn start_kv_router_background(
    component: Component,
    consumer_uuid: String,
    kv_events_tx: mpsc::Sender<RouterEvent>,
    remove_worker_tx: mpsc::Sender<WorkerId>,
    maybe_get_workers_tx: Option<mpsc::Sender<GetWorkersRequest>>,
    maybe_snapshot_tx: Option<mpsc::Sender<DumpRequest>>,
    cancellation_token: CancellationToken,
    router_snapshot_threshold: Option<u32>,
    router_reset_states: bool,
) -> Result<()> {
    // Set up NATS connections
    let stream_name = Slug::slugify(&format!("{}.{}", component.subject(), KV_EVENT_SUBJECT))
        .to_string()
        .replace("_", "-");
    let nats_server =
        std::env::var("NATS_SERVER").unwrap_or_else(|_| "nats://localhost:4222".to_string());

    // Create NatsQueue for event consumption
    let mut nats_queue = NatsQueue::new_with_consumer(
        stream_name.clone(),
        nats_server.clone(),
        std::time::Duration::from_secs(60), // 1 minute timeout
        consumer_uuid.clone(),
    );
    nats_queue.connect_with_reset(router_reset_states).await?;

    // Always create NATS client (needed for both reset and snapshots)
    let client_options = dynamo_runtime::transports::nats::Client::builder()
        .server(&nats_server)
        .build()?;
    let nats_client = client_options.connect().await?;

    // Get etcd client (needed for both snapshots and router watching)
    let etcd_client = component
        .drt()
        .etcd_client()
        .ok_or_else(|| anyhow::anyhow!("etcd client not available"))?;

    // Create bucket name for snapshots/state
    let bucket_name = Slug::slugify(&format!("{}-{RADIX_STATE_BUCKET}", component.subject()))
        .to_string()
        .replace("_", "-");

    // Create RWLock for snapshot coordination
    let lock_prefix = format!("{}/{}", ROUTER_SNAPSHOT_LOCK, component.subject());
    let snapshot_rwlock = DistributedRWLock::new(lock_prefix);

    // Handle initial state based on router_reset_states flag
    if router_reset_states {
        // Delete the bucket to reset state
        tracing::info!("Resetting router state, deleting bucket: {bucket_name}");
        if let Err(e) = nats_client.object_store_delete_bucket(&bucket_name).await {
            tracing::warn!("Failed to delete bucket (may not exist): {e:?}");
        }
    } else {
        // Try to download initial state from object store with read lock
        let url = url::Url::parse(&format!(
            "nats://{}/{bucket_name}/{RADIX_STATE_FILE}",
            nats_client.addr()
        ))?;

        // Acquire read lock with default timeout
        if let Ok(_read_guard) = snapshot_rwlock
            .read_lock_with_wait(&etcd_client, &consumer_uuid, None)
            .await
        {
            tracing::debug!("Acquired read lock for snapshot download");

            // Download snapshot while holding read lock
            match nats_client
                .object_store_download_data::<Vec<RouterEvent>>(&url)
                .await
            {
                Ok(events) => {
                    tracing::info!(
                        "Successfully downloaded {} events from object store",
                        events.len()
                    );
                    // Send all events to the indexer
                    for event in events {
                        if let Err(e) = kv_events_tx.send(event).await {
                            tracing::warn!("Failed to send initial event to indexer: {e:?}");
                        }
                    }
                    tracing::info!("Successfully sent all initial events to indexer");
                }
                Err(_) => {
                    tracing::debug!(
                        "Failed to download snapshots. This is normal for freshly started Router replicas."
                    );
                }
            }
        } else {
            tracing::warn!("Could not acquire read lock for snapshot download (timeout or error)");
        }
    }

    // Cleanup orphaned consumers on startup
    cleanup_orphaned_consumers(&mut nats_queue, &etcd_client, &component, &consumer_uuid).await;

    // Watch for router deletions to clean up orphaned consumers
    let (_prefix_str, _watcher, mut router_replicas_rx) = etcd_client
        .kv_get_and_watch_prefix(&format!("{}/", KV_ROUTERS_ROOT_PATH))
        .await?
        .dissolve();

    // Get the generate endpoint and watch for instance deletions
    let generate_endpoint = component.endpoint("generate");
    let (_instance_prefix, _instance_watcher, mut instance_event_rx) = etcd_client
        .kv_get_and_watch_prefix(generate_endpoint.etcd_root())
        .await?
        .dissolve();

    // Get instances_rx for tracking current workers
    let client = generate_endpoint.client().await?;
    let instances_rx = match client.instance_source.as_ref() {
        dynamo_runtime::component::InstanceSource::Dynamic(rx) => rx.clone(),
        dynamo_runtime::component::InstanceSource::Static => {
            anyhow::bail!("Expected dynamic instance source for KV routing");
        }
    };

    // Only set up snapshot-related resources if snapshot_tx, get_workers_tx, and threshold are provided
    let snapshot_resources = if let (Some(get_workers_tx), Some(snapshot_tx), Some(_)) = (
        maybe_get_workers_tx,
        maybe_snapshot_tx,
        router_snapshot_threshold,
    ) {
        Some(SnapshotResources {
            nats_client,
            bucket_name,
            rwlock: snapshot_rwlock.clone(),
            instances_rx,
            get_workers_tx,
            snapshot_tx,
        })
    } else {
        None
    };

    tokio::spawn(async move {
        let mut check_interval = tokio::time::interval(Duration::from_secs(1));
        check_interval.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);

        loop {
            tokio::select! {
                biased;

                _ = cancellation_token.cancelled() => {
                    tracing::debug!("KV Router background task received cancellation signal");
                    // Clean up the queue and remove the durable consumer
                    // TODO: durable consumer cannot cleanup if ungraceful shutdown (crash)
                    if let Err(e) = nats_queue.shutdown(None).await {
                        tracing::warn!("Failed to shutdown NatsQueue: {e}");
                    }
                    break;
                }

                // Handle generate endpoint instance deletion events
                Some(event) = instance_event_rx.recv() => {
                    let WatchEvent::Delete(kv) = event else {
                        continue;
                    };

                    let key = String::from_utf8_lossy(kv.key());

                    let Some(worker_id_str) = key.split(&['/', ':'][..]).next_back() else {
                        tracing::warn!("Could not extract worker ID from instance key: {key}");
                        continue;
                    };

                    // Parse as hexadecimal (base 16)
                    let Ok(worker_id) = u64::from_str_radix(worker_id_str, 16) else {
                        tracing::warn!("Could not parse worker ID from instance key: {key}");
                        continue;
                    };

                    tracing::info!("Generate endpoint instance deleted, removing worker {worker_id}");
                    if let Err(e) = remove_worker_tx.send(worker_id).await {
                        tracing::warn!("Failed to send worker removal for worker {worker_id}: {e}");
                    }
                }

                // Handle event consumption
                result = nats_queue.dequeue_task(None) => {
                    match result {
                        Ok(Some(bytes)) => {
                            let event: RouterEvent = match serde_json::from_slice(&bytes) {
                                Ok(event) => event,
                                Err(e) => {
                                    tracing::warn!("Failed to deserialize RouterEvent: {e:?}");
                                    continue;
                                }
                            };

                            // Forward the RouterEvent to the indexer
                            if let Err(e) = kv_events_tx.send(event).await {
                                tracing::warn!(
                                    "failed to send kv event to indexer; shutting down: {e:?}"
                                );
                                break;
                            }
                        },
                        Ok(None) => {
                            tracing::trace!("Dequeue timeout, continuing");
                        },
                        Err(e) => {
                            tracing::error!("Failed to dequeue task: {e:?}");
                            tokio::time::sleep(std::time::Duration::from_millis(100)).await;
                        }
                    }
                }

                // Handle periodic stream checking and purging (only if snapshot_resources is provided)
                _ = check_interval.tick() => {
                    let Some(resources) = snapshot_resources.as_ref() else {
                        continue;
                    };

                    // Check total messages in the stream
                    let Ok(message_count) = nats_queue.get_stream_messages().await else {
                        tracing::warn!("Failed to get stream message count");
                        continue;
                    };

                    // Guard clause: skip if message count is too low
                    let threshold = router_snapshot_threshold.unwrap_or(u32::MAX) as u64;
                    if message_count <= threshold {
                        continue;
                    }

                    tracing::info!("Stream has {message_count} messages, attempting to acquire write lock for purge and snapshot");

                    // Perform snapshot upload and purge (acquires write lock internally)
                    match resources.purge_then_snapshot(
                        &etcd_client,
                        &mut nats_queue,
                        &remove_worker_tx,
                    ).await {
                        Ok(_) => tracing::info!("Successfully performed purge and snapshot"),
                        Err(e) => tracing::debug!("Could not perform purge and snapshot: {e:?}"),
                    }
                }

                // Handle router deletion events
                Some(event) = router_replicas_rx.recv() => {
                    let WatchEvent::Delete(kv) = event else {
                        // We only care about deletions for cleaning up consumers
                        continue;
                    };

                    let key = String::from_utf8_lossy(kv.key());
                    tracing::info!("Detected router replica deletion: {key}");

                    // Only process deletions for routers on the same component
                    if !key.contains(component.path().as_str()) {
                        tracing::trace!(
                            "Skipping router deletion from different component (key: {key}, subscriber component: {})",
                            component.path()
                        );
                        continue;
                    }

                    // Extract the router UUID from the key
                    let Some(router_uuid) = key.split('/').next_back() else {
                        tracing::warn!("Could not extract UUID from router key: {key}");
                        continue;
                    };

                    // The consumer UUID is the router UUID
                    let consumer_to_delete = router_uuid.to_string();

                    tracing::info!("Attempting to delete orphaned consumer: {consumer_to_delete}");

                    // Create a unique cleanup lock for this specific consumer
                    let cleanup_lock_name = format!("{}/{}/{}", ROUTER_CLEANUP_LOCK, component.subject(), consumer_to_delete);
                    let cleanup_rwlock = DistributedRWLock::new(cleanup_lock_name);

                    // Try to acquire cleanup write lock (non-blocking) before deleting consumer
                    if let Some(_cleanup_guard) = cleanup_rwlock.try_write_lock(&etcd_client).await {
                        tracing::debug!(
                            "Acquired cleanup lock for deleting consumer: {consumer_to_delete}"
                        );

                        // Delete the consumer
                        if let Err(e) = nats_queue.shutdown(Some(consumer_to_delete.clone())).await {
                            tracing::warn!("Failed to delete consumer {consumer_to_delete}: {e}");
                        } else {
                            tracing::info!("Successfully deleted orphaned consumer: {consumer_to_delete}");
                        }

                        // Cleanup lock is automatically released when _cleanup_guard goes out of scope
                    } else {
                        tracing::debug!(
                            "Could not acquire cleanup lock for consumer {consumer_to_delete}"
                        );
                    }
                }
            }
        }

        // Clean up the queue and remove the durable consumer
        if let Err(e) = nats_queue.shutdown(None).await {
            tracing::warn!("Failed to shutdown NatsQueue: {e}");
        }
    });

    Ok(())
}

/// Cleanup orphaned NATS consumers that no longer have corresponding etcd router entries
async fn cleanup_orphaned_consumers(
    nats_queue: &mut NatsQueue,
    etcd_client: &EtcdClient,
    component: &Component,
    consumer_uuid: &str,
) {
    let Ok(consumers) = nats_queue.list_consumers().await else {
        return;
    };

    let router_prefix = format!("{}/{}/", KV_ROUTERS_ROOT_PATH, component.path());
    let Ok(router_entries) = etcd_client.kv_get_prefix(&router_prefix).await else {
        return;
    };

    let active_uuids: HashSet<String> = router_entries
        .iter()
        .filter_map(|kv| {
            String::from_utf8_lossy(kv.key())
                .split('/')
                .next_back()
                .map(str::to_string)
        })
        .collect();

    for consumer in consumers {
        if consumer == consumer_uuid {
            // Never delete myself (extra/redundant safeguard)
            continue;
        }
        if !active_uuids.contains(&consumer) {
            tracing::info!("Cleaning up orphaned consumer: {consumer}");
            let _ = nats_queue.shutdown(Some(consumer)).await;
        }
    }
}
