use std::{path::PathBuf, sync::Arc, time::Duration};

use anyhow::anyhow;
use exocore_core::{
    cell::Cell,
    futures::{block_on, owned_spawn, sleep, spawn_blocking, spawn_future, BatchingStream},
    time::Clock,
    utils::backoff::BackoffCalculator,
};
use exocore_protos::{
    apps::{in_message::InMessageType, out_message::OutMessageType, InMessage, OutMessage},
    prost::Message,
    store::{EntityQuery, MutationRequest},
};
use exocore_store::store::Store;
use futures::{
    channel::mpsc,
    future::{pending, select_all, FutureExt},
    lock::Mutex,
    Future, SinkExt, StreamExt,
};

use super::wasmtime::WasmTimeRuntime;
use crate::{Config, Error};

const MSG_BUFFER_SIZE: usize = 5000;
const RUNTIME_MSG_BATCH_SIZE: usize = 1000;
const APP_MIN_TICK_TIME: Duration = Duration::from_millis(100);

/// Exocore applications host.
///
/// Executes applications that have a WASM module in a background thread per
/// applications and handles incoming and outgoing messages to the module for
/// store and communication.
pub struct Applications<S: Store> {
    config: Config,
    cell: Cell,
    clock: Clock,
    store: S,
    apps: Vec<Application>,
}

impl<S: Store> Applications<S> {
    pub async fn new(
        config: Config,
        clock: Clock,
        cell: Cell,
        store: S,
    ) -> Result<Applications<S>, Error> {
        let mut apps = Vec::new();
        for app in cell.applications().applications() {
            let cell_app = app.application();

            let app_manifest = cell_app.manifest();
            if app_manifest.module.is_none() {
                continue;
            };

            let module_path = cell_app
                .module_path()
                .ok_or_else(|| anyhow!("Couldn't find module path"))?;

            let app = Application {
                cell: cell.clone(),
                cell_app: cell_app.clone(),
                module_path,
            };
            app.cell_app
                .validate()
                .map_err(|err| anyhow!("Couldn't validate module: {}", err))?;

            apps.push(app);
        }

        Ok(Applications {
            config,
            cell,
            clock,
            store,
            apps,
        })
    }

    /// Starts and runs applications.
    pub async fn run(self) -> Result<(), Error> {
        if self.apps.is_empty() {
            info!("{}: No apps to start. Blocking forever.", self.cell);
            pending::<()>().await;
            return Ok(());
        }

        let mut spawned_apps = Vec::new();
        for app in self.apps {
            spawned_apps.push(owned_spawn(Self::start_app_loop(
                self.clock.clone(),
                self.config,
                app,
                self.store.clone(),
            )));
        }

        // wait for any applications to terminate
        let _ = select_all(spawned_apps).await;

        Ok(())
    }

    async fn start_app_loop(clock: Clock, config: Config, app: Application, store: S) {
        let mut backoff = BackoffCalculator::new(clock, config.restart_backoff);
        loop {
            info!(
                "{}: Starting application (version {})",
                app,
                app.cell_app.version()
            );

            let store = store.clone();
            Self::start_app(&app, store).await;

            backoff.increment_failure();

            let restart_delay = backoff.backoff_duration();
            error!(
                "{}: Application has quit. Restarting in {:?}...",
                app, restart_delay
            );
            sleep(restart_delay).await;
        }
    }

    async fn start_app(app: &Application, store: S) {
        let (in_sender, in_receiver) = mpsc::channel(MSG_BUFFER_SIZE);
        let (out_sender, mut out_receiver) = mpsc::channel(MSG_BUFFER_SIZE);

        // Spawn the application module runtime on a separate thread.
        let runtime_spawn = {
            let env = Arc::new(WiredEnvironment {
                log_prefix: app.to_string(),
                sender: std::sync::Mutex::new(out_sender),
            });

            let app_module_path = app.module_path.clone();
            let app_prefix = app.to_string();
            spawn_blocking(move || -> Result<(), Error> {
                let mut app_runtime = WasmTimeRuntime::from_file(app_module_path, env)?;
                let mut batch_receiver = BatchingStream::new(in_receiver, RUNTIME_MSG_BATCH_SIZE);

                let mut started = false;
                let mut next_tick = sleep(APP_MIN_TICK_TIME);
                loop {
                    let in_messages: Option<Vec<InMessage>> = block_on(async {
                        futures::select! {
                            _ = next_tick.fuse() => Some(vec![]),
                            msgs = batch_receiver.next().fuse() => msgs,
                        }
                    });

                    let in_messages = if let Some(in_messages) = in_messages {
                        in_messages
                    } else {
                        info!(
                            "{}: In message receiver returned none. Stopping app runtime",
                            app_prefix
                        );
                        return Ok(());
                    };
                    let in_messages_count = in_messages.len();

                    for in_message in in_messages {
                        app_runtime.send_message(in_message)?;
                    }

                    let next_tick_duration = app_runtime.tick()?.unwrap_or(APP_MIN_TICK_TIME);
                    next_tick = sleep(next_tick_duration);

                    debug!(
                        "{}: App ticked. {} incoming message, next tick in {:?}",
                        app_prefix, in_messages_count, next_tick_duration
                    );

                    if !started {
                        info!("{}: Application started", app_prefix);
                        started = true;
                    }
                }
            })
        };

        // Spawn a task to handle store requests coming from the application
        let store_worker = {
            let store = store.clone();
            let app_prefix = app.to_string();
            async move {
                let in_sender = Arc::new(Mutex::new(in_sender));
                while let Some(message) = out_receiver.next().await {
                    match OutMessageType::from_i32(message.r#type) {
                        Some(OutMessageType::StoreEntityQuery) => {
                            let store = store.clone();
                            handle_store_message(
                                message.rendez_vous_id,
                                InMessageType::StoreEntityResults,
                                in_sender.clone(),
                                move || handle_entity_query(message, store),
                            )
                        }
                        Some(OutMessageType::StoreMutationRequest) => {
                            let store = store.clone();
                            handle_store_message(
                                message.rendez_vous_id,
                                InMessageType::StoreMutationResult,
                                in_sender.clone(),
                                move || handle_entity_mutation(message, store),
                            )
                        }
                        other => {
                            error!(
                                "{}: Got an unknown message type {:?} with id {}",
                                app_prefix, other, message.r#type
                            );
                        }
                    }
                }

                Ok::<(), Error>(())
            }
        };

        futures::select! {
            res = runtime_spawn.fuse() => {
                info!("{}: App runtime spawn has stopped: {:?}", app, res);
            }
            _ = store_worker.fuse() => {
                info!("{}: Store worker task has stopped", app);
            }
        };
    }
}

fn handle_store_message<F, O>(
    rendez_vous_id: u32,
    reply_type: InMessageType,
    in_sender: Arc<Mutex<mpsc::Sender<InMessage>>>,
    func: F,
) where
    F: (FnOnce() -> O) + Send + 'static,
    O: Future<Output = Result<Vec<u8>, Error>> + Send + 'static,
{
    spawn_future(async move {
        let mut msg = InMessage {
            r#type: reply_type.into(),
            rendez_vous_id,
            ..Default::default()
        };

        let res = func();
        match res.await {
            Ok(res) => msg.data = res,
            Err(err) => msg.error = err.to_string(),
        }

        let mut in_sender = in_sender.lock().await;
        let _ = in_sender.send(msg).await;
    });
}

async fn handle_entity_query<S: Store>(
    out_message: OutMessage,
    store: S,
) -> Result<Vec<u8>, Error> {
    let query = EntityQuery::decode(out_message.data.as_ref())?;
    let res = store.query(query);
    let res = res.await?;

    Ok(res.encode_to_vec())
}

async fn handle_entity_mutation<S: Store>(
    out_message: OutMessage,
    store: S,
) -> Result<Vec<u8>, Error> {
    let mutation = MutationRequest::decode(out_message.data.as_ref())?;
    let res = store.mutate(mutation);
    let res = res.await?;

    Ok(res.encode_to_vec())
}

struct WiredEnvironment {
    log_prefix: String,
    sender: std::sync::Mutex<mpsc::Sender<exocore_protos::apps::OutMessage>>,
}

impl super::wasmtime::HostEnvironment for WiredEnvironment {
    fn handle_message(&self, msg: exocore_protos::apps::OutMessage) {
        let mut sender = self.sender.lock().unwrap();
        if let Err(err) = sender.try_send(msg) {
            error!("Couldn't send message via channel: {}", err)
        }
    }

    fn handle_log(&self, level: log::Level, msg: &str) {
        log!(level, "{}: WASM: {}", self.log_prefix, msg);
    }
}

struct Application {
    cell: Cell,
    cell_app: exocore_core::cell::Application,
    module_path: PathBuf,
}

impl std::fmt::Display for Application {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{} App{{{}}}", self.cell, self.cell_app.name())
    }
}