use std::collections::{HashMap, HashSet};
use tauri::{AppHandle, Emitter};
use tokio::sync::{mpsc, watch};

#[derive(Debug)]
pub enum DownloadCmd {
    CaptureUrls(Vec<String>),
    FrontendReady(bool),
}

#[derive(Clone, Debug, PartialEq)]
pub enum DownloadEvent {
    CapturedUrls(String),
}

#[derive(Clone)]
pub struct DownloadCoordinator {
    tx: mpsc::Sender<DownloadCmd>,
    media_tx: mpsc::Sender<MediaCmd>,
}

impl DownloadCoordinator {
    pub fn spawn(app_handle: AppHandle) -> Self {
        Self::spawn_with_events(CoordinatorEventSink::Tauri(app_handle))
    }

    pub fn spawn_headless() -> (Self, mpsc::UnboundedReceiver<DownloadEvent>) {
        let (event_tx, event_rx) = mpsc::unbounded_channel();
        (
            Self::spawn_with_events(CoordinatorEventSink::Headless(event_tx)),
            event_rx,
        )
    }

    fn spawn_with_events(events: CoordinatorEventSink) -> Self {
        let (tx, rx) = mpsc::channel(128);
        let (media_tx, media_rx) = mpsc::channel(32);
        tauri::async_runtime::spawn(run_coordinator(events, rx, media_rx));
        Self { tx, media_tx }
    }

    pub async fn send(&self, command: DownloadCmd) -> Result<(), String> {
        self.tx
            .send(command)
            .await
            .map_err(|_| "download coordinator is unavailable".to_string())
    }

    pub async fn register_media(
        &self,
        id: String,
        lifecycle_generation: u64,
    ) -> Result<watch::Receiver<bool>, String> {
        let (cancel_tx, cancel_rx) = watch::channel(false);
        self.media_tx
            .send(MediaCmd::Register {
                id,
                lifecycle_generation,
                cancel_tx,
            })
            .await
            .map_err(|_| "download coordinator is unavailable".to_string())?;
        Ok(cancel_rx)
    }

    pub async fn pause_media(&self, id: String, lifecycle_generation: u64) -> Result<(), String> {
        self.media_tx
            .send(MediaCmd::Pause {
                id,
                lifecycle_generation,
            })
            .await
            .map_err(|_| "download coordinator is unavailable".to_string())
    }

    pub async fn pause_media_with_ack(
        &self,
        id: String,
        lifecycle_generation: u64,
        ack: tokio::sync::oneshot::Sender<bool>,
    ) -> Result<(), String> {
        self.media_tx
            .send(MediaCmd::PauseWithAck {
                id,
                lifecycle_generation,
                ack,
            })
            .await
            .map_err(|_| "download coordinator is unavailable".to_string())
    }

    pub async fn finish_media(&self, id: String, lifecycle_generation: u64) {
        let _ = self
            .media_tx
            .send(MediaCmd::Finished {
                id,
                lifecycle_generation,
            })
            .await;
    }
}

#[derive(Clone)]
enum CoordinatorEventSink {
    Tauri(AppHandle),
    Headless(mpsc::UnboundedSender<DownloadEvent>),
}

impl CoordinatorEventSink {
    fn emit_captured_urls(&self, payload: String) -> bool {
        match self {
            Self::Tauri(app_handle) => app_handle.emit("deep-link-add-download", payload).is_ok(),
            Self::Headless(event_tx) => event_tx.send(DownloadEvent::CapturedUrls(payload)).is_ok(),
        }
    }
}

enum MediaCmd {
    Register {
        id: String,
        lifecycle_generation: u64,
        cancel_tx: watch::Sender<bool>,
    },
    Pause {
        id: String,
        lifecycle_generation: u64,
    },
    PauseWithAck {
        id: String,
        lifecycle_generation: u64,
        ack: tokio::sync::oneshot::Sender<bool>,
    },
    Finished {
        id: String,
        lifecycle_generation: u64,
    },
}

async fn run_coordinator(
    events: CoordinatorEventSink,
    mut command_rx: mpsc::Receiver<DownloadCmd>,
    mut media_rx: mpsc::Receiver<MediaCmd>,
) {
    let mut active_media = HashMap::<String, (u64, watch::Sender<bool>)>::new();
    let mut cancelled_media_generations = HashMap::<String, u64>::new();
    let mut pending_media_acks =
        HashMap::<(String, u64), tokio::sync::oneshot::Sender<bool>>::new();
    let mut pending_captured_urls = Vec::<String>::new();
    let mut frontend_ready = false;
    let mut command_open = true;
    let mut media_open = true;

    while command_open || media_open {
        tokio::select! {
            command = command_rx.recv(), if command_open => {
                match command {
                    Some(command) => match command {
                        DownloadCmd::CaptureUrls(urls) => {
                            append_unique_urls(&mut pending_captured_urls, urls);
                            if frontend_ready && !pending_captured_urls.is_empty() {
                                let payload = pending_captured_urls.join("\n");
                                if events.emit_captured_urls(payload) {
                                    pending_captured_urls.clear();
                                }
                            }
                        }
                        DownloadCmd::FrontendReady(ready) => {
                            frontend_ready = ready;
                            if ready && !pending_captured_urls.is_empty() {
                                let payload = pending_captured_urls.join("\n");
                                if events.emit_captured_urls(payload) {
                                    pending_captured_urls.clear();
                                }
                            }
                        }
                    },
                    None => command_open = false,
                }
            }
            command = media_rx.recv(), if media_open => {
                match command {
                    Some(command) => match command {
                    MediaCmd::Register { id, lifecycle_generation, cancel_tx } => {
                        if active_media
                            .get(&id)
                            .is_some_and(|(generation, _)| *generation > lifecycle_generation)
                        {
                            let _ = cancel_tx.send(true);
                            continue;
                        }
                        let pending_cancel = cancelled_media_generations.get(&id).copied();
                        if pending_cancel.is_some_and(|generation| generation < lifecycle_generation) {
                            cancelled_media_generations.remove(&id);
                        }
                        let cancelled = pending_cancel.is_some_and(|generation| generation >= lifecycle_generation);
                        if let Some((_, previous)) = active_media.insert(id.clone(), (lifecycle_generation, cancel_tx)) {
                            let _ = previous.send(true);
                        }
                        if cancelled {
                            if let Some((_, cancel_tx)) = active_media.get(&id) {
                                let _ = cancel_tx.send(true);
                            }
                            if pending_cancel == Some(lifecycle_generation) {
                                cancelled_media_generations.remove(&id);
                            }
                        }
                    }
                    MediaCmd::Pause { id, lifecycle_generation } => {
                        if active_media.get(&id).is_some_and(|(generation, _)| *generation == lifecycle_generation) {
                            if let Some((_, cancel_tx)) = active_media.remove(&id) {
                                let _ = cancel_tx.send(true);
                            }
                        } else {
                            cancelled_media_generations
                                .entry(id)
                                .and_modify(|generation| *generation = (*generation).max(lifecycle_generation))
                                .or_insert(lifecycle_generation);
                        }
                    }
                    MediaCmd::PauseWithAck { id, lifecycle_generation, ack } => {
                        if active_media.get(&id).is_some_and(|(generation, _)| *generation == lifecycle_generation) {
                            if let Some((_, cancel_tx)) = active_media.remove(&id) {
                                let _ = cancel_tx.send(true);
                                pending_media_acks.insert((id, lifecycle_generation), ack);
                            }
                        } else {
                            cancelled_media_generations
                                .entry(id)
                                .and_modify(|generation| *generation = (*generation).max(lifecycle_generation))
                                .or_insert(lifecycle_generation);
                            // No runner was registered for this generation.
                            // The caller may retire the cancellation tombstone
                            // while it still owns the per-download lock.
                            let _ = ack.send(false);
                        }
                    }
                    MediaCmd::Finished { id, lifecycle_generation } => {
                        if active_media.get(&id).is_some_and(|(generation, _)| *generation == lifecycle_generation) {
                            active_media.remove(&id);
                        }
                        if let Some(ack) =
                            pending_media_acks.remove(&(id.clone(), lifecycle_generation))
                        {
                            // A pending acknowledgement means the runner was
                            // registered and has now observed cancellation.
                            let _ = ack.send(true);
                        }
                        if cancelled_media_generations
                            .get(&id)
                            .is_some_and(|generation| *generation <= lifecycle_generation)
                        {
                            cancelled_media_generations.remove(&id);
                        }
                    }
                    },
                    None => media_open = false,
                }
            }
        }
    }

    for (_, (_, cancel_tx)) in active_media {
        let _ = cancel_tx.send(true);
    }
}

fn append_unique_urls(target: &mut Vec<String>, urls: Vec<String>) {
    let mut seen = target.iter().cloned().collect::<HashSet<_>>();
    target.extend(urls.into_iter().filter(|url| seen.insert(url.clone())));
}

pub(crate) fn format_speed(bytes_per_second: f64) -> String {
    if bytes_per_second >= 1024.0 * 1024.0 {
        format!("{:.1} MB/s", bytes_per_second / (1024.0 * 1024.0))
    } else if bytes_per_second >= 1024.0 {
        format!("{:.1} KB/s", bytes_per_second / 1024.0)
    } else {
        format!("{bytes_per_second:.0} B/s")
    }
}

pub(crate) fn format_size(bytes: f64) -> String {
    if bytes >= 1024.0 * 1024.0 * 1024.0 {
        format!("{:.2} GB", bytes / (1024.0 * 1024.0 * 1024.0))
    } else if bytes >= 1024.0 * 1024.0 {
        format!("{:.1} MB", bytes / (1024.0 * 1024.0))
    } else if bytes >= 1024.0 {
        format!("{:.1} KB", bytes / 1024.0)
    } else {
        format!("{bytes:.0} B")
    }
}

pub(crate) fn format_duration(seconds: f64) -> String {
    if seconds >= 3600.0 {
        format!("{:.0}h {:.0}m", seconds / 3600.0, (seconds % 3600.0) / 60.0)
    } else if seconds >= 60.0 {
        format!("{:.0}m {:.0}s", seconds / 60.0, seconds % 60.0)
    } else {
        format!("{seconds:.0}s")
    }
}

#[cfg(test)]
mod tests {
    use super::{CoordinatorEventSink, DownloadCmd, DownloadCoordinator, DownloadEvent};
    use tokio::sync::mpsc;
    use std::time::Duration;

    #[tokio::test]
    async fn coordinator_exits_when_both_command_channels_close() {
        let (event_tx, _event_rx) = mpsc::unbounded_channel();
        let (command_tx, command_rx) = mpsc::channel(1);
        let (media_tx, media_rx) = mpsc::channel(1);
        let coordinator = tokio::spawn(super::run_coordinator(
            CoordinatorEventSink::Headless(event_tx),
            command_rx,
            media_rx,
        ));

        drop(command_tx);
        drop(media_tx);

        tokio::time::timeout(Duration::from_secs(1), coordinator)
            .await
            .expect("coordinator did not exit after both channels closed")
            .expect("coordinator task panicked");
    }

    #[tokio::test]
    async fn buffers_captured_urls_until_frontend_is_ready() {
        let (coordinator, mut events) = DownloadCoordinator::spawn_headless();
        coordinator
            .send(DownloadCmd::CaptureUrls(vec![
                "https://example.com/startup.zip".to_string(),
            ]))
            .await
            .unwrap();

        assert!(
            tokio::time::timeout(Duration::from_millis(20), events.recv())
                .await
                .is_err()
        );

        coordinator
            .send(DownloadCmd::FrontendReady(true))
            .await
            .unwrap();

        assert_eq!(
            tokio::time::timeout(Duration::from_secs(1), events.recv())
                .await
                .unwrap()
                .unwrap(),
            DownloadEvent::CapturedUrls("https://example.com/startup.zip".to_string())
        );
    }

    #[tokio::test]
    async fn stale_media_finish_cannot_remove_a_newer_lifecycle() {
        let (coordinator, _events) = DownloadCoordinator::spawn_headless();
        let mut old_cancel = coordinator.register_media("same-id".to_string(), 1).await.unwrap();
        let mut new_cancel = coordinator.register_media("same-id".to_string(), 2).await.unwrap();

        tokio::time::timeout(Duration::from_secs(1), old_cancel.changed())
            .await
            .unwrap()
            .unwrap();
        coordinator.finish_media("same-id".to_string(), 1).await;

        let (ack_tx, ack_rx) = tokio::sync::oneshot::channel();
        coordinator
            .pause_media_with_ack("same-id".to_string(), 2, ack_tx)
            .await
            .unwrap();
        coordinator.finish_media("same-id".to_string(), 2).await;
        tokio::time::timeout(Duration::from_secs(1), ack_rx)
            .await
            .unwrap()
            .unwrap();
        assert!(*new_cancel.borrow_and_update());
    }

    #[tokio::test]
    async fn pause_before_media_registration_cancels_the_late_lifecycle() {
        let (coordinator, _events) = DownloadCoordinator::spawn_headless();
        let (ack_tx, ack_rx) = tokio::sync::oneshot::channel();

        coordinator
            .pause_media_with_ack("late-media".to_string(), 7, ack_tx)
            .await
            .unwrap();
        tokio::time::timeout(Duration::from_secs(1), ack_rx)
            .await
            .unwrap()
            .unwrap();

        let mut cancel_rx = coordinator
            .register_media("late-media".to_string(), 7)
            .await
            .unwrap();
        tokio::time::timeout(Duration::from_secs(1), async {
            while !*cancel_rx.borrow_and_update() {
                cancel_rx.changed().await.unwrap();
            }
        })
        .await
        .expect("late media registration was not cancelled");
    }

    #[tokio::test]
    async fn unregistered_media_pause_tombstone_can_be_reconciled() {
        let (coordinator, _events) = DownloadCoordinator::spawn_headless();
        let (ack_tx, ack_rx) = tokio::sync::oneshot::channel();

        coordinator
            .pause_media_with_ack("abandoned-media".to_string(), 9, ack_tx)
            .await
            .unwrap();
        assert!(!ack_rx.await.unwrap());

        // Once the queue lifecycle is invalidated under its control lock,
        // it can retire the tombstone without allowing a late runner to
        // start. A later registration must therefore remain uncancelled.
        coordinator
            .finish_media("abandoned-media".to_string(), 9)
            .await;
        let cancel_rx = coordinator
            .register_media("abandoned-media".to_string(), 9)
            .await
            .unwrap();
        assert!(!*cancel_rx.borrow());
    }

    #[tokio::test]
    async fn stale_media_registration_cannot_replace_a_newer_lifecycle() {
        let (coordinator, _events) = DownloadCoordinator::spawn_headless();
        let mut new_cancel = coordinator
            .register_media("same-id".to_string(), 2)
            .await
            .unwrap();
        let mut old_cancel = coordinator
            .register_media("same-id".to_string(), 1)
            .await
            .unwrap();

        tokio::time::timeout(Duration::from_secs(1), old_cancel.changed())
            .await
            .unwrap()
            .unwrap();
        assert!(*old_cancel.borrow_and_update());

        let (ack_tx, ack_rx) = tokio::sync::oneshot::channel();
        coordinator
            .pause_media_with_ack("same-id".to_string(), 2, ack_tx)
            .await
            .unwrap();
        coordinator.finish_media("same-id".to_string(), 2).await;
        tokio::time::timeout(Duration::from_secs(1), ack_rx)
            .await
            .unwrap()
            .unwrap();
        assert!(*new_cancel.borrow_and_update());
    }

    #[tokio::test]
    async fn media_pause_ack_is_preserved_across_lifecycle_replacement() {
        let (coordinator, _events) = DownloadCoordinator::spawn_headless();
        let _old_cancel = coordinator
            .register_media("same-id".to_string(), 1)
            .await
            .unwrap();
        let (old_ack_tx, old_ack_rx) = tokio::sync::oneshot::channel();
        coordinator
            .pause_media_with_ack("same-id".to_string(), 1, old_ack_tx)
            .await
            .unwrap();

        let _new_cancel = coordinator
            .register_media("same-id".to_string(), 2)
            .await
            .unwrap();
        let (new_ack_tx, new_ack_rx) = tokio::sync::oneshot::channel();
        coordinator
            .pause_media_with_ack("same-id".to_string(), 2, new_ack_tx)
            .await
            .unwrap();

        coordinator.finish_media("same-id".to_string(), 1).await;
        tokio::time::timeout(Duration::from_secs(1), old_ack_rx)
            .await
            .unwrap()
            .unwrap();
        coordinator.finish_media("same-id".to_string(), 2).await;
        tokio::time::timeout(Duration::from_secs(1), new_ack_rx)
            .await
            .unwrap()
            .unwrap();
    }
}