//! This modules abstracts data sources and merges them in a single virtual one
//!
//! It is also responsible for syncing them together

use std::error::Error;
use std::collections::HashSet;
use std::marker::PhantomData;
use std::sync::{Arc, Mutex};
use std::fmt::{Display, Formatter};

use url::Url;
use itertools::Itertools;

use crate::traits::{BaseCalendar, CalDavSource, DavCalendar};
use crate::traits::CompleteCalendar;
use crate::item::SyncStatus;

pub mod sync_progress;
use sync_progress::SyncProgress;
use sync_progress::{FeedbackSender, SyncEvent};

/// How many items will be batched in a single HTTP request when downloading from the server
#[cfg(not(test))]
const DOWNLOAD_BATCH_SIZE: usize = 30;
/// How many items will be batched in a single HTTP request when downloading from the server
#[cfg(test)]
const DOWNLOAD_BATCH_SIZE: usize = 3;

// I am too lazy to actually make `fetch_and_apply` generic over an async closure.
// Let's work around by passing an enum, so that `fetch_and_apply` will know what to do
enum BatchDownloadType {
    RemoteAdditions,
    RemoteChanges,
}

impl Display for BatchDownloadType {
    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
        match self {
            Self::RemoteAdditions => write!(f, "remote additions"),
            Self::RemoteChanges => write!(f, "remote changes"),
        }
    }
}


/// A data source that combines two `CalDavSource`s, which is able to sync both sources.
///
/// Usually, you will only need to use a provider between a server and a local cache, that is to say a [`CalDavProvider`](crate::CalDavProvider), i.e. a `Provider<Cache, CachedCalendar, Client, RemoteCalendar>`. \
/// However, providers can be used for integration tests, where the remote source is mocked by a `Cache`.
#[derive(Debug)]
pub struct Provider<L, T, R, U>
where
    L: CalDavSource<T>,
    T: CompleteCalendar + Sync + Send,
    R: CalDavSource<U>,
    U: DavCalendar + Sync + Send,
{
    /// The remote source (usually a server)
    remote: R,
    /// The local cache
    local: L,

    phantom_t: PhantomData<T>,
    phantom_u: PhantomData<U>,
}

impl<L, T, R, U> Provider<L, T, R, U>
where
    L: CalDavSource<T>,
    T: CompleteCalendar + Sync + Send,
    R: CalDavSource<U>,
    U: DavCalendar + Sync + Send,
{
    /// Create a provider.
    ///
    /// `remote` is usually a [`Client`](crate::client::Client), `local` is usually a [`Cache`](crate::cache::Cache).
    /// However, both can be interchangeable. The only difference is that `remote` always wins in case of a sync conflict
    pub fn new(remote: R, local: L) -> Self {
        Self { remote, local,
            phantom_t: PhantomData, phantom_u: PhantomData,
        }
    }

    /// Returns the data source described as `local`
    pub fn local(&self)  -> &L { &self.local }
    /// Returns the data source described as `local`
    pub fn local_mut(&mut self)  -> &mut L { &mut self.local }
    /// Returns the data source described as `remote`.
    ///
    /// Apart from tests, there are very few (if any) reasons to access `remote` directly.
    /// Usually, you should rather use the `local` source, which (usually) is a much faster local cache.
    /// To be sure `local` accurately mirrors the `remote` source, you can run [`Provider::sync`]
    pub fn remote(&self) -> &R { &self.remote }

    /// Performs a synchronisation between `local` and `remote`, and provide feeedback to the user about the progress.
    ///
    /// This bidirectional sync applies additions/deletions made on a source to the other source.
    /// In case of conflicts (the same item has been modified on both ends since the last sync, `remote` always wins).
    ///
    /// It returns whether the sync was totally successful (details about errors are logged using the `log::*` macros).
    /// In case errors happened, the sync might have been partially executed but your data will never be correupted (either locally nor in the server).
    /// Simply run this function again, it will re-start a sync, picking up where it failed.
    pub async fn sync_with_feedback(&mut self, feedback_sender: FeedbackSender) -> bool {
        let mut progress = SyncProgress::new_with_feedback_channel(feedback_sender);
        self.run_sync(&mut progress).await
    }

    /// Performs a synchronisation between `local` and `remote`, without giving any feedback.
    ///
    /// See [`Self::sync_with_feedback`]
    pub async fn sync(&mut self) -> bool {
        let mut progress = SyncProgress::new();
        self.run_sync(&mut progress).await
    }

    async fn run_sync(&mut self, progress: &mut SyncProgress) -> bool {
        if let Err(err) = self.run_sync_inner(progress).await {
            progress.error(&format!("Sync terminated because of an error: {}", err));
        }
        progress.feedback(SyncEvent::Finished{ success: progress.is_success() });
        progress.is_success()
    }

    async fn run_sync_inner(&mut self, progress: &mut SyncProgress) -> Result<(), Box<dyn Error>> {
        progress.info("Starting a sync.");
        progress.feedback(SyncEvent::Started);

        let mut handled_calendars = HashSet::new();

        // Sync every remote calendar
        let cals_remote = self.remote.get_calendars().await?;
        for (cal_url, cal_remote) in cals_remote {
            let counterpart = match self.get_or_insert_local_counterpart_calendar(&cal_url, cal_remote.clone()).await {
                Err(err) => {
                    progress.warn(&format!("Unable to get or insert local counterpart calendar for {} ({}). Skipping this time", cal_url, err));
                    continue;
                },
                Ok(arc) => arc,
            };

            if let Err(err) = Self::sync_calendar_pair(counterpart, cal_remote, progress).await {
                progress.warn(&format!("Unable to sync calendar {}: {}, skipping this time.", cal_url, err));
                continue;
            }
            handled_calendars.insert(cal_url);
        }

        // Sync every local calendar that would not be in the remote yet
        let cals_local = self.local.get_calendars().await?;
        for (cal_url, cal_local) in cals_local {
            if handled_calendars.contains(&cal_url) {
                continue;
            }

            let counterpart = match self.get_or_insert_remote_counterpart_calendar(&cal_url, cal_local.clone()).await {
                Err(err) => {
                    progress.warn(&format!("Unable to get or insert remote counterpart calendar for {} ({}). Skipping this time", cal_url, err));
                    continue;
                },
                Ok(arc) => arc,
            };

            if let Err(err) = Self::sync_calendar_pair(cal_local, counterpart, progress).await {
                progress.warn(&format!("Unable to sync calendar {}: {}, skipping this time.", cal_url, err));
                continue;
            }
        }

        progress.info("Sync ended");

        Ok(())
    }


    async fn get_or_insert_local_counterpart_calendar(&mut self, cal_url: &Url, needle: Arc<Mutex<U>>) -> Result<Arc<Mutex<T>>, Box<dyn Error>> {
        get_or_insert_counterpart_calendar("local", &mut self.local, cal_url, needle).await
    }
    async fn get_or_insert_remote_counterpart_calendar(&mut self, cal_url: &Url, needle: Arc<Mutex<T>>) -> Result<Arc<Mutex<U>>, Box<dyn Error>> {
        get_or_insert_counterpart_calendar("remote", &mut self.remote, cal_url, needle).await
    }


    async fn sync_calendar_pair(cal_local: Arc<Mutex<T>>, cal_remote: Arc<Mutex<U>>, progress: &mut SyncProgress) -> Result<(), Box<dyn Error>> {
        let mut cal_remote = cal_remote.lock().unwrap();
        let mut cal_local = cal_local.lock().unwrap();
        let cal_name = cal_local.name().to_string();

        progress.info(&format!("Syncing calendar {}", cal_name));
        progress.reset_counter();
        progress.feedback(SyncEvent::InProgress{
            calendar: cal_name.clone(),
            items_done_already: 0,
            details: "started".to_string()
        });

        // Step 1 - find the differences
        progress.debug("Finding the differences to sync...");
        let mut local_del = HashSet::new();
        let mut remote_del = HashSet::new();
        let mut local_changes = HashSet::new();
        let mut remote_changes = HashSet::new();
        let mut local_additions = HashSet::new();
        let mut remote_additions = HashSet::new();

        let remote_items = cal_remote.get_item_version_tags().await?;
        progress.feedback(SyncEvent::InProgress{
            calendar: cal_name.clone(),
            items_done_already: 0,
            details: format!("{} remote items", remote_items.len()),
        });

        let mut local_items_to_handle = cal_local.get_item_urls().await?;
        for (url, remote_tag) in remote_items {
            progress.trace(&format!("***** Considering remote item {}...", url));
            match cal_local.get_item_by_url(&url).await {
                None => {
                    // This was created on the remote
                    progress.debug(&format!("*   {} is a remote addition", url));
                    remote_additions.insert(url);
                },
                Some(local_item) => {
                    if local_items_to_handle.remove(&url) == false {
                        progress.error(&format!("Inconsistent state: missing task {} from the local tasks", url));
                    }

                    match local_item.sync_status() {
                        SyncStatus::NotSynced => {
                            progress.error(&format!("URL reuse between remote and local sources ({}). Ignoring this item in the sync", url));
                            continue;
                        },
                        SyncStatus::Synced(local_tag) => {
                            if &remote_tag != local_tag {
                                // This has been modified on the remote
                                progress.debug(&format!("*   {} is a remote change", url));
                                remote_changes.insert(url);
                            }
                        },
                        SyncStatus::LocallyModified(local_tag) => {
                            if &remote_tag == local_tag {
                                // This has been changed locally
                                progress.debug(&format!("*   {} is a local change", url));
                                local_changes.insert(url);
                            } else {
                                progress.info(&format!("Conflict: task {} has been modified in both sources. Using the remote version.", url));
                                progress.debug(&format!("*   {} is considered a remote change", url));
                                remote_changes.insert(url);
                            }
                        },
                        SyncStatus::LocallyDeleted(local_tag) => {
                            if &remote_tag == local_tag {
                                // This has been locally deleted
                                progress.debug(&format!("*   {} is a local deletion", url));
                                local_del.insert(url);
                            } else {
                                progress.info(&format!("Conflict: task {} has been locally deleted and remotely modified. Reverting to the remote version.", url));
                                progress.debug(&format!("*   {} is a considered a remote change", url));
                                remote_changes.insert(url);
                            }
                        },
                    }
                }
            }
        }

        // Also iterate on the local tasks that are not on the remote
        for url in local_items_to_handle {
            progress.trace(&format!("##### Considering local item {}...", url));
            let local_item = match cal_local.get_item_by_url(&url).await {
                None => {
                    progress.error(&format!("Inconsistent state: missing task {} from the local tasks", url));
                    continue;
                },
                Some(item) => item,
            };

            match local_item.sync_status() {
                SyncStatus::Synced(_) => {
                    // This item has been removed from the remote
                    progress.debug(&format!("#   {} is a deletion from the server", url));
                    remote_del.insert(url);
                },
                SyncStatus::NotSynced => {
                    // This item has just been locally created
                    progress.debug(&format!("#   {} has been locally created", url));
                    local_additions.insert(url);
                },
                SyncStatus::LocallyDeleted(_) => {
                    // This item has been deleted from both sources
                    progress.debug(&format!("#   {} has been deleted from both sources", url));
                    remote_del.insert(url);
                },
                SyncStatus::LocallyModified(_) => {
                    progress.info(&format!("Conflict: item {} has been deleted from the server and locally modified. Deleting the local copy", url));
                    remote_del.insert(url);
                },
            }
        }


        // Step 2 - commit changes
        progress.trace("Committing changes...");
        for url_del in local_del {
            progress.debug(&format!("> Pushing local deletion {} to the server", url_del));
            progress.increment_counter(1);
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                items_done_already: progress.counter(),
                details: Self::item_name(&cal_local, &url_del).await,
            });

            match cal_remote.delete_item(&url_del).await {
                Err(err) => {
                    progress.warn(&format!("Unable to delete remote item {}: {}", url_del, err));
                },
                Ok(()) => {
                    // Change the local copy from "marked to deletion" to "actually deleted"
                    if let Err(err) = cal_local.immediately_delete_item(&url_del).await {
                        progress.error(&format!("Unable to permanently delete local item {}: {}", url_del, err));
                    }
                },
            }
        }

        for url_del in remote_del {
            progress.debug(&format!("> Applying remote deletion {} locally", url_del));
            progress.increment_counter(1);
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                items_done_already: progress.counter(),
                details: Self::item_name(&cal_local, &url_del).await,
            });
            if let Err(err) = cal_local.immediately_delete_item(&url_del).await {
                progress.warn(&format!("Unable to delete local item {}: {}", url_del, err));
            }
        }

        Self::apply_remote_additions(
            remote_additions,
            &mut *cal_local,
            &mut *cal_remote,
            progress,
            &cal_name
        ).await;

        Self::apply_remote_changes(
            remote_changes,
            &mut *cal_local,
            &mut *cal_remote,
            progress,
            &cal_name
        ).await;


        for url_add in local_additions {
            progress.debug(&format!("> Pushing local addition {} to the server", url_add));
            progress.increment_counter(1);
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                items_done_already: progress.counter(),
                details: Self::item_name(&cal_local, &url_add).await,
            });
            match cal_local.get_item_by_url_mut(&url_add).await {
                None => {
                    progress.error(&format!("Inconsistency: created item {} has been marked for upload but is locally missing", url_add));
                    continue;
                },
                Some(item) => {
                    match cal_remote.add_item(item.clone()).await {
                        Err(err) => progress.error(&format!("Unable to add item {} to remote calendar: {}", url_add, err)),
                        Ok(new_ss) => {
                            // Update local sync status
                            item.set_sync_status(new_ss);
                        },
                    }
                },
            };
        }

        for url_change in local_changes {
            progress.debug(&format!("> Pushing local change {} to the server", url_change));
            progress.increment_counter(1);
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                items_done_already: progress.counter(),
                details: Self::item_name(&cal_local, &url_change).await,
            });
            match cal_local.get_item_by_url_mut(&url_change).await {
                None => {
                    progress.error(&format!("Inconsistency: modified item {} has been marked for upload but is locally missing", url_change));
                    continue;
                },
                Some(item) => {
                    match cal_remote.update_item(item.clone()).await {
                        Err(err) => progress.error(&format!("Unable to update item {} in remote calendar: {}", url_change, err)),
                        Ok(new_ss) => {
                            // Update local sync status
                            item.set_sync_status(new_ss);
                        },
                    };
                }
            };
        }

        Ok(())
    }


    async fn item_name(cal: &T, url: &Url) -> String {
        cal.get_item_by_url(url).await.map(|item| item.name()).unwrap_or_default().to_string()
    }

    async fn apply_remote_additions(
        mut remote_additions: HashSet<Url>,
        cal_local: &mut T,
        cal_remote: &mut U,
        progress: &mut SyncProgress,
        cal_name: &str
    ) {
        for batch in remote_additions.drain().chunks(DOWNLOAD_BATCH_SIZE).into_iter() {
            Self::fetch_batch_and_apply(BatchDownloadType::RemoteAdditions, batch, cal_local, cal_remote, progress, cal_name).await;
        }
    }

    async fn apply_remote_changes(
        mut remote_changes: HashSet<Url>,
        cal_local: &mut T,
        cal_remote: &mut U,
        progress: &mut SyncProgress,
        cal_name: &str
    ) {
        for batch in remote_changes.drain().chunks(DOWNLOAD_BATCH_SIZE).into_iter() {
            Self::fetch_batch_and_apply(BatchDownloadType::RemoteChanges, batch, cal_local, cal_remote, progress, cal_name).await;
        }
    }

    async fn fetch_batch_and_apply<I: Iterator<Item = Url>>(
        batch_type: BatchDownloadType,
        remote_additions: I,
        cal_local: &mut T,
        cal_remote: &mut U,
        progress: &mut SyncProgress,
        cal_name: &str
    ) {
        progress.debug(&format!("> Applying a batch of {} locally", batch_type) /* too bad Chunks does not implement ExactSizeIterator, that could provide useful debug info. See https://github.com/rust-itertools/itertools/issues/171 */);

        let list_of_additions: Vec<Url> = remote_additions.map(|url| url.clone()).collect();
        match cal_remote.get_items_by_url(&list_of_additions).await {
            Err(err) => {
                progress.warn(&format!("Unable to get the batch of {} {:?}: {}. Skipping them.", batch_type, list_of_additions, err));
            },
            Ok(items) => {
                for item in items {
                    match item {
                        None => {
                            progress.error(&format!("Inconsistency: an item from the batch has vanished from the remote end"));
                            continue;
                        },
                        Some(new_item) => {
                            let local_update_result = match batch_type {
                                BatchDownloadType::RemoteAdditions => cal_local.add_item(new_item.clone()).await,
                                BatchDownloadType::RemoteChanges => cal_local.update_item(new_item.clone()).await,
                            };
                            if let Err(err) = local_update_result {
                                progress.error(&format!("Not able to add item {} to local calendar: {}", new_item.url(), err));
                            }
                        },
                    }
                }

                // Notifying every item at the same time would not make sense. Let's notify only one of them
                let one_item_name = match list_of_additions.get(0) {
                    Some(url) => Self::item_name(&cal_local, &url).await,
                    None => String::from("<unable to get the name of the first batched item>"),
                };
                progress.increment_counter(list_of_additions.len());
                progress.feedback(SyncEvent::InProgress{
                    calendar: cal_name.to_string(),
                    items_done_already: progress.counter(),
                    details: one_item_name,
                });
            },
        }
    }
}


async fn get_or_insert_counterpart_calendar<H, N, I>(haystack_descr: &str, haystack: &mut H, cal_url: &Url, needle: Arc<Mutex<N>>)
    -> Result<Arc<Mutex<I>>, Box<dyn Error>>
where
    H: CalDavSource<I>,
    I: BaseCalendar,
    N: BaseCalendar,
{
    loop {
        if let Some(cal) = haystack.get_calendar(&cal_url).await {
            break Ok(cal);
        }

        // This calendar does not exist locally yet, let's add it
        log::debug!("Adding a {} calendar {}", haystack_descr, cal_url);
        let src = needle.lock().unwrap();
        let name = src.name().to_string();
        let supported_comps = src.supported_components();
        let color = src.color();
        if let Err(err) = haystack.create_calendar(
            cal_url.clone(),
            name,
            supported_comps,
            color.cloned(),
        ).await{
            return Err(err);
        }
    }
}