//! 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 url::Url;

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};

/// 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.feedback(SyncEvent::InProgress{
            calendar: cal_name.clone(),
            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(),
            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.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                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.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                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));
            }
        }

        for url_add in remote_additions {
            progress.debug(&format!("> Applying remote addition {} locally", url_add));
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                details: Self::item_name(&cal_local, &url_add).await,
            });
            match cal_remote.get_item_by_url(&url_add).await {
                Err(err) => {
                    progress.warn(&format!("Unable to get remote item {}: {}. Skipping it.", url_add, err));
                    continue;
                },
                Ok(item) => match item {
                    None => {
                        progress.error(&format!("Inconsistency: new item {} has vanished from the remote end", url_add));
                        continue;
                    },
                    Some(new_item) => {
                        if let Err(err) = cal_local.add_item(new_item.clone()).await {
                            progress.error(&format!("Not able to add item {} to local calendar: {}", url_add, err));
                        }
                    },
                },
            }
        }

        for url_change in remote_changes {
            progress.debug(&format!("> Applying remote change {} locally", url_change));
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                details: Self::item_name(&cal_local, &url_change).await,
            });
            match cal_remote.get_item_by_url(&url_change).await {
                Err(err) => {
                    progress.warn(&format!("Unable to get remote item {}: {}. Skipping it", url_change, err));
                    continue;
                },
                Ok(item) => match item {
                    None => {
                        progress.error(&format!("Inconsistency: modified item {} has vanished from the remote end", url_change));
                        continue;
                    },
                    Some(item) => {
                        if let Err(err) = cal_local.update_item(item.clone()).await {
                            progress.error(&format!("Unable to update item {} in local calendar: {}", url_change, err));
                        }
                    },
                }
            }
        }


        for url_add in local_additions {
            progress.debug(&format!("> Pushing local addition {} to the server", url_add));
            progress.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                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.feedback(SyncEvent::InProgress{
                calendar: cal_name.clone(),
                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 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);
        }
    }
}