pub mod data;
use std::{
    collections::HashSet,
    mem::size_of,
    sync::{Arc, Mutex},
};

pub use data::Data;
use lru_cache::LruCache;
use ruma::{EventId, RoomId};

use crate::{services, utils, Result};

use self::data::StateDiff;

pub struct Service {
    pub db: &'static dyn Data,

    pub stateinfo_cache: Mutex<
        LruCache<
            u64,
            Vec<(
                u64,                           // sstatehash
                HashSet<CompressedStateEvent>, // full state
                HashSet<CompressedStateEvent>, // added
                HashSet<CompressedStateEvent>, // removed
            )>,
        >,
    >,
}

pub type CompressedStateEvent = [u8; 2 * size_of::<u64>()];

impl Service {
    /// Returns a stack with info on shortstatehash, full state, added diff and removed diff for the selected shortstatehash and each parent layer.
    #[tracing::instrument(skip(self))]
    pub fn load_shortstatehash_info(
        &self,
        shortstatehash: u64,
    ) -> Result<
        Vec<(
            u64,                           // sstatehash
            HashSet<CompressedStateEvent>, // full state
            HashSet<CompressedStateEvent>, // added
            HashSet<CompressedStateEvent>, // removed
        )>,
    > {
        if let Some(r) = self
            .stateinfo_cache
            .lock()
            .unwrap()
            .get_mut(&shortstatehash)
        {
            return Ok(r.clone());
        }

        let StateDiff {
            parent,
            added,
            removed,
        } = self.db.get_statediff(shortstatehash)?;

        if let Some(parent) = parent {
            let mut response = self.load_shortstatehash_info(parent)?;
            let mut state = response.last().unwrap().1.clone();
            state.extend(added.iter().copied());
            for r in &removed {
                state.remove(r);
            }

            response.push((shortstatehash, state, added, removed));

            Ok(response)
        } else {
            let response = vec![(shortstatehash, added.clone(), added, removed)];
            self.stateinfo_cache
                .lock()
                .unwrap()
                .insert(shortstatehash, response.clone());
            Ok(response)
        }
    }

    pub fn compress_state_event(
        &self,
        shortstatekey: u64,
        event_id: &EventId,
    ) -> Result<CompressedStateEvent> {
        let mut v = shortstatekey.to_be_bytes().to_vec();
        v.extend_from_slice(
            &services()
                .rooms
                .short
                .get_or_create_shorteventid(event_id)?
                .to_be_bytes(),
        );
        Ok(v.try_into().expect("we checked the size above"))
    }

    /// Returns shortstatekey, event id
    pub fn parse_compressed_state_event(
        &self,
        compressed_event: &CompressedStateEvent,
    ) -> Result<(u64, Arc<EventId>)> {
        Ok((
            utils::u64_from_bytes(&compressed_event[0..size_of::<u64>()])
                .expect("bytes have right length"),
            services().rooms.short.get_eventid_from_short(
                utils::u64_from_bytes(&compressed_event[size_of::<u64>()..])
                    .expect("bytes have right length"),
            )?,
        ))
    }

    /// Creates a new shortstatehash that often is just a diff to an already existing
    /// shortstatehash and therefore very efficient.
    ///
    /// There are multiple layers of diffs. The bottom layer 0 always contains the full state. Layer
    /// 1 contains diffs to states of layer 0, layer 2 diffs to layer 1 and so on. If layer n > 0
    /// grows too big, it will be combined with layer n-1 to create a new diff on layer n-1 that's
    /// based on layer n-2. If that layer is also too big, it will recursively fix above layers too.
    ///
    /// * `shortstatehash` - Shortstatehash of this state
    /// * `statediffnew` - Added to base. Each vec is shortstatekey+shorteventid
    /// * `statediffremoved` - Removed from base. Each vec is shortstatekey+shorteventid
    /// * `diff_to_sibling` - Approximately how much the diff grows each time for this layer
    /// * `parent_states` - A stack with info on shortstatehash, full state, added diff and removed diff for each parent layer
    #[tracing::instrument(skip(
        self,
        statediffnew,
        statediffremoved,
        diff_to_sibling,
        parent_states
    ))]
    pub fn save_state_from_diff(
        &self,
        shortstatehash: u64,
        statediffnew: HashSet<CompressedStateEvent>,
        statediffremoved: HashSet<CompressedStateEvent>,
        diff_to_sibling: usize,
        mut parent_states: Vec<(
            u64,                           // sstatehash
            HashSet<CompressedStateEvent>, // full state
            HashSet<CompressedStateEvent>, // added
            HashSet<CompressedStateEvent>, // removed
        )>,
    ) -> Result<()> {
        let diffsum = statediffnew.len() + statediffremoved.len();

        if parent_states.len() > 3 {
            // Number of layers
            // To many layers, we have to go deeper
            let parent = parent_states.pop().unwrap();

            let mut parent_new = parent.2;
            let mut parent_removed = parent.3;

            for removed in statediffremoved {
                if !parent_new.remove(&removed) {
                    // It was not added in the parent and we removed it
                    parent_removed.insert(removed);
                }
                // Else it was added in the parent and we removed it again. We can forget this change
            }

            for new in statediffnew {
                if !parent_removed.remove(&new) {
                    // It was not touched in the parent and we added it
                    parent_new.insert(new);
                }
                // Else it was removed in the parent and we added it again. We can forget this change
            }

            self.save_state_from_diff(
                shortstatehash,
                parent_new,
                parent_removed,
                diffsum,
                parent_states,
            )?;

            return Ok(());
        }

        if parent_states.is_empty() {
            // There is no parent layer, create a new state
            self.db.save_statediff(
                shortstatehash,
                StateDiff {
                    parent: None,
                    added: statediffnew,
                    removed: statediffremoved,
                },
            )?;

            return Ok(());
        };

        // Else we have two options.
        // 1. We add the current diff on top of the parent layer.
        // 2. We replace a layer above

        let parent = parent_states.pop().unwrap();
        let parent_diff = parent.2.len() + parent.3.len();

        if diffsum * diffsum >= 2 * diff_to_sibling * parent_diff {
            // Diff too big, we replace above layer(s)
            let mut parent_new = parent.2;
            let mut parent_removed = parent.3;

            for removed in statediffremoved {
                if !parent_new.remove(&removed) {
                    // It was not added in the parent and we removed it
                    parent_removed.insert(removed);
                }
                // Else it was added in the parent and we removed it again. We can forget this change
            }

            for new in statediffnew {
                if !parent_removed.remove(&new) {
                    // It was not touched in the parent and we added it
                    parent_new.insert(new);
                }
                // Else it was removed in the parent and we added it again. We can forget this change
            }

            self.save_state_from_diff(
                shortstatehash,
                parent_new,
                parent_removed,
                diffsum,
                parent_states,
            )?;
        } else {
            // Diff small enough, we add diff as layer on top of parent
            self.db.save_statediff(
                shortstatehash,
                StateDiff {
                    parent: Some(parent.0),
                    added: statediffnew,
                    removed: statediffremoved,
                },
            )?;
        }

        Ok(())
    }

    /// Returns the new shortstatehash, and the state diff from the previous room state
    pub fn save_state(
        &self,
        room_id: &RoomId,
        new_state_ids_compressed: HashSet<CompressedStateEvent>,
    ) -> Result<(
        u64,
        HashSet<CompressedStateEvent>,
        HashSet<CompressedStateEvent>,
    )> {
        let previous_shortstatehash = services().rooms.state.get_room_shortstatehash(room_id)?;

        let state_hash = utils::calculate_hash(
            &new_state_ids_compressed
                .iter()
                .map(|bytes| &bytes[..])
                .collect::<Vec<_>>(),
        );

        let (new_shortstatehash, already_existed) = services()
            .rooms
            .short
            .get_or_create_shortstatehash(&state_hash)?;

        if Some(new_shortstatehash) == previous_shortstatehash {
            return Ok((new_shortstatehash, HashSet::new(), HashSet::new()));
        }

        let states_parents = previous_shortstatehash
            .map_or_else(|| Ok(Vec::new()), |p| self.load_shortstatehash_info(p))?;

        let (statediffnew, statediffremoved) = if let Some(parent_stateinfo) = states_parents.last()
        {
            let statediffnew: HashSet<_> = new_state_ids_compressed
                .difference(&parent_stateinfo.1)
                .copied()
                .collect();

            let statediffremoved: HashSet<_> = parent_stateinfo
                .1
                .difference(&new_state_ids_compressed)
                .copied()
                .collect();

            (statediffnew, statediffremoved)
        } else {
            (new_state_ids_compressed, HashSet::new())
        };

        if !already_existed {
            self.save_state_from_diff(
                new_shortstatehash,
                statediffnew.clone(),
                statediffremoved.clone(),
                2, // every state change is 2 event changes on average
                states_parents,
            )?;
        };

        Ok((new_shortstatehash, statediffnew, statediffremoved))
    }
}