use std::{
    array,
    collections::HashSet,
    mem::size_of,
    sync::{Arc, Mutex},
};

use lru_cache::LruCache;
use ruma::{EventId, RoomId};

use crate::{
    observability::{FoundIn, Lookup, METRICS},
    services, utils, Result,
};

pub(crate) mod data;

pub(crate) use data::Data;
use data::StateDiff;

use super::short::{ShortEventId, ShortStateHash, ShortStateKey};

#[derive(Clone)]
pub(crate) struct CompressedStateLayer {
    pub(crate) shortstatehash: ShortStateHash,
    pub(crate) full_state: Arc<HashSet<CompressedStateEvent>>,
    pub(crate) added: Arc<HashSet<CompressedStateEvent>>,
    pub(crate) removed: Arc<HashSet<CompressedStateEvent>>,
}

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

    #[allow(clippy::type_complexity)]
    pub(crate) stateinfo_cache:
        Mutex<LruCache<ShortStateHash, Vec<CompressedStateLayer>>>,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub(crate) struct CompressedStateEvent {
    pub(crate) state: ShortStateKey,
    pub(crate) event: ShortEventId,
}

impl CompressedStateEvent {
    pub(crate) fn as_bytes(
        &self,
    ) -> [u8; size_of::<ShortStateKey>() + size_of::<ShortEventId>()] {
        let mut bytes = self
            .state
            .get()
            .to_be_bytes()
            .into_iter()
            .chain(self.event.get().to_be_bytes());
        array::from_fn(|_| bytes.next().unwrap())
    }

    pub(crate) fn from_bytes(
        bytes: [u8; size_of::<ShortStateKey>() + size_of::<ShortEventId>()],
    ) -> Self {
        let state = ShortStateKey::new(u64::from_be_bytes(
            bytes[0..8].try_into().unwrap(),
        ));
        let event = ShortEventId::new(u64::from_be_bytes(
            bytes[8..16].try_into().unwrap(),
        ));

        Self {
            state,
            event,
        }
    }
}

impl Service {
    /// Returns a stack with info on shortstatehash, full state, added diff and
    /// removed diff for the selected shortstatehash and each parent layer.
    #[allow(clippy::type_complexity)]
    #[tracing::instrument(skip(self))]
    pub(crate) fn load_shortstatehash_info(
        &self,
        shortstatehash: ShortStateHash,
    ) -> Result<Vec<CompressedStateLayer>> {
        let lookup = Lookup::StateInfo;

        if let Some(r) =
            self.stateinfo_cache.lock().unwrap().get_mut(&shortstatehash)
        {
            METRICS.record_lookup(lookup, FoundIn::Cache);
            return Ok(r.clone());
        }

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

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

            response.push(CompressedStateLayer {
                shortstatehash,
                full_state: Arc::new(state),
                added,
                removed: Arc::new(removed),
            });
            response
        } else {
            vec![CompressedStateLayer {
                shortstatehash,
                full_state: added.clone(),
                added,
                removed,
            }]
        };

        METRICS.record_lookup(lookup, FoundIn::Database);
        self.stateinfo_cache
            .lock()
            .unwrap()
            .insert(shortstatehash, response.clone());

        Ok(response)
    }

    // Allowed because this function uses `services()`
    #[allow(clippy::unused_self)]
    pub(crate) fn compress_state_event(
        &self,
        shortstatekey: ShortStateKey,
        event_id: &EventId,
    ) -> Result<CompressedStateEvent> {
        Ok(CompressedStateEvent {
            state: shortstatekey,
            event: services()
                .rooms
                .short
                .get_or_create_shorteventid(event_id)?,
        })
    }

    /// Returns shortstatekey, event id
    // Allowed because this function uses `services()`
    #[allow(clippy::unused_self)]
    pub(crate) fn parse_compressed_state_event(
        &self,
        compressed_event: &CompressedStateEvent,
    ) -> Result<(ShortStateKey, Arc<EventId>)> {
        Ok((
            compressed_event.state,
            services()
                .rooms
                .short
                .get_eventid_from_short(compressed_event.event)?,
        ))
    }

    /// 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
    #[allow(clippy::type_complexity)]
    #[tracing::instrument(skip(
        self,
        statediffnew,
        statediffremoved,
        diff_to_sibling,
        parent_states
    ))]
    pub(crate) fn save_state_from_diff(
        &self,
        shortstatehash: ShortStateHash,
        statediffnew: Arc<HashSet<CompressedStateEvent>>,
        statediffremoved: Arc<HashSet<CompressedStateEvent>>,
        diff_to_sibling: usize,
        mut parent_states: Vec<CompressedStateLayer>,
    ) -> 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.added).clone();
            let mut parent_removed = (*parent.removed).clone();

            for removed in statediffremoved.iter() {
                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.iter() {
                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,
                Arc::new(parent_new),
                Arc::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.added.len() + parent.removed.len();

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

            for removed in statediffremoved.iter() {
                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.iter() {
                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,
                Arc::new(parent_new),
                Arc::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.shortstatehash),
                    added: statediffnew,
                    removed: statediffremoved,
                },
            )?;
        }

        Ok(())
    }

    /// Returns the new shortstatehash, and the state diff from the previous
    /// room state
    #[allow(clippy::type_complexity)]
    #[tracing::instrument(skip(self, new_state_ids_compressed))]
    pub(crate) fn save_state(
        &self,
        room_id: &RoomId,
        new_state_ids_compressed: Arc<HashSet<CompressedStateEvent>>,
    ) -> Result<(
        ShortStateHash,
        Arc<HashSet<CompressedStateEvent>>,
        Arc<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(CompressedStateEvent::as_bytes),
        );

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

        if Some(new_shortstatehash) == previous_shortstatehash {
            return Ok((
                new_shortstatehash,
                Arc::new(HashSet::new()),
                Arc::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.full_state)
                    .copied()
                    .collect();

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

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

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

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