use std::{
	collections::{HashMap, HashSet},
	fmt::{Debug, Write},
	mem::size_of,
	sync::{Arc, Mutex},
};

use arrayvec::ArrayVec;
use conduwuit::{
	at, checked, err, expected, utils,
	utils::{bytes, math::usize_from_f64, stream::IterStream},
	Result,
};
use database::Map;
use futures::{Stream, StreamExt};
use lru_cache::LruCache;
use ruma::{EventId, RoomId};

use crate::{
	rooms,
	rooms::short::{ShortEventId, ShortId, ShortStateHash, ShortStateKey},
	Dep,
};

pub struct Service {
	pub stateinfo_cache: Mutex<StateInfoLruCache>,
	db: Data,
	services: Services,
}

struct Services {
	short: Dep<rooms::short::Service>,
	state: Dep<rooms::state::Service>,
}

struct Data {
	shortstatehash_statediff: Arc<Map>,
}

#[derive(Clone)]
struct StateDiff {
	parent: Option<ShortStateHash>,
	added: Arc<CompressedState>,
	removed: Arc<CompressedState>,
}

#[derive(Clone, Default)]
pub struct ShortStateInfo {
	pub shortstatehash: ShortStateHash,
	pub full_state: Arc<CompressedState>,
	pub added: Arc<CompressedState>,
	pub removed: Arc<CompressedState>,
}

#[derive(Clone, Default)]
pub struct HashSetCompressStateEvent {
	pub shortstatehash: ShortStateHash,
	pub added: Arc<CompressedState>,
	pub removed: Arc<CompressedState>,
}

type StateInfoLruCache = LruCache<ShortStateHash, ShortStateInfoVec>;
type ShortStateInfoVec = Vec<ShortStateInfo>;
type ParentStatesVec = Vec<ShortStateInfo>;

pub(crate) type CompressedState = HashSet<CompressedStateEvent>;
pub(crate) type CompressedStateEvent = [u8; 2 * size_of::<ShortId>()];

impl crate::Service for Service {
	fn build(args: crate::Args<'_>) -> Result<Arc<Self>> {
		let config = &args.server.config;
		let cache_capacity =
			f64::from(config.stateinfo_cache_capacity) * config.cache_capacity_modifier;
		Ok(Arc::new(Self {
			stateinfo_cache: LruCache::new(usize_from_f64(cache_capacity)?).into(),
			db: Data {
				shortstatehash_statediff: args.db["shortstatehash_statediff"].clone(),
			},
			services: Services {
				short: args.depend::<rooms::short::Service>("rooms::short"),
				state: args.depend::<rooms::state::Service>("rooms::state"),
			},
		}))
	}

	fn memory_usage(&self, out: &mut dyn Write) -> Result {
		let (cache_len, ents) = {
			let cache = self.stateinfo_cache.lock().expect("locked");
			let ents = cache.iter().map(at!(1)).flat_map(|vec| vec.iter()).fold(
				HashMap::new(),
				|mut ents, ssi| {
					for cs in &[&ssi.added, &ssi.removed, &ssi.full_state] {
						ents.insert(Arc::as_ptr(cs), compressed_state_size(cs));
					}

					ents
				},
			);

			(cache.len(), ents)
		};

		let ents_len = ents.len();
		let bytes = ents.values().copied().fold(0_usize, usize::saturating_add);

		let bytes = bytes::pretty(bytes);
		writeln!(out, "stateinfo_cache: {cache_len} {ents_len} ({bytes})")?;

		Ok(())
	}

	fn clear_cache(&self) { self.stateinfo_cache.lock().expect("locked").clear(); }

	fn name(&self) -> &str { crate::service::make_name(std::module_path!()) }
}

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(name = "load", level = "debug", skip(self))]
	pub async fn load_shortstatehash_info(
		&self,
		shortstatehash: ShortStateHash,
	) -> Result<ShortStateInfoVec> {
		if let Some(r) = self.stateinfo_cache.lock()?.get_mut(&shortstatehash) {
			return Ok(r.clone());
		}

		let stack = self.new_shortstatehash_info(shortstatehash).await?;

		self.cache_shortstatehash_info(shortstatehash, stack.clone())
			.await?;

		Ok(stack)
	}

	/// Returns a stack with info on shortstatehash, full state, added diff and
	/// removed diff for the selected shortstatehash and each parent layer.
	#[tracing::instrument(
		name = "cache",
		level = "debug",
		skip_all,
		fields(
			?shortstatehash,
			stack = stack.len(),
		),
	)]
	async fn cache_shortstatehash_info(
		&self,
		shortstatehash: ShortStateHash,
		stack: ShortStateInfoVec,
	) -> Result {
		self.stateinfo_cache.lock()?.insert(shortstatehash, stack);

		Ok(())
	}

	async fn new_shortstatehash_info(
		&self,
		shortstatehash: ShortStateHash,
	) -> Result<ShortStateInfoVec> {
		let StateDiff { parent, added, removed } = self.get_statediff(shortstatehash).await?;

		let Some(parent) = parent else {
			return Ok(vec![ShortStateInfo {
				shortstatehash,
				full_state: added.clone(),
				added,
				removed,
			}]);
		};

		let mut stack = Box::pin(self.load_shortstatehash_info(parent)).await?;
		let top = stack.last().expect("at least one frame");

		let mut full_state = (*top.full_state).clone();
		full_state.extend(added.iter().copied());

		let removed = (*removed).clone();
		for r in &removed {
			full_state.remove(r);
		}

		stack.push(ShortStateInfo {
			shortstatehash,
			added,
			removed: Arc::new(removed),
			full_state: Arc::new(full_state),
		});

		Ok(stack)
	}

	pub fn compress_state_events<'a, I>(
		&'a self,
		state: I,
	) -> impl Stream<Item = CompressedStateEvent> + Send + 'a
	where
		I: Iterator<Item = (&'a ShortStateKey, &'a EventId)> + Clone + Debug + Send + 'a,
	{
		let event_ids = state.clone().map(at!(1));

		let short_event_ids = self
			.services
			.short
			.multi_get_or_create_shorteventid(event_ids);

		state
			.stream()
			.map(at!(0))
			.zip(short_event_ids)
			.map(|(shortstatekey, shorteventid)| {
				compress_state_event(*shortstatekey, shorteventid)
			})
	}

	pub async fn compress_state_event(
		&self,
		shortstatekey: ShortStateKey,
		event_id: &EventId,
	) -> CompressedStateEvent {
		let shorteventid = self
			.services
			.short
			.get_or_create_shorteventid(event_id)
			.await;

		compress_state_event(shortstatekey, shorteventid)
	}

	/// 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
	pub fn save_state_from_diff(
		&self,
		shortstatehash: ShortStateHash,
		statediffnew: Arc<HashSet<CompressedStateEvent>>,
		statediffremoved: Arc<HashSet<CompressedStateEvent>>,
		diff_to_sibling: usize,
		mut parent_states: ParentStatesVec,
	) -> Result {
		let statediffnew_len = statediffnew.len();
		let statediffremoved_len = statediffremoved.len();
		let diffsum = checked!(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().expect("parent must have a state");

			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.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().expect("parent must have a state");
		let parent_added_len = parent.added.len();
		let parent_removed_len = parent.removed.len();
		let parent_diff = checked!(parent_added_len + parent_removed_len)?;

		if checked!(diffsum * diffsum)? >= checked!(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.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
	#[tracing::instrument(skip(self, new_state_ids_compressed), level = "debug")]
	pub async fn save_state(
		&self,
		room_id: &RoomId,
		new_state_ids_compressed: Arc<HashSet<CompressedStateEvent>>,
	) -> Result<HashSetCompressStateEvent> {
		let previous_shortstatehash = self
			.services
			.state
			.get_room_shortstatehash(room_id)
			.await
			.ok();

		let state_hash =
			utils::calculate_hash(new_state_ids_compressed.iter().map(|bytes| &bytes[..]));

		let (new_shortstatehash, already_existed) = self
			.services
			.short
			.get_or_create_shortstatehash(&state_hash)
			.await;

		if Some(new_shortstatehash) == previous_shortstatehash {
			return Ok(HashSetCompressStateEvent {
				shortstatehash: new_shortstatehash,
				..Default::default()
			});
		}

		let states_parents = if let Some(p) = previous_shortstatehash {
			self.load_shortstatehash_info(p).await.unwrap_or_default()
		} else {
			ShortStateInfoVec::new()
		};

		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(),
				2, // every state change is 2 event changes on average
				states_parents,
			)?;
		};

		Ok(HashSetCompressStateEvent {
			shortstatehash: new_shortstatehash,
			added: statediffnew,
			removed: statediffremoved,
		})
	}

	#[tracing::instrument(skip(self), level = "debug", name = "get")]
	async fn get_statediff(&self, shortstatehash: ShortStateHash) -> Result<StateDiff> {
		const BUFSIZE: usize = size_of::<ShortStateHash>();
		const STRIDE: usize = size_of::<ShortStateHash>();

		let value = self
			.db
			.shortstatehash_statediff
			.aqry::<BUFSIZE, _>(&shortstatehash)
			.await
			.map_err(|e| {
				err!(Database("Failed to find StateDiff from short {shortstatehash:?}: {e}"))
			})?;

		let parent = utils::u64_from_bytes(&value[0..size_of::<u64>()])
			.ok()
			.take_if(|parent| *parent != 0);

		debug_assert!(value.len() % STRIDE == 0, "value not aligned to stride");
		let num_values = value.len() / STRIDE;

		let mut add_mode = true;
		let mut added = HashSet::with_capacity(num_values);
		let mut removed = HashSet::with_capacity(num_values);

		let mut i = STRIDE;
		while let Some(v) = value.get(i..expected!(i + 2 * STRIDE)) {
			if add_mode && v.starts_with(&0_u64.to_be_bytes()) {
				add_mode = false;
				i = expected!(i + STRIDE);
				continue;
			}
			if add_mode {
				added.insert(v.try_into()?);
			} else {
				removed.insert(v.try_into()?);
			}
			i = expected!(i + 2 * STRIDE);
		}

		added.shrink_to_fit();
		removed.shrink_to_fit();
		Ok(StateDiff {
			parent,
			added: Arc::new(added),
			removed: Arc::new(removed),
		})
	}

	fn save_statediff(&self, shortstatehash: ShortStateHash, diff: &StateDiff) {
		let mut value = Vec::<u8>::with_capacity(
			2_usize
				.saturating_add(diff.added.len())
				.saturating_add(diff.removed.len()),
		);

		let parent = diff.parent.unwrap_or(0_u64);
		value.extend_from_slice(&parent.to_be_bytes());

		for new in diff.added.iter() {
			value.extend_from_slice(&new[..]);
		}

		if !diff.removed.is_empty() {
			value.extend_from_slice(&0_u64.to_be_bytes());
			for removed in diff.removed.iter() {
				value.extend_from_slice(&removed[..]);
			}
		}

		self.db
			.shortstatehash_statediff
			.insert(&shortstatehash.to_be_bytes(), &value);
	}
}

#[inline]
#[must_use]
fn compress_state_event(
	shortstatekey: ShortStateKey,
	shorteventid: ShortEventId,
) -> CompressedStateEvent {
	const SIZE: usize = size_of::<CompressedStateEvent>();

	let mut v = ArrayVec::<u8, SIZE>::new();
	v.extend(shortstatekey.to_be_bytes());
	v.extend(shorteventid.to_be_bytes());
	v.as_ref()
		.try_into()
		.expect("failed to create CompressedStateEvent")
}

#[inline]
#[must_use]
pub fn parse_compressed_state_event(
	compressed_event: CompressedStateEvent,
) -> (ShortStateKey, ShortEventId) {
	use utils::u64_from_u8;

	let shortstatekey = u64_from_u8(&compressed_event[0..size_of::<ShortStateKey>()]);
	let shorteventid = u64_from_u8(&compressed_event[size_of::<ShortStateKey>()..]);

	(shortstatekey, shorteventid)
}

#[inline]
fn compressed_state_size(compressed_state: &CompressedState) -> usize {
	compressed_state
		.len()
		.checked_mul(size_of::<CompressedStateEvent>())
		.expect("CompressedState size overflow")
}