use std::collections::HashMap;
use std::sync::Arc;
use std::sync::atomic::{AtomicI64, Ordering};

use dashmap::DashMap;
use tokio::sync::Mutex;
use uuid::Uuid;

use crate::{ChannelError, ChannelResult, security::require_cluster};

const SEQ_KEY_PREFIX: &str = "room:seq:";
const DEFAULT_SEGMENT_SIZE: u64 = 1024;
const MAX_REFRESH_RETRIES: u32 = 3;

const BOOTSTRAP_SCRIPT: &str = r#"
local key = KEYS[1]
local db_max = tonumber(ARGV[1])
local current = tonumber(redis.call('GET', key) or '0')
if current < db_max then
    redis.call('SET', key, db_max)
end
return tonumber(redis.call('GET', key))
"#;

struct SegmentState {
    end: i64,
    next: AtomicI64,
}

pub struct SeqAllocator(Arc<SeqAllocatorInner>);

struct SeqAllocatorInner {
    cache: cache::AppCache,
    db: db::AppDatabase,
    segments: DashMap<Uuid, Arc<SegmentState>>,
    refresh_locks: DashMap<Uuid, Arc<Mutex<()>>>,
    segment_size: u64,
}

impl Clone for SeqAllocator {
    fn clone(&self) -> Self {
        Self(Arc::clone(&self.0))
    }
}

impl SeqAllocator {
    pub fn new(cache: cache::AppCache, db: db::AppDatabase) -> Self {
        Self::with_segment_size(cache, db, DEFAULT_SEGMENT_SIZE)
    }

    pub fn with_segment_size(
        cache: cache::AppCache,
        db: db::AppDatabase,
        size: u64,
    ) -> Self {
        Self(Arc::new(SeqAllocatorInner {
            cache,
            db,
            segments: DashMap::new(),
            refresh_locks: DashMap::new(),
            segment_size: if size > 0 { size } else { DEFAULT_SEGMENT_SIZE },
        }))
    }

    pub async fn seq(&self, room: Uuid) -> ChannelResult<i64> {
        for _ in 0..MAX_REFRESH_RETRIES {
            if let Some(seq) = self.try_allocate(&room) {
                return Ok(seq);
            }

            let lock = self.get_refresh_lock(room);
            let _guard = lock.lock().await;

            if let Some(seq) = self.try_allocate(&room) {
                return Ok(seq);
            }

            self.refresh_segment(room).await?;
            self.0.refresh_locks.remove(&room);
        }

        Err(ChannelError::Internal(
            "seq allocation exhausted".to_string(),
        ))
    }

    pub async fn bootstrap(&self, room: Uuid) -> ChannelResult<i64> {
        let db_max = self.db_max_seq(room).await?;
        if db_max == 0 {
            return Ok(0);
        }

        let key = format!("{}{}", SEQ_KEY_PREFIX, room);
        let cluster = require_cluster(&self.0.cache)?;
        let mut conn = cluster.conn();

        let current: i64 = redis::Cmd::new()
            .arg("EVAL")
            .arg(BOOTSTRAP_SCRIPT)
            .arg(1)
            .arg(&key)
            .arg(db_max)
            .query_async(&mut conn)
            .await
            .map_err(|e| ChannelError::Cache(cache::CacheError::Redis(e)))?;

        self.0.segments.remove(&room);
        Ok(current)
    }

    pub async fn bootstrap_all(
        &self,
        rooms: Vec<Uuid>,
    ) -> ChannelResult<HashMap<Uuid, i64>> {
        let mut results = HashMap::with_capacity(rooms.len());
        for room in rooms {
            results.insert(room, self.bootstrap(room).await?);
        }
        Ok(results)
    }

    fn try_allocate(&self, room: &Uuid) -> Option<i64> {
        let state = self.0.segments.get(room)?;
        loop {
            let current = state.next.load(Ordering::Acquire);
            if current >= state.end {
                return None;
            }
            if state
                .next
                .compare_exchange_weak(current, current + 1, Ordering::AcqRel, Ordering::Acquire)
                .is_ok()
            {
                return Some(current);
            }
        }
    }

    fn get_refresh_lock(&self, room: Uuid) -> Arc<Mutex<()>> {
        Arc::clone(
            self.0
                .refresh_locks
                .entry(room)
                .or_insert_with(|| Arc::new(Mutex::new(())))
                .value(),
        )
    }

    async fn refresh_segment(&self, room: Uuid) -> ChannelResult<()> {
        let key = format!("{}{}", SEQ_KEY_PREFIX, room);
        let cluster = require_cluster(&self.0.cache)?;
        let mut conn = cluster.conn();

        let counter: i64 = redis::Cmd::new()
            .arg("INCRBY")
            .arg(&key)
            .arg(self.0.segment_size as i64)
            .query_async(&mut conn)
            .await
            .map_err(|e| ChannelError::Cache(cache::CacheError::Redis(e)))?;

        let start = counter - self.0.segment_size as i64 + 1;
        let end = counter + 1;

        self.0.segments.insert(
            room,
            Arc::new(SegmentState {
                end,
                next: AtomicI64::new(start),
            }),
        );

        Ok(())
    }

    async fn db_max_seq(&self, room: Uuid) -> ChannelResult<i64> {
        let row: (i64,) = db::sqlx::query_as(
            "SELECT COALESCE(MAX(seq), 0) FROM room_message WHERE room = $1 AND deleted_at IS NULL",
        )
        .bind(room)
        .fetch_one(self.0.db.reader())
        .await?;
        Ok(row.0)
    }
}