use std::collections::HashMap;
use std::sync::Arc;

use futures::StreamExt;
use serde::Deserialize;
use serde::Serialize;
use thiserror::Error;

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum ServiceKind {
    Aggregate,
    Projection,
    Runner,
}

#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
pub struct RoutingConfig {
    pub revision: u64,

    pub aggregate_placement: HashMap<String, String>,
    pub projection_placement: HashMap<String, String>,
    pub runner_placement: HashMap<String, String>,

    pub aggregate_shards: HashMap<String, Vec<String>>,
    pub projection_shards: HashMap<String, Vec<String>>,
    pub runner_shards: HashMap<String, Vec<String>>,
}

impl RoutingConfig {
    pub fn empty() -> Self {
        Self {
            revision: 0,
            aggregate_placement: HashMap::new(),
            projection_placement: HashMap::new(),
            runner_placement: HashMap::new(),
            aggregate_shards: HashMap::new(),
            projection_shards: HashMap::new(),
            runner_shards: HashMap::new(),
        }
    }
}

#[derive(Debug, Clone, Serialize)]
pub struct RoutingTable {
    pub revision: u64,
    aggregate_placement: HashMap<String, String>,
    projection_placement: HashMap<String, String>,
    runner_placement: HashMap<String, String>,
    aggregate_shards: HashMap<String, Vec<String>>,
    projection_shards: HashMap<String, Vec<String>>,
    runner_shards: HashMap<String, Vec<String>>,
}

impl From<RoutingConfig> for RoutingTable {
    fn from(value: RoutingConfig) -> Self {
        Self {
            revision: value.revision,
            aggregate_placement: value.aggregate_placement,
            projection_placement: value.projection_placement,
            runner_placement: value.runner_placement,
            aggregate_shards: value.aggregate_shards,
            projection_shards: value.projection_shards,
            runner_shards: value.runner_shards,
        }
    }
}

#[derive(Debug, Error, Clone, PartialEq, Eq)]
pub enum RoutingError {
    #[error("unknown tenant")]
    UnknownTenant,
    #[error("missing shard directory entry")]
    MissingShard,
    #[error("no endpoints for shard")]
    EmptyShard,
}

#[derive(Clone)]
pub struct RouterState {
    table: Arc<tokio::sync::RwLock<Arc<RoutingTable>>>,
    source: Arc<dyn RoutingSource>,
}

impl std::fmt::Debug for RouterState {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("RouterState").finish_non_exhaustive()
    }
}

impl RouterState {
    pub async fn new(source: Arc<dyn RoutingSource>) -> Result<Self, RoutingSourceError> {
        let cfg = source.load().await?;
        Ok(Self {
            table: Arc::new(tokio::sync::RwLock::new(Arc::new(cfg.into()))),
            source,
        })
    }

    pub async fn snapshot(&self) -> Arc<RoutingTable> {
        self.table.read().await.clone()
    }

    pub async fn reload(&self) -> Result<(), RoutingSourceError> {
        let cfg = self.source.load().await?;
        let next = Arc::new(RoutingTable::from(cfg));
        *self.table.write().await = next;
        Ok(())
    }

    pub fn start_watcher(&self) -> tokio::task::JoinHandle<()> {
        let this = self.clone();
        tokio::spawn(async move {
            let mut stream = match this.source.watch().await {
                Ok(s) => s,
                Err(_) => return,
            };

            while let Some(msg) = stream.next().await {
                if msg.is_err() {
                    continue;
                }
                let _ = this.reload().await;
            }
        })
    }

    pub async fn resolve(
        &self,
        tenant_id: &str,
        kind: ServiceKind,
    ) -> Result<String, RoutingError> {
        let table = self.snapshot().await;
        let result = table.resolve(tenant_id, kind);
        metrics::counter!(
            "gateway_routing_resolutions_total",
            "kind" => kind_label(kind),
            "result" => if result.is_ok() { "ok" } else { "err" }
        )
        .increment(1);
        result
    }
}

fn kind_label(kind: ServiceKind) -> &'static str {
    match kind {
        ServiceKind::Aggregate => "aggregate",
        ServiceKind::Projection => "projection",
        ServiceKind::Runner => "runner",
    }
}

impl RoutingTable {
    pub fn resolve(&self, tenant_id: &str, kind: ServiceKind) -> Result<String, RoutingError> {
        let shard_id = match kind {
            ServiceKind::Aggregate => self.aggregate_placement.get(tenant_id),
            ServiceKind::Projection => self.projection_placement.get(tenant_id),
            ServiceKind::Runner => self.runner_placement.get(tenant_id),
        }
        .ok_or(RoutingError::UnknownTenant)?;

        let endpoints = match kind {
            ServiceKind::Aggregate => self.aggregate_shards.get(shard_id),
            ServiceKind::Projection => self.projection_shards.get(shard_id),
            ServiceKind::Runner => self.runner_shards.get(shard_id),
        }
        .ok_or(RoutingError::MissingShard)?;

        endpoints.first().cloned().ok_or(RoutingError::EmptyShard)
    }
}

#[derive(Debug, Error)]
pub enum RoutingSourceError {
    #[error("source error: {0}")]
    Source(String),
    #[error("decode error: {0}")]
    Decode(String),
}

#[async_trait::async_trait]
pub trait RoutingSource: Send + Sync {
    async fn load(&self) -> Result<RoutingConfig, RoutingSourceError>;
    async fn watch(
        &self,
    ) -> Result<
        std::pin::Pin<Box<dyn futures::Stream<Item = Result<(), RoutingSourceError>> + Send>>,
        RoutingSourceError,
    >;
}

#[derive(Clone)]
pub struct FixedSource {
    cfg: RoutingConfig,
}

impl FixedSource {
    pub fn new(cfg: RoutingConfig) -> Self {
        Self { cfg }
    }
}

#[async_trait::async_trait]
impl RoutingSource for FixedSource {
    async fn load(&self) -> Result<RoutingConfig, RoutingSourceError> {
        Ok(self.cfg.clone())
    }

    async fn watch(
        &self,
    ) -> Result<
        std::pin::Pin<Box<dyn futures::Stream<Item = Result<(), RoutingSourceError>> + Send>>,
        RoutingSourceError,
    > {
        Ok(Box::pin(futures::stream::empty()))
    }
}

#[derive(Clone)]
pub struct StaticFileSource {
    path: String,
}

impl StaticFileSource {
    pub fn new(path: impl Into<String>) -> Self {
        Self { path: path.into() }
    }
}

#[async_trait::async_trait]
impl RoutingSource for StaticFileSource {
    async fn load(&self) -> Result<RoutingConfig, RoutingSourceError> {
        let raw = tokio::fs::read_to_string(&self.path)
            .await
            .map_err(|e| RoutingSourceError::Source(e.to_string()))?;

        if self.path.ends_with(".json") {
            serde_json::from_str::<RoutingConfig>(&raw)
                .map_err(|e| RoutingSourceError::Decode(e.to_string()))
        } else {
            let yaml: serde_yaml::Value = serde_yaml::from_str(&raw)
                .map_err(|e| RoutingSourceError::Decode(e.to_string()))?;
            let json = serde_json::to_value(yaml)
                .map_err(|e| RoutingSourceError::Decode(e.to_string()))?;
            serde_json::from_value::<RoutingConfig>(json)
                .map_err(|e| RoutingSourceError::Decode(e.to_string()))
        }
    }

    async fn watch(
        &self,
    ) -> Result<
        std::pin::Pin<Box<dyn futures::Stream<Item = Result<(), RoutingSourceError>> + Send>>,
        RoutingSourceError,
    > {
        Ok(Box::pin(futures::stream::empty()))
    }
}

#[derive(Clone)]
pub struct NatsKvSource {
    kv: async_nats::jetstream::kv::Store,
    key: String,
}

impl NatsKvSource {
    pub async fn connect(
        nats_url: impl Into<String>,
        bucket: impl Into<String>,
        key: impl Into<String>,
    ) -> Result<Self, RoutingSourceError> {
        let nats_url = nats_url.into();
        let bucket = bucket.into();
        let key = key.into();

        let client = async_nats::connect(nats_url)
            .await
            .map_err(|e| RoutingSourceError::Source(e.to_string()))?;
        let jetstream = async_nats::jetstream::new(client);

        let kv = match jetstream.get_key_value(&bucket).await {
            Ok(kv) => kv,
            Err(_) => jetstream
                .create_key_value(async_nats::jetstream::kv::Config {
                    bucket: bucket.clone(),
                    ..Default::default()
                })
                .await
                .map_err(|e| RoutingSourceError::Source(e.to_string()))?,
        };

        Ok(Self { kv, key })
    }
}

#[async_trait::async_trait]
impl RoutingSource for NatsKvSource {
    async fn load(&self) -> Result<RoutingConfig, RoutingSourceError> {
        let entry = self
            .kv
            .entry(&self.key)
            .await
            .map_err(|e| RoutingSourceError::Source(e.to_string()))?;

        let Some(entry) = entry else {
            return Ok(RoutingConfig::empty());
        };

        serde_json::from_slice::<RoutingConfig>(&entry.value)
            .map_err(|e| RoutingSourceError::Decode(e.to_string()))
    }

    async fn watch(
        &self,
    ) -> Result<
        std::pin::Pin<Box<dyn futures::Stream<Item = Result<(), RoutingSourceError>> + Send>>,
        RoutingSourceError,
    > {
        let key = self.key.clone();
        let watch = self
            .kv
            .watch(&key)
            .await
            .map_err(|e| RoutingSourceError::Source(e.to_string()))?;

        Ok(Box::pin(watch.filter_map(|entry| async move {
            match entry {
                Ok(entry) => match entry.operation {
                    async_nats::jetstream::kv::Operation::Put => Some(Ok(())),
                    async_nats::jetstream::kv::Operation::Delete
                    | async_nats::jetstream::kv::Operation::Purge => None,
                },
                Err(e) => Some(Err(RoutingSourceError::Source(e.to_string()))),
            }
        })))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn assert_send_sync<T: Send + Sync>() {}

    #[test]
    fn router_state_is_send_sync() {
        assert_send_sync::<RouterState>();
    }

    #[tokio::test]
    async fn resolves_endpoints_for_tenant_service_kind() {
        let cfg = RoutingConfig {
            revision: 1,
            aggregate_placement: HashMap::from([("t1".to_string(), "a".to_string())]),
            projection_placement: HashMap::from([("t1".to_string(), "p".to_string())]),
            runner_placement: HashMap::from([("t1".to_string(), "r".to_string())]),
            aggregate_shards: HashMap::from([("a".to_string(), vec!["http://a".to_string()])]),
            projection_shards: HashMap::from([("p".to_string(), vec!["http://p".to_string()])]),
            runner_shards: HashMap::from([("r".to_string(), vec!["http://r".to_string()])]),
        };

        let source: Arc<dyn RoutingSource> = Arc::new(TestSource::new(cfg));
        let router = RouterState::new(source).await.unwrap();

        assert_eq!(
            router.resolve("t1", ServiceKind::Aggregate).await.unwrap(),
            "http://a"
        );
        assert_eq!(
            router.resolve("t1", ServiceKind::Projection).await.unwrap(),
            "http://p"
        );
        assert_eq!(
            router.resolve("t1", ServiceKind::Runner).await.unwrap(),
            "http://r"
        );
    }

    #[tokio::test]
    async fn unknown_tenant_is_typed_error() {
        let source: Arc<dyn RoutingSource> = Arc::new(TestSource::new(RoutingConfig::empty()));
        let router = RouterState::new(source).await.unwrap();
        let err = router
            .resolve("missing", ServiceKind::Aggregate)
            .await
            .unwrap_err();
        assert_eq!(err, RoutingError::UnknownTenant);
    }

    #[tokio::test]
    async fn hot_reload_swaps_table_atomically() {
        let cfg1 = RoutingConfig {
            revision: 1,
            aggregate_placement: HashMap::from([("t1".to_string(), "a".to_string())]),
            projection_placement: HashMap::new(),
            runner_placement: HashMap::new(),
            aggregate_shards: HashMap::from([("a".to_string(), vec!["http://a1".to_string()])]),
            projection_shards: HashMap::new(),
            runner_shards: HashMap::new(),
        };
        let cfg2 = RoutingConfig {
            revision: 2,
            aggregate_placement: HashMap::from([("t1".to_string(), "a".to_string())]),
            projection_placement: HashMap::new(),
            runner_placement: HashMap::new(),
            aggregate_shards: HashMap::from([("a".to_string(), vec!["http://a2".to_string()])]),
            projection_shards: HashMap::new(),
            runner_shards: HashMap::new(),
        };

        let test_source = Arc::new(TestSource::new(cfg1));
        let router = RouterState::new(test_source.clone()).await.unwrap();

        let before = router.resolve("t1", ServiceKind::Aggregate).await.unwrap();
        assert_eq!(before, "http://a1");

        test_source.set(cfg2).await;
        router.reload().await.unwrap();

        let after = router.resolve("t1", ServiceKind::Aggregate).await.unwrap();
        assert_eq!(after, "http://a2");
    }

    #[derive(Clone)]
    struct TestSource {
        cfg: Arc<tokio::sync::RwLock<RoutingConfig>>,
    }

    impl TestSource {
        fn new(cfg: RoutingConfig) -> Self {
            Self {
                cfg: Arc::new(tokio::sync::RwLock::new(cfg)),
            }
        }

        async fn set(&self, cfg: RoutingConfig) {
            *self.cfg.write().await = cfg;
        }
    }

    #[async_trait::async_trait]
    impl RoutingSource for TestSource {
        async fn load(&self) -> Result<RoutingConfig, RoutingSourceError> {
            Ok(self.cfg.read().await.clone())
        }

        async fn watch(
            &self,
        ) -> Result<
            std::pin::Pin<Box<dyn futures::Stream<Item = Result<(), RoutingSourceError>> + Send>>,
            RoutingSourceError,
        > {
            Ok(Box::pin(futures::stream::empty()))
        }
    }
}