//! The host kernel, responsible for dispatching requests to the local host

use std::convert::{TryFrom, TryInto};
use std::pin::Pin;

use bytes::Bytes;
use futures::future::Future;
use log::debug;
use safecast::{TryCastFrom, TryCastInto};

use tc_error::*;
use tcgeneric::*;

use crate::cluster::Cluster;
use crate::object::InstanceExt;
use crate::route::Public;
use crate::scalar::*;
use crate::state::*;
use crate::txn::*;

mod hosted;

use hosted::Hosted;

const HYPOTHETICAL: PathLabel = path_label(&["transact", "hypothetical"]);

type ExeScope<'a> = crate::scalar::Scope<'a, State>;

/// The host kernel, responsible for dispatching requests to the local host
pub struct Kernel {
    actor: Actor,
    hosted: Hosted,
}

impl Kernel {
    /// Construct a new `Kernel` to host the given [`Cluster`]s.
    pub fn new<I: IntoIterator<Item = InstanceExt<Cluster>>>(clusters: I) -> Self {
        Self {
            actor: Actor::new(Link::default().into()),
            hosted: clusters.into_iter().collect(),
        }
    }

    /// Return a list of hosted clusters
    pub fn hosted(&self) -> impl Iterator<Item = &InstanceExt<Cluster>> {
        self.hosted.clusters()
    }

    /// Route a GET request.
    pub async fn get(&self, txn: &Txn, path: &[PathSegment], key: Value) -> TCResult<State> {
        if path.is_empty() {
            if key.is_none() {
                Ok(Value::from(Bytes::copy_from_slice(self.actor.public_key().as_bytes())).into())
            } else {
                Err(TCError::method_not_allowed(TCPath::from(path)))
            }
        } else if let Some(class) = StateType::from_path(path) {
            let err = format!("Cannot cast into {} from {}", class, key);
            State::Scalar(Scalar::Value(key))
                .into_type(class)
                .ok_or_else(|| TCError::unsupported(err))
        } else if let Some((suffix, cluster)) = self.hosted.get(path) {
            debug!(
                "GET {}: {} from cluster {}",
                TCPath::from(suffix),
                key,
                cluster
            );

            cluster.get(&txn, suffix, key).await
        } else if &path[0] == "error" && path.len() == 2 {
            let message = String::try_cast_from(key, |v| {
                TCError::bad_request("cannot cast into error message string from", v)
            })?;

            if let Some(err_type) = error_type(&path[1]) {
                Err(TCError::new(err_type, message))
            } else {
                Err(TCError::not_found(TCPath::from(path)))
            }
        } else {
            Err(TCError::not_found(TCPath::from(path)))
        }
    }

    /// Route a PUT request.
    pub async fn put(
        &self,
        txn: &Txn,
        path: &[PathSegment],
        key: Value,
        value: State,
    ) -> TCResult<()> {
        if path.is_empty() {
            if key.is_none() {
                if Link::can_cast_from(&value) {
                    // It's a synchronization message for a hypothetical transaction
                    return Ok(());
                }
            }

            Err(TCError::method_not_allowed(TCPath::from(path)))
        } else if let Some(class) = StateType::from_path(path) {
            Err(TCError::method_not_allowed(class))
        } else if let Some((suffix, cluster)) = self.hosted.get(path) {
            debug!(
                "PUT {}: {} <- {} to cluster {}",
                TCPath::from(suffix),
                key,
                value,
                cluster
            );

            execute(txn, cluster, |txn, cluster| async move {
                cluster.put(&txn, suffix, key, value).await
            })
            .await
        } else {
            Err(TCError::not_found(TCPath::from(path)))
        }
    }

    /// Route a POST request.
    pub async fn post(&self, txn: &Txn, path: &[PathSegment], data: State) -> TCResult<State> {
        if path.is_empty() {
            if Map::try_from(data)?.is_empty() {
                // it's a "commit" instruction for a hypothetical transaction
                Ok(State::default())
            } else {
                Err(TCError::method_not_allowed(TCPath::from(path)))
            }
        } else if path == &HYPOTHETICAL[..] {
            let txn = txn.clone().claim(&self.actor, TCPathBuf::default()).await?;
            let context = Map::<State>::default();

            if PostOp::can_cast_from(&data) {
                OpDef::call(data.opt_cast_into().unwrap(), txn, context).await
            } else {
                data.resolve(&ExeScope::new(&State::default(), context), &txn)
                    .await
            }
        } else if let Some((suffix, cluster)) = self.hosted.get(path) {
            let params: Map<State> = data.try_into()?;

            debug!(
                "POST {}: {} to cluster {}",
                TCPath::from(suffix),
                params,
                cluster
            );

            if suffix.is_empty() && params.is_empty() {
                // it's a "commit" instruction
                cluster.post(&txn, suffix, params).await
            } else {
                execute(txn, cluster, |txn, cluster| async move {
                    cluster.post(&txn, suffix, params).await
                })
                .await
            }
        } else {
            Err(TCError::not_found(TCPath::from(path)))
        }
    }

    /// Route a DELETE request.
    pub async fn delete(&self, txn: &Txn, path: &[PathSegment], key: Value) -> TCResult<()> {
        if path.is_empty() || StateType::from_path(path).is_some() {
            Err(TCError::method_not_allowed(TCPath::from(path)))
        } else if let Some((suffix, cluster)) = self.hosted.get(path) {
            debug!(
                "DELETE {}: {} from cluster {}",
                TCPath::from(suffix),
                key,
                cluster
            );

            cluster.delete(&txn, suffix, key).await
        } else if &path[0] == "error" && path.len() == 2 {
            if error_type(&path[1]).is_some() {
                Err(TCError::method_not_allowed(TCPath::from(path)))
            } else {
                Err(TCError::not_found(TCPath::from(path)))
            }
        } else {
            Err(TCError::not_found(TCPath::from(path)))
        }
    }
}

fn execute<
    'a,
    R: Send,
    Fut: Future<Output = TCResult<R>> + Send,
    F: FnOnce(Txn, &'a InstanceExt<Cluster>) -> Fut + Send + 'a,
>(
    txn: &'a Txn,
    cluster: &'a InstanceExt<Cluster>,
    handler: F,
) -> Pin<Box<dyn Future<Output = TCResult<R>> + Send + 'a>> {
    Box::pin(async move {
        if let Some(owner) = txn.owner() {
            if cluster.path() == &owner.path()[..] {
                debug!(
                    "{} owns this transaction, no need to notify",
                    TCPath::from(cluster.path())
                );
            } else {
                txn.put(owner.clone(), Value::None, cluster.link().clone().into())
                    .await?;
            }

            handler(txn.clone(), cluster).await
        } else {
            // Claim and execute the transaction
            let txn = cluster.claim(&txn).await?;
            let result = handler(txn.clone(), cluster).await;

            if result.is_ok() {
                cluster.distribute_commit(txn).await?;
            } else {
                cluster.distribute_rollback(txn).await;
            }

            result
        }
    })
}

fn error_type(err_type: &Id) -> Option<ErrorType> {
    match err_type.as_str() {
        "bad_gateway" => Some(ErrorType::BadGateway),
        "bad_request" => Some(ErrorType::BadRequest),
        "conflict" => Some(ErrorType::Conflict),
        "forbidden" => Some(ErrorType::Forbidden),
        "internal" => Some(ErrorType::Internal),
        "method_not_allowed" => Some(ErrorType::MethodNotAllowed),
        "not_found" => Some(ErrorType::NotFound),
        "not_implemented" => Some(ErrorType::NotImplemented),
        "timeout" => Some(ErrorType::Timeout),
        "unauthorized" => Some(ErrorType::Unauthorized),
        _ => None,
    }
}