# apc.rs - source
Source of the Rust file `maplibre/src/io/apc.rs`.

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    use std::{
        any::Any,
        cell::RefCell,
        fmt::Debug,
        future::Future,
        marker::PhantomData,
        pin::Pin,
        sync::{
            mpsc,
            mpsc::{Receiver, Sender},
        },
        vec::IntoIter,
    };

    use serde::{Deserialize, Serialize};
    use thiserror::Error;

    use crate::{
        coords::WorldTileCoords, define_label, environment::OffscreenKernelEnvironment,
        io::scheduler::Scheduler, style::Style,
    };

    define_label!(MessageTag);

    impl MessageTag for u32 {
        fn dyn_clone(&self) -> Box<dyn MessageTag> {
            Box::new(*self)
        }
    }

    #[derive(Error, Debug)]
    pub enum MessageError {
        #[error("the message did not contain the expected data")]
        CastError(Box<dyn Any>),
    }

    /// The result of the tessellation of a tile. This is sent as a message from a worker to the caller
    /// of an [`AsyncProcedure`].
    #[derive(Debug)]
    pub struct Message {
        tag: &'static dyn MessageTag,
        transferable: Box<dyn Any + Send>,
    }

    impl Message {
        pub fn new(tag: &'static dyn MessageTag, transferable: Box<dyn Any + Send>) -> Self {
            Self { tag, transferable }
        }

        pub fn into_transferable<T: 'static>(self) -> Box<T> {
            self.transferable
                .downcast::<T>()
                .expect("message has wrong tag")
        }

        pub fn has_tag(&self, tag: &'static dyn MessageTag) -> bool {
            self.tag == tag
        }

        pub fn tag(&self) -> &'static dyn MessageTag {
            self.tag
        }
    }

    pub trait IntoMessage {
        fn into(self) -> Message;
    }

    /// Inputs for an [`AsyncProcedure`]
    #[derive(Clone, Serialize, Deserialize)]
    pub enum Input {
        TileRequest {
            coords: WorldTileCoords,
            style: Style, // TODO
        },
        NotYetImplemented, // TODO: Placeholder, should be removed when second input is added
    }

    #[derive(Error, Debug)]
    pub enum SendError {
        #[error("could not transmit data")]
        Transmission,
    }

    /// Allows sending messages from workers to back to the caller.
    pub trait Context: 'static {
        /// Send a message back to the caller.
        fn send<T: IntoMessage>(&self, message: T) -> Result<(), SendError>;
    }

    #[derive(Error, Debug)]
    pub enum ProcedureError {
        /// The [`Input`] is not compatible with the procedure
        #[error("provided input is not compatible with procedure")]
        IncompatibleInput,
        #[error("execution of procedure failed")]
        Execution(Box<dyn std::error::Error>),
        #[error("sending data failed")]
        Send(SendError),
    }

    #[cfg(feature = "thread-safe-futures")]
    pub type AsyncProcedureFuture =
        Pin<Box<(dyn Future<Output = Result<(), ProcedureError>> + Send + 'static)>>;
    #[cfg(not(feature = "thread-safe-futures"))]
    pub type AsyncProcedureFuture =
        Pin<Box<(dyn Future<Output = Result<(), ProcedureError>> + 'static)>>;

    #[derive(Error, Debug)]
    pub enum CallError {
        #[error("scheduling work failed")]
        Schedule,
        #[error("serializing data failed")]
        Serialize(Box<dyn std::error::Error>),
        #[error("deserializing failed")]
        Deserialize(Box<dyn std::error::Error>),
        #[error("deserializing input failed")]
        DeserializeInput(Box<dyn std::error::Error>),
    }

    /// Type definitions for asynchronous procedure calls. These functions can be called in an
    /// [`AsyncProcedureCall`]. Functions of this type are required to be statically available at
    /// compile time. It is explicitly not possible to use closures, as they would require special
    /// serialization which is currently not supported.
    pub type AsyncProcedure<K, C> = fn(input: Input, context: C, kernel: K) -> AsyncProcedureFuture;

    /// APCs define an interface for performing work asynchronously.
    /// This work can be implemented through procedures which can be called asynchronously, hence the
    /// name AsyncProcedureCall or APC for short.
    ///
    /// APCs serve as an abstraction for doing work on a separate thread, and then getting responses
    /// back. An asynchronous procedure call can for example be performed by using message passing. In
    /// fact this could theoretically work over a network socket.
    ///
    /// It is possible to schedule work on a  remote host by calling [`AsyncProcedureCall::call()`]
    /// and getting the results back by calling the non-blocking function
    /// [`AsyncProcedureCall::receive()`]. The [`AsyncProcedureCall::receive()`] function returns a
    /// struct which implements [`Transferables`].
    ///
    /// ## Transferables
    ///
    /// Based on whether the current platform supports shared-memory or not, the implementation of APCs
    /// might want to send the whole data from the worker to the caller back or just pointers to that
    /// data. The [`Transferables`] trait allows developers to define that and use different data
    /// layouts for different platforms.
    ///
    /// ## Message Passing vs APC
    ///
    /// One might wonder why this is called [`AsyncProcedureCall`] instead of `MessagePassingInterface`.
    /// The reason for this is quite simple. We are actually referencing and calling procedures which
    /// are defined in different threads, processes or hosts. That means, that an [`AsyncProcedureCall`]
    /// is actually distinct from a `MessagePassingInterface`.
    ///
    ///
    /// ## Current Implementations
    ///
    /// We currently have two implementation for APCs. One uses the Tokio async runtime on native
    /// targets in [`SchedulerAsyncProcedureCall`].
    /// For the web we implemented an alternative way to call APCs which is called
    /// [`PassingAsyncProcedureCall`]. This implementation does not depend on shared-memory compared to
    /// [`SchedulerAsyncProcedureCall`]. In fact, on the web we are currently not depending on
    /// shared-memory because that feature is hidden behind feature flags in browsers
    /// (see [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer)).
    ///
    ///
    // TODO: Rename to AsyncProcedureCaller?
    pub trait AsyncProcedureCall<K: OffscreenKernelEnvironment>: 'static {
        type Context: Context + Send + Clone;

        type ReceiveIterator<F: FnMut(&Message) -> bool>: Iterator<Item = Message>;

        /// Try to receive a message non-blocking.
        fn receive<F: FnMut(&Message) -> bool>(&self, filter: F) -> Self::ReceiveIterator<F>;

        /// Call an [`AsyncProcedure`] using some [`Input`]. This function is non-blocking and
        /// returns immediately.
        fn call(
            &self,
            input: Input,
            procedure: AsyncProcedure<K, Self::Context>,
        ) -> Result<(), CallError>;
    }

    #[derive(Clone)]
    pub struct SchedulerContext {
        sender: Sender<Message>,
    }

    impl Context for SchedulerContext {
        fn send<T: IntoMessage>(&self, message: T) -> Result<(), SendError> {
            self.sender
                .send(message.into())
                .map_err(|_e| SendError::Transmission)
        }
    }

    pub struct SchedulerAsyncProcedureCall<K: OffscreenKernelEnvironment, S: Scheduler> {
        channel: (Sender<Message>, Receiver<Message>),
        buffer: RefCell<Vec<Message>>,
        scheduler: S,
        phantom_k: PhantomData<K>,
    }

    impl<K: OffscreenKernelEnvironment, S: Scheduler> SchedulerAsyncProcedureCall<K, S> {
        pub fn new(scheduler: S) -> Self {
            Self {
                channel: mpsc::channel(),
                buffer: RefCell::new(Vec::new()),
                phantom_k: PhantomData::default(),
                scheduler,
            }
        }
    }

    impl<K: OffscreenKernelEnvironment, S: Scheduler> AsyncProcedureCall<K>
        for SchedulerAsyncProcedureCall<K, S>
    {
        type Context = SchedulerContext;
        type ReceiveIterator<F: FnMut(&Message) -> bool> = IntoIter<Message>;

        fn receive<F: FnMut(&Message) -> bool>(&self, mut filter: F) -> Self::ReceiveIterator<F> {
            let mut buffer = self.buffer.borrow_mut();
            let mut ret = Vec::new();

            // FIXME tcs: Verify this!
            let mut index = 0usize;
            let mut max_len = buffer.len();
            while index < max_len {
                if filter(&buffer[index]) {
                    ret.push(buffer.swap_remove(index));
                    max_len -= 1;
                }
                index += 1;
            }

            // TODO: (optimize) Using while instead of if means that we are processing all that is
            // TODO: available this might cause frame drops.
            while let Ok(message) = self.channel.1.try_recv() {
                tracing::debug!("Data reached main thread: {message:?}");
                log::debug!("Data reached main thread: {message:?}");

                if filter(&message) {
                    ret.push(message);
                } else {
                    buffer.push(message)
                }
            }

            ret.into_iter()
        }

        fn call(
            &self,
            input: Input,
            procedure: AsyncProcedure<K, Self::Context>,
        ) -> Result<(), CallError> {
            let sender = self.channel.0.clone();

            self.scheduler
                .schedule(move || async move {
                    log::info!("Processing on thread: {:?}", std::thread::current().name());

                    procedure(input, SchedulerContext { sender }, K::create())
                        .await
                        .unwrap();
                })
                .map_err(|_e| CallError::Schedule)
        }
    }

    #[cfg(test)]
    pub mod tests {
        use crate::io::apc::{Context, IntoMessage, SendError};

        pub struct DummyContext;

        impl Context for DummyContext {
            fn send<T: IntoMessage>(&self, _message: T) -> Result<(), SendError> {
                Ok(())
            }
        }
    }