miden_processor/host/

handlers.rs

use alloc::{
    boxed::Box,
    collections::{BTreeMap, btree_map::Entry},
    sync::Arc,
    vec::Vec,
};
use core::{error::Error, fmt, fmt::Debug};

use miden_core::events::{EventId, EventName, SystemEvent};

use crate::{ExecutionError, ProcessorState, advice::AdviceMutation};

// EVENT HANDLER TRAIT
// ================================================================================================

/// An [`EventHandler`] defines a function that that can be called from the processor which can
/// read the VM state and modify the state of the advice provider.
///
/// A struct implementing this trait can access its own state, but any output it produces must
/// be stored in the process's advice provider.
pub trait EventHandler: Send + Sync + 'static {
    /// Handles the event when triggered.
    fn on_event(&self, process: &ProcessorState) -> Result<Vec<AdviceMutation>, EventError>;
}

/// Default implementation for both free functions and closures with signature
/// `fn(&ProcessorState) -> Result<(), HandlerError>`
impl<F> EventHandler for F
where
    F: for<'a> Fn(&'a ProcessorState) -> Result<Vec<AdviceMutation>, EventError>
        + Send
        + Sync
        + 'static,
{
    fn on_event(&self, process: &ProcessorState) -> Result<Vec<AdviceMutation>, EventError> {
        self(process)
    }
}

/// A handler which ignores the process state and leaves the `AdviceProvider` unchanged.
pub struct NoopEventHandler;

impl EventHandler for NoopEventHandler {
    fn on_event(&self, _process: &ProcessorState) -> Result<Vec<AdviceMutation>, EventError> {
        Ok(Vec::new())
    }
}

// EVENT ERROR
// ================================================================================================

/// A generic [`Error`] wrapper allowing handlers to return errors to the Host caller.
///
/// Error handlers can define their own [`Error`] type which can be seamlessly converted
/// into this type since it is a [`Box`].
///
/// # Example
///
/// ```rust, ignore
/// pub struct MyError{ /* ... */ };
///
/// fn try_something() -> Result<(), MyError> { /* ... */ }
///
/// fn my_handler(process: &mut ProcessorState) -> Result<(), HandlerError> {
///     // ...
///     try_something()?;
///     // ...
///     Ok(())
/// }
/// ```
pub type EventError = Box<dyn Error + Send + Sync + 'static>;

// EVENT HANDLER REGISTRY
// ================================================================================================

/// Registry for maintaining event handlers.
///
/// # Example
///
/// ```rust, ignore
/// impl Host for MyHost {
///     fn on_event(
///         &mut self,
///         process: &mut ProcessorState,
///         event_id: u32,
///     ) -> Result<(), EventError> {
///         if self
///             .event_handlers
///             .handle_event(event_id, process)
///             .map_err(|err| EventError::HandlerError { id: event_id, err })?
///         {
///             // the event was handled by the registered event handlers; just return
///             return Ok(());
///         }
///
///         // implement custom event handling
///
///         Err(EventError::UnhandledEvent { id: event_id })
///     }
/// }
/// ```
#[derive(Default)]
pub struct EventHandlerRegistry {
    handlers: BTreeMap<EventId, (EventName, Arc<dyn EventHandler>)>,
}

impl EventHandlerRegistry {
    pub fn new() -> Self {
        Self { handlers: BTreeMap::new() }
    }

    /// Registers an [`EventHandler`] with a given event name.
    ///
    /// The [`EventId`] is computed from the event name during registration.
    ///
    /// # Errors
    /// Returns an error if:
    /// - The event is a reserved system event
    /// - A handler with the same event ID is already registered
    pub fn register(
        &mut self,
        event: EventName,
        handler: Arc<dyn EventHandler>,
    ) -> Result<(), ExecutionError> {
        // Check if the event is a reserved system event
        if SystemEvent::from_name(event.as_str()).is_some() {
            return Err(crate::errors::HostError::ReservedEventNamespace { event }.into());
        }

        // Compute EventId from the event name
        let id = event.to_event_id();
        match self.handlers.entry(id) {
            Entry::Vacant(e) => e.insert((event, handler)),
            Entry::Occupied(_) => {
                return Err(crate::errors::HostError::DuplicateEventHandler { event }.into());
            },
        };
        Ok(())
    }

    /// Unregisters a handler with the given identifier, returning a flag whether a handler with
    /// that identifier was previously registered.
    pub fn unregister(&mut self, id: EventId) -> bool {
        self.handlers.remove(&id).is_some()
    }

    /// Returns the [`EventName`] registered for `id`, if any.
    pub fn resolve_event(&self, id: EventId) -> Option<&EventName> {
        self.handlers.get(&id).map(|(event, _)| event)
    }

    /// Handles the event if the registry contains a handler with the same identifier.
    ///
    /// Returns an `Option<_>` indicating whether the event was handled. Returns `None` if the
    /// event was not handled, `Some(mutations)` if it was handled successfully, and propagates
    /// handler errors to the caller.
    pub fn handle_event(
        &self,
        id: EventId,
        process: &ProcessorState,
    ) -> Result<Option<Vec<AdviceMutation>>, EventError> {
        if let Some((_event_name, handler)) = self.handlers.get(&id) {
            let mutations = handler.on_event(process)?;
            return Ok(Some(mutations));
        }

        Ok(None)
    }
}

impl Debug for EventHandlerRegistry {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let events: Vec<_> = self.handlers.values().map(|(event, _)| event).collect();
        f.debug_struct("EventHandlerRegistry").field("handlers", &events).finish()
    }
}