qubit-executor 0.3.2

/*******************************************************************************
 *
 *    Copyright (c) 2025 - 2026 Haixing Hu.
 *
 *    SPDX-License-Identifier: Apache-2.0
 *
 *    Licensed under the Apache License, Version 2.0.
 *
 ******************************************************************************/
use oneshot::{
    Receiver,
    TryRecvError,
};
use std::{
    future::IntoFuture,
    sync::Arc,
};

use super::{
    TaskExecutionError,
    TaskResult,
    task_handle_future::TaskHandleFuture,
    task_result_handle::TaskResultHandle,
    task_state::TaskState,
    try_get::TryGet,
};
use crate::hook::TaskId;

/// Lightweight result handle for a submitted callable task.
///
/// `TaskHandle` owns the receiving endpoint for exactly one task result. It can
/// block through [`Self::get`], poll non-blockingly through [`Self::try_get`],
/// or be awaited by value.
pub struct TaskHandle<R, E> {
    /// Shared task state used for status and cancellation.
    pub(crate) state: Arc<TaskState<R, E>>,
    /// One-shot receiver for the final task result.
    receiver: Receiver<TaskResult<R, E>>,
}

impl<R, E> TaskHandle<R, E> {
    /// Creates a task handle from a one-shot result receiver.
    ///
    /// # Parameters
    ///
    /// * `receiver` - Receiver that yields the final task result.
    ///
    /// # Returns
    ///
    /// A task result handle.
    #[inline]
    pub(crate) const fn new(
        state: Arc<TaskState<R, E>>,
        receiver: Receiver<TaskResult<R, E>>,
    ) -> Self {
        Self { state, receiver }
    }

    /// Returns the identifier assigned to this task.
    ///
    /// # Returns
    ///
    /// The task id stored in the shared task state.
    #[inline]
    pub fn task_id(&self) -> TaskId {
        self.state.task_id
    }

    /// Marks the task accepted and emits the accepted hook once.
    #[inline]
    pub(crate) fn accept(&self) {
        let _accepted_now = self.state.accept();
    }

    /// Waits for the task to finish and returns its final result.
    ///
    /// This method blocks the current thread until a result is available.
    ///
    /// # Returns
    ///
    /// `Ok(R)` if the task succeeds. If the accepted task returns `Err(E)`,
    /// panics, is cancelled, or loses its completion endpoint before producing
    /// a value, the corresponding [`crate::TaskExecutionError`] is returned.
    #[inline]
    pub fn get(self) -> TaskResult<R, E> {
        self.receiver
            .recv()
            .unwrap_or(Err(TaskExecutionError::Dropped))
    }

    /// Attempts to retrieve the final result without blocking.
    ///
    /// # Returns
    ///
    /// [`TryGet::Ready`] with the final result when available, otherwise
    /// [`TryGet::Pending`] containing this handle.
    #[inline]
    pub fn try_get(self) -> TryGet<Self, R, E> {
        let Self { state, receiver } = self;
        match receiver.try_recv() {
            Ok(result) => TryGet::Ready(result),
            Err(TryRecvError::Empty) => TryGet::Pending(Self { state, receiver }),
            Err(TryRecvError::Disconnected) => TryGet::Ready(Err(TaskExecutionError::Dropped)),
        }
    }

    /// Returns whether the task has installed a terminal state.
    ///
    /// # Returns
    ///
    /// `true` after the task succeeds, fails, panics, is cancelled, or loses
    /// its completion endpoint. The final result send may still be racing with
    /// this status observation.
    #[inline]
    pub fn is_done(&self) -> bool {
        self.state.status().is_done()
    }
}

impl<R, E> TaskResultHandle<R, E> for TaskHandle<R, E>
where
    R: Send,
    E: Send,
{
    /// Returns whether the tracked task state is terminal.
    #[inline]
    fn is_done(&self) -> bool {
        Self::is_done(self)
    }

    /// Blocks until the result channel yields a task result.
    #[inline]
    fn get(self) -> TaskResult<R, E> {
        Self::get(self)
    }

    /// Attempts to read the result channel without blocking.
    #[inline]
    fn try_get(self) -> TryGet<Self, R, E> {
        Self::try_get(self)
    }
}

impl<R, E> IntoFuture for TaskHandle<R, E> {
    type Output = TaskResult<R, E>;
    type IntoFuture = TaskHandleFuture<R, E>;

    /// Converts this handle into a future resolving to the task result.
    #[inline]
    fn into_future(self) -> Self::IntoFuture {
        TaskHandleFuture::new(IntoFuture::into_future(self.receiver))
    }
}