qubit-cas 0.4.0

Typed compare-and-swap executor with retry-aware conflict handling
Documentation 
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/*******************************************************************************
 *
 *    Copyright (c) 2025 - 2026 Haixing Hu.
 *
 *    SPDX-License-Identifier: Apache-2.0
 *
 *    Licensed under the Apache License, Version 2.0.
 *
 ******************************************************************************/
//! CAS decision values returned by user operations.

use std::sync::Arc;

/// Decision returned by one CAS operation attempt.
///
/// A CAS operation reads a consistent snapshot of `T` and returns one of four
/// outcomes:
///
/// - [`Update`](Self::Update): install a new state by compare-and-swap.
/// - [`Finish`](Self::Finish): treat the attempt as successful without writing.
/// - [`Retry`](Self::Retry): report a retryable business failure.
/// - [`Abort`](Self::Abort): report a terminal business failure.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum CasDecision<T, R, E> {
    /// Writes a new state and returns a business output.
    Update {
        /// Replacement state to install when compare-and-swap succeeds.
        next: Arc<T>,
        /// Business output returned to the caller on success.
        output: R,
    },

    /// Completes successfully without writing a new state.
    Finish {
        /// Business output returned to the caller.
        output: R,
    },

    /// Requests another CAS attempt.
    Retry(E),

    /// Aborts the CAS flow immediately.
    Abort(E),
}

impl<T, R, E> CasDecision<T, R, E> {
    /// Creates an update decision from an owned value.
    ///
    /// # Parameters
    /// - `next`: Replacement state to wrap in [`Arc`] and install.
    /// - `output`: Business output returned on success.
    ///
    /// # Returns
    /// A [`CasDecision::Update`] value.
    #[inline]
    pub fn update(next: T, output: R) -> Self {
        Self::Update {
            next: Arc::new(next),
            output,
        }
    }

    /// Creates an update decision from an `Arc<T>`.
    ///
    /// # Parameters
    /// - `next`: Replacement state to install.
    /// - `output`: Business output returned on success.
    ///
    /// # Returns
    /// A [`CasDecision::Update`] value.
    #[inline]
    pub fn update_arc(next: Arc<T>, output: R) -> Self {
        Self::Update { next, output }
    }

    /// Creates a finish decision.
    ///
    /// # Parameters
    /// - `output`: Business output returned to the caller.
    ///
    /// # Returns
    /// A [`CasDecision::Finish`] value.
    #[inline]
    pub fn finish(output: R) -> Self {
        Self::Finish { output }
    }

    /// Creates a retry decision.
    ///
    /// # Parameters
    /// - `error`: Retryable business failure.
    ///
    /// # Returns
    /// A [`CasDecision::Retry`] value.
    #[inline]
    pub fn retry(error: E) -> Self {
        Self::Retry(error)
    }

    /// Creates an abort decision.
    ///
    /// # Parameters
    /// - `error`: Terminal business failure.
    ///
    /// # Returns
    /// A [`CasDecision::Abort`] value.
    #[inline]
    pub fn abort(error: E) -> Self {
        Self::Abort(error)
    }
}