sim-lib-control 0.1.2

use sim_kernel::Ref;

/// The next observable transition from a resumable coroutine frame.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum CoroutineFrameStep<T = Ref> {
    /// The frame produced a value from its producer side.
    Produced(T),
    /// The frame consumed a value on its consumer side.
    Consumed(T),
    /// Both sides are exhausted.
    Complete,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum CoroutineFrameTurn {
    Produce,
    Consume,
}

/// A resumable producer/consumer frame for control libraries.
///
/// The frame alternates between produced and consumed values while either side
/// has work remaining. It carries no language-specific status names, so codec
/// and language layers can map the generic steps into their own surfaces.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct CoroutineFrame<T = Ref> {
    produced: Vec<T>,
    consumed: Vec<T>,
    produced_index: usize,
    consumed_index: usize,
    turn: CoroutineFrameTurn,
}

impl<T> CoroutineFrame<T> {
    /// Builds a frame from producer-side and consumer-side values.
    pub fn new(produced: Vec<T>, consumed: Vec<T>) -> Self {
        Self {
            produced,
            consumed,
            produced_index: 0,
            consumed_index: 0,
            turn: CoroutineFrameTurn::Produce,
        }
    }

    /// Resumes the frame, returning the next producer or consumer transition.
    pub fn resume(&mut self) -> CoroutineFrameStep<T>
    where
        T: Clone,
    {
        loop {
            match self.turn {
                CoroutineFrameTurn::Produce => {
                    self.turn = CoroutineFrameTurn::Consume;
                    if let Some(value) = self.produced.get(self.produced_index).cloned() {
                        self.produced_index += 1;
                        return CoroutineFrameStep::Produced(value);
                    }
                }
                CoroutineFrameTurn::Consume => {
                    self.turn = CoroutineFrameTurn::Produce;
                    if let Some(value) = self.consumed.get(self.consumed_index).cloned() {
                        self.consumed_index += 1;
                        return CoroutineFrameStep::Consumed(value);
                    }
                }
            }

            if self.is_complete() {
                return CoroutineFrameStep::Complete;
            }
        }
    }

    /// Returns whether both producer and consumer sides are exhausted.
    pub fn is_complete(&self) -> bool {
        self.produced_index >= self.produced.len() && self.consumed_index >= self.consumed.len()
    }
}

/// Identifies which of a coroutine's two cooperating lanes yielded a value.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum CoroutineLane {
    /// The first lane.
    First,
    /// The second lane.
    Second,
}

/// Outcome of resuming a [`Coroutine`]: a yielded value, or exhaustion.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum CoroutineStep {
    /// A lane yielded a value and control returned to the driver.
    Yielded {
        /// The lane that produced this value.
        lane: CoroutineLane,
        /// The yielded value.
        value: Ref,
    },
    /// Both lanes are drained; the coroutine has nothing left to yield.
    Exhausted,
}

/// Two cooperating value streams that yield by alternating between lanes.
///
/// Models symmetric coroutine control: each [`Coroutine::resume`] hands control
/// to the next lane, falling through to the other when one is drained.
///
/// # Examples
///
/// ```
/// use sim_kernel::{Ref, Symbol};
/// use sim_lib_control::{Coroutine, CoroutineLane, CoroutineStep};
///
/// let a = Ref::Symbol(Symbol::new("a"));
/// let b = Ref::Symbol(Symbol::new("b"));
/// let mut co = Coroutine::alternating(vec![a.clone()], vec![b.clone()]);
/// assert_eq!(
///     co.resume(),
///     CoroutineStep::Yielded { lane: CoroutineLane::First, value: a }
/// );
/// assert_eq!(
///     co.resume(),
///     CoroutineStep::Yielded { lane: CoroutineLane::Second, value: b }
/// );
/// assert_eq!(co.resume(), CoroutineStep::Exhausted);
/// ```
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Coroutine {
    first: Vec<Ref>,
    second: Vec<Ref>,
    first_index: usize,
    second_index: usize,
    next_lane: CoroutineLane,
}

impl Coroutine {
    /// Builds a coroutine that alternates yields between the `first` and
    /// `second` lanes, starting with the first.
    pub fn alternating(first: Vec<Ref>, second: Vec<Ref>) -> Self {
        Self {
            first,
            second,
            first_index: 0,
            second_index: 0,
            next_lane: CoroutineLane::First,
        }
    }

    /// Resumes the coroutine, yielding the next value from the active lane (or
    /// the other lane if the active one is drained), or
    /// [`CoroutineStep::Exhausted`] when both are empty.
    pub fn resume(&mut self) -> CoroutineStep {
        let step = match self.next_lane {
            CoroutineLane::First => self.resume_first().or_else(|| self.resume_second()),
            CoroutineLane::Second => self.resume_second().or_else(|| self.resume_first()),
        };
        step.unwrap_or(CoroutineStep::Exhausted)
    }

    fn resume_first(&mut self) -> Option<CoroutineStep> {
        let value = self.first.get(self.first_index).cloned()?;
        self.first_index += 1;
        self.next_lane = CoroutineLane::Second;
        Some(CoroutineStep::Yielded {
            lane: CoroutineLane::First,
            value,
        })
    }

    fn resume_second(&mut self) -> Option<CoroutineStep> {
        let value = self.second.get(self.second_index).cloned()?;
        self.second_index += 1;
        self.next_lane = CoroutineLane::First;
        Some(CoroutineStep::Yielded {
            lane: CoroutineLane::Second,
            value,
        })
    }
}