//! Strategies that determine the behaviour of locks when encountering contention.

/// A trait implemented by spinning relax strategies.
pub trait RelaxStrategy {
    /// Perform the relaxing operation during a period of contention.
    fn relax();
}

/// A strategy that rapidly spins while informing the CPU that it should power down non-essential components via
/// [`core::hint::spin_loop`].
///
/// Note that spinning is a 'dumb' strategy and most schedulers cannot correctly differentiate it from useful work,
/// thereby misallocating even more CPU time to the spinning process. This is known as
/// ['priority inversion'](https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html).
///
/// If you see signs that priority inversion is occurring, consider switching to [`Yield`] or, even better, not using a
/// spinlock at all and opting for a proper scheduler-aware lock. Remember also that different targets, operating
/// systems, schedulers, and even the same scheduler with different workloads will exhibit different behaviour. Just
/// because priority inversion isn't occurring in your tests does not mean that it will not occur. Use a scheduler-
/// aware lock if at all possible.
pub struct Spin;

impl RelaxStrategy for Spin {
    #[inline(always)]
    fn relax() {
        // Use the deprecated spin_loop_hint() to ensure that we don't get
        // a higher MSRV than we need to.
        #[allow(deprecated)]
        core::sync::atomic::spin_loop_hint();
    }
}

/// A strategy that yields the current time slice to the scheduler in favour of other threads or processes.
///
/// This is generally used as a strategy for minimising power consumption and priority inversion on targets that have a
/// standard library available. Note that such targets have scheduler-integrated concurrency primitives available, and
/// you should generally use these instead, except in rare circumstances.
#[cfg(feature = "std")]
pub struct Yield;

#[cfg(feature = "std")]
impl RelaxStrategy for Yield {
    #[inline(always)]
    fn relax() {
        std::thread::yield_now();
    }
}

/// A strategy that rapidly spins, without telling the CPU to do any powering down.
///
/// You almost certainly do not want to use this. Use [`Spin`] instead. It exists for completeness and for targets
/// that, for some reason, miscompile or do not support spin hint intrinsics despite attempting to generate code for
/// them (i.e: this is a workaround for possible compiler bugs).
pub struct Loop;

impl RelaxStrategy for Loop {
    #[inline(always)]
    fn relax() {}
}
