extern crate core as std;

use std::fmt;
use std::marker::PhantomData;
use std::mem::ManuallyDrop;
use std::ops::{Deref, DerefMut};
use std::ptr;

/// Controls in which cases the associated code should be run
pub trait Strategy {
    /// Return `true` if the guard’s associated code should run
    /// (in the context where this method is called).
    fn should_run() -> bool;
}

/// Always run on scope exit.
///
/// “Always” run: on regular exit from a scope or on unwinding from a panic.
/// Can not run on abort, process exit, and other catastrophic events where
/// destructors don’t run.
#[derive(Debug)]
pub enum Always {}

impl Strategy for Always {
    #[inline(always)]
    fn should_run() -> bool {
        true
    }
}

/// Macro to create a `ScopeGuard` (always run).
///
/// The macro takes statements, which are the body of a closure
/// that will run when the scope is exited.
#[macro_export]
macro_rules! defer {
    ($($t:tt)*) => {
        let _guard = $crate::external::scopeguard::guard((), |()| { $($t)* });
    };
}

pub(crate) use defer;

/// `ScopeGuard` is a scope guard that may own a protected value.
///
/// If you place a guard in a local variable, the closure can
/// run regardless how you leave the scope — through regular return or panic
/// (except if panic or other code aborts; so as long as destructors run).
/// It is run only once.
///
/// The `S` parameter for [`Strategy`](trait.Strategy.html) determines if
/// the closure actually runs.
///
/// The guard's closure will be called with the held value in the destructor.
///
/// The `ScopeGuard` implements `Deref` so that you can access the inner value.
pub struct ScopeGuard<T, F, S = Always>
where
    F: FnOnce(T),
    S: Strategy,
{
    value: ManuallyDrop<T>,
    dropfn: ManuallyDrop<F>,
    // fn(S) -> S is used, so that the S is not taken into account for auto traits.
    strategy: PhantomData<fn(S) -> S>,
}

impl<T, F, S> ScopeGuard<T, F, S>
where
    F: FnOnce(T),
    S: Strategy,
{
    /// Create a `ScopeGuard` that owns `v` (accessible through deref) and calls
    /// `dropfn` when its destructor runs.
    ///
    /// The `Strategy` decides whether the scope guard's closure should run.
    #[inline]
    #[must_use]
    pub fn with_strategy(v: T, dropfn: F) -> ScopeGuard<T, F, S> {
        ScopeGuard {
            value: ManuallyDrop::new(v),
            dropfn: ManuallyDrop::new(dropfn),
            strategy: PhantomData,
        }
    }

    /// “Defuse” the guard and extract the value without calling the closure.
    ///
    /// ```
    /// extern crate scopeguard;
    ///
    /// use scopeguard::{guard, ScopeGuard};
    ///
    /// fn conditional() -> bool { true }
    ///
    /// fn main() {
    ///     let mut guard = guard(Vec::new(), |mut v| v.clear());
    ///     guard.push(1);
    ///
    ///     if conditional() {
    ///         // a condition maybe makes us decide to
    ///         // “defuse” the guard and get back its inner parts
    ///         let value = ScopeGuard::into_inner(guard);
    ///     } else {
    ///         // guard still exists in this branch
    ///     }
    /// }
    /// ```
    #[inline]
    #[allow(unused)]
    pub fn into_inner(guard: Self) -> T {
        // Cannot move out of `Drop`-implementing types,
        // so `ptr::read` the value and forget the guard.
        let mut guard = ManuallyDrop::new(guard);
        unsafe {
            let value = ptr::read(&*guard.value);
            // Drop the closure after `value` has been read, so that if the
            // closure's `drop` function panics, unwinding still tries to drop
            // `value`.
            ManuallyDrop::drop(&mut guard.dropfn);
            value
        }
    }
}

/// Create a new `ScopeGuard` owning `v` and with deferred closure `dropfn`.
#[inline]
#[must_use]
pub fn guard<T, F>(v: T, dropfn: F) -> ScopeGuard<T, F, Always>
where
    F: FnOnce(T),
{
    ScopeGuard::with_strategy(v, dropfn)
}

// ScopeGuard can be Sync even if F isn't because the closure is
// not accessible from references.
// The guard does not store any instance of S, so it is also irrelevant.
unsafe impl<T, F, S> Sync for ScopeGuard<T, F, S>
where
    T: Sync,
    F: FnOnce(T),
    S: Strategy,
{
}

impl<T, F, S> Deref for ScopeGuard<T, F, S>
where
    F: FnOnce(T),
    S: Strategy,
{
    type Target = T;

    fn deref(&self) -> &T {
        &*self.value
    }
}

impl<T, F, S> DerefMut for ScopeGuard<T, F, S>
where
    F: FnOnce(T),
    S: Strategy,
{
    fn deref_mut(&mut self) -> &mut T {
        &mut *self.value
    }
}

impl<T, F, S> Drop for ScopeGuard<T, F, S>
where
    F: FnOnce(T),
    S: Strategy,
{
    fn drop(&mut self) {
        // This is OK because the fields are `ManuallyDrop`s
        // which will not be dropped by the compiler.
        let (value, dropfn) = unsafe { (ptr::read(&*self.value), ptr::read(&*self.dropfn)) };
        if S::should_run() {
            dropfn(value);
        }
    }
}

impl<T, F, S> fmt::Debug for ScopeGuard<T, F, S>
where
    T: fmt::Debug,
    F: FnOnce(T),
    S: Strategy,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct(stringify!(ScopeGuard))
            .field("value", &*self.value)
            .finish()
    }
}