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//! This module is full of hackery and dark magic.
//! Either spend a day fixing it and quietly submit a PR or don't mention it to anybody.

use core::cell::UnsafeCell;
use core::{mem, ptr};

#[inline(always)]
pub const fn ptr_size_bits() -> usize {
    mem::size_of::<usize>() * 8
}

#[inline(always)]
pub fn map_in_place_2<T, U, F: FnOnce(U, T) -> T>((k, v): (U, &mut T), f: F) {
    unsafe {
        // # Safety
        //
        // If the closure panics, we must abort otherwise we could double drop `T`
        let _promote_panic_to_abort = AbortOnPanic;

        ptr::write(v, f(k, ptr::read(v)));
    }
}

/// # Safety
///
/// Requires that you ensure the reference does not become invalid.
/// The object has to outlive the reference.
#[inline(always)]
pub unsafe fn change_lifetime_const<'a, 'b, T>(x: &'a T) -> &'b T {
    &*(x as *const T)
}

/// # Safety
///
/// Requires that you ensure the reference does not become invalid.
/// The object has to outlive the reference.
#[inline(always)]
pub unsafe fn change_lifetime_mut<'a, 'b, T>(x: &'a mut T) -> &'b mut T {
    &mut *(x as *mut T)
}

/// A simple wrapper around `T`
///
/// This is to prevent UB when using `HashMap::get_key_value`, because
/// `HashMap` doesn't expose an api to get the key and value, where
/// the value is a `&mut T`.
///
/// See [#10](https://github.com/xacrimon/dashmap/issues/10) for details
///
/// This type is meant to be an implementation detail, but must be exposed due to the `Dashmap::shards`
#[repr(transparent)]
pub struct SharedValue<T> {
    value: UnsafeCell<T>,
}

impl<T: Clone> Clone for SharedValue<T> {
    fn clone(&self) -> Self {
        let inner = self.get().clone();
        Self {
            value: UnsafeCell::new(inner),
        }
    }
}

unsafe impl<T: Send> Send for SharedValue<T> {}
unsafe impl<T: Sync> Sync for SharedValue<T> {}

impl<T> SharedValue<T> {
    /// Create a new `SharedValue<T>`
    #[inline]
    pub const fn new(value: T) -> Self {
        Self {
            value: UnsafeCell::new(value),
        }
    }

    /// Get a shared reference to `T`
    #[inline]
    pub fn get(&self) -> &T {
        unsafe { &*self.value.get() }
    }

    /// Get an unique reference to `T`
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        unsafe { &mut *self.value.get() }
    }

    /// Unwraps the value
    #[inline]
    pub fn into_inner(self) -> T {
        self.value.into_inner()
    }

    /// Get a mutable raw pointer to the underlying value
    #[inline]
    pub(crate) fn as_ptr(&self) -> *mut T {
        self.value.get()
    }
}

struct AbortOnPanic;

impl Drop for AbortOnPanic {
    #[inline]
    fn drop(&mut self) {
        cfg_if::cfg_if! {
            if #[cfg(feature = "no_std")] {
                // Note: This is hard, as core/no_std has no concept of threads or knowledge of panicking.
                // An alternative would be to do this:
                //
                // ```rust
                // // Elsewhere in the library/host binary
                // use core::sync::atomic::{AtomicBool, Ordering};
                //
                // static UNWINDING: AtomicBool = AtomicBool::new(false);
                //
                // #[panic_handler]
                // fn panic(info: &PanicInfo) -> ! {
                //      UNWINDING.store(true, Ordering::Relaxed);
                //
                //      unsafe {
                //          core::intrinsics::abort();
                //      }
                // }
                //
                // // In AbortOnPanic::drop
                // if UNWINDING.load(Ordering::Relaxed) {
                //      unsafe {
                //          core::intrinsics::abort();
                //      }
                // }
                // ```
                //
                // Now, this isn't an ideal solution for multiple reasons, as it uses intrinsics which require a feature
                // and can be overwritten by the user without them even knowing. That being said, *most* users of no_std
                // do tend to use panic = "abort", which solves this problem for us by aborting on panics.
            } else {
                if std::thread::panicking() {
                    std::process::abort()
                }
            }
        }
    }
}