//! Synchronization primitives for lazy evaluation.

//!

//! Implementation adapted from the `SyncLazy` type of the standard library. See:

//! <https://doc.rust-lang.org/std/lazy/struct.SyncLazy.html>

use crate::{once::Once, RelaxStrategy, Spin};

use core::{cell::Cell, fmt, ops::Deref};

/// A value which is initialized on the first access.

///

/// This type is a thread-safe `Lazy`, and can be used in statics.

///

/// # Examples

///

/// ```

/// use std::collections::HashMap;

/// use spin::Lazy;

///

/// static HASHMAP: Lazy<HashMap<i32, String>> = Lazy::new(|| {

///     println!("initializing");

///     let mut m = HashMap::new();

///     m.insert(13, "Spica".to_string());

///     m.insert(74, "Hoyten".to_string());

///     m

/// });

///

/// fn main() {

///     println!("ready");

///     std::thread::spawn(|| {

///         println!("{:?}", HASHMAP.get(&13));

///     }).join().unwrap();

///     println!("{:?}", HASHMAP.get(&74));

///

///     // Prints:

///     //   ready

///     //   initializing

///     //   Some("Spica")

///     //   Some("Hoyten")

/// }

/// ```

pub struct Lazy<T, F = fn() -> T, R = Spin> {

    cell: Once<T, R>,

    init: Cell<Option<F>>,

}

impl<T: fmt::Debug, F, R> fmt::Debug for Lazy<T, F, R> {

    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

        f.debug_struct("Lazy")

            .field("cell", &self.cell)

            .field("init", &"..")

            .finish()

    }

}

// We never create a `&F` from a `&Lazy<T, F>` so it is fine

// to not impl `Sync` for `F`

// we do create a `&mut Option<F>` in `force`, but this is

// properly synchronized, so it only happens once

// so it also does not contribute to this impl.

unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}

// auto-derived `Send` impl is OK.

impl<T, F, R> Lazy<T, F, R> {

    /// Creates a new lazy value with the given initializing

    /// function.

    pub const fn new(f: F) -> Self {

        Self {

            cell: Once::new(),

            init: Cell::new(Some(f)),

        }

    }

    /// Retrieves a mutable pointer to the inner data.

    ///

    /// This is especially useful when interfacing with low level code or FFI where the caller

    /// explicitly knows that it has exclusive access to the inner data. Note that reading from

    /// this pointer is UB until initialized or directly written to.

    pub fn as_mut_ptr(&self) -> *mut T {

        self.cell.as_mut_ptr()

    }

}

impl<T, F: FnOnce() -> T, R: RelaxStrategy> Lazy<T, F, R> {

    /// Forces the evaluation of this lazy value and

    /// returns a reference to result. This is equivalent

    /// to the `Deref` impl, but is explicit.

    ///

    /// # Examples

    ///

    /// ```

    /// use spin::Lazy;

    ///

    /// let lazy = Lazy::new(|| 92);

    ///

    /// assert_eq!(Lazy::force(&lazy), &92);

    /// assert_eq!(&*lazy, &92);

    /// ```

    pub fn force(this: &Self) -> &T {

        this.cell.call_once(|| match this.init.take() {

            Some(f) => f(),

            None => panic!("Lazy instance has previously been poisoned"),

        })

    }

}

impl<T, F: FnOnce() -> T, R: RelaxStrategy> Deref for Lazy<T, F, R> {

    type Target = T;

    fn deref(&self) -> &T {

        Self::force(self)

    }

}

impl<T: Default, R> Default for Lazy<T, fn() -> T, R> {

    /// Creates a new lazy value using `Default` as the initializing function.

    fn default() -> Self {

        Self::new(T::default)

    }

}