//! This module works around limitations of the Rust programming language, and provides missing
//! functionalities that we may expect the Rust programming language and its standard libraries
//! to provide.

pub mod rev_group;
pub mod zeroed_alloc;

/// Const function for min value of two usize numbers.
pub const fn min_of_usize(a: usize, b: usize) -> usize {
    if a > b {
        b
    } else {
        a
    }
}

#[rustversion::nightly]
pub use core::intrinsics::{likely, unlikely};

// likely() and unlikely() compiler hints in stable Rust
// [1]: https://github.com/rust-lang/hashbrown/blob/a41bd76de0a53838725b997c6085e024c47a0455/src/raw/mod.rs#L48-L70
// [2]: https://users.rust-lang.org/t/compiler-hint-for-unlikely-likely-for-if-branches/62102/3
#[rustversion::not(nightly)]
#[cold]
fn cold() {}

#[rustversion::not(nightly)]
pub fn likely(b: bool) -> bool {
    if !b {
        cold();
    }
    b
}
#[rustversion::not(nightly)]
pub fn unlikely(b: bool) -> bool {
    if b {
        cold();
    }
    b
}

use std::cell::UnsafeCell;
use std::mem::MaybeUninit;
use std::sync::Once;

/// InitializeOnce creates an uninitialized value that needs to be manually initialized later. InitializeOnce
/// guarantees the value is only initialized once. This type is used to allow more efficient reads.
/// Unlike the `lazy_static!` which checks whether the static is initialized
/// in every read, InitializeOnce has no extra check for reads.
pub struct InitializeOnce<T: 'static> {
    v: UnsafeCell<MaybeUninit<T>>,
    /// This is used to guarantee `init_fn` is only called once.
    once: Once,
}

impl<T> InitializeOnce<T> {
    pub const fn new() -> Self {
        InitializeOnce {
            v: UnsafeCell::new(MaybeUninit::uninit()),
            once: Once::new(),
        }
    }

    /// Initialize the value. This should be called before ever using the struct.
    /// If this method is called by multiple threads, the first thread will
    /// initialize the value, and the other threads will be blocked until the
    /// initialization is done (`Once` returns).
    pub fn initialize_once(&self, init_fn: &'static dyn Fn() -> T) {
        self.once.call_once(|| {
            unsafe { &mut *self.v.get() }.write(init_fn());
        });
        debug_assert!(self.once.is_completed());
    }

    /// Get the value. This should only be used after initialize_once()
    pub fn get_ref(&self) -> &T {
        // We only assert in debug builds.
        debug_assert!(self.once.is_completed());
        unsafe { (*self.v.get()).assume_init_ref() }
    }

    /// Get a mutable reference to the value.
    /// This is currently only used for SFTMap during plan creation (single threaded),
    /// and before the plan creation is done, the binding cannot use MMTK at all.
    ///
    /// # Safety
    /// The caller needs to make sure there is no race when mutating the value.
    #[allow(clippy::mut_from_ref)]
    pub unsafe fn get_mut(&self) -> &mut T {
        // We only assert in debug builds.
        debug_assert!(self.once.is_completed());
        unsafe { (*self.v.get()).assume_init_mut() }
    }
}

impl<T> std::ops::Deref for InitializeOnce<T> {
    type Target = T;
    fn deref(&self) -> &Self::Target {
        self.get_ref()
    }
}

unsafe impl<T> Sync for InitializeOnce<T> {}

/// This implements `std::array::from_fn` introduced in Rust 1.63.
/// We should replace this with the standard counterpart after bumping MSRV,
/// but we also need to evaluate whether it would use too much stack space (see code comments).
pub(crate) fn array_from_fn<T, const N: usize, F>(mut cb: F) -> [T; N]
where
    F: FnMut(usize) -> T,
{
    // Note on unsafety: An alternative to the unsafe implementation below is creating a fixed
    // array of `[0, 1, 2, ..., N-1]` and using the `.map(cb)` method to create the result.
    // However, the `array::map` method implemented in the standard library consumes a surprisingly
    // large amount of stack space.  For VMs that run on confined stacks, such as JikesRVM, that
    // would cause stack overflow.  Therefore we implement it manually using unsafe primitives.
    let mut result_array: MaybeUninit<[T; N]> = MaybeUninit::zeroed();
    let array_ptr = result_array.as_mut_ptr();
    for index in 0..N {
        let item = cb(index);
        unsafe {
            std::ptr::addr_of_mut!((*array_ptr)[index]).write(item);
        }
    }
    unsafe { result_array.assume_init() }
}

/// Create a formatted string that makes the best effort idenfying the current process and thread.
pub fn debug_process_thread_id() -> String {
    let pid = unsafe { libc::getpid() };
    #[cfg(target_os = "linux")]
    {
        // `gettid()` is Linux-specific.
        let tid = unsafe { libc::gettid() };
        format!("PID: {}, TID: {}", pid, tid)
    }
    #[cfg(not(target_os = "linux"))]
    {
        // TODO: When we support other platforms, use platform-specific methods to get thread
        // identifiers.
        format!("PID: {}", pid)
    }
}

#[cfg(test)]
mod initialize_once_tests {
    use super::*;

    #[test]
    fn test_threads_compete_initialize() {
        use std::sync::atomic::AtomicUsize;
        use std::sync::atomic::Ordering;
        use std::thread;

        // Create multiple threads to initialize the same `InitializeOnce` value
        const N_THREADS: usize = 1000;
        // The test value
        static I: InitializeOnce<usize> = InitializeOnce::new();
        // Count how many times the function is called
        static INITIALIZE_COUNT: AtomicUsize = AtomicUsize::new(0);
        // The function to create initial value
        fn initialize_usize() -> usize {
            INITIALIZE_COUNT.fetch_add(1, Ordering::SeqCst);
            42
        }

        let mut threads = vec![];
        for _ in 1..N_THREADS {
            threads.push(thread::spawn(|| {
                I.initialize_once(&initialize_usize);
                // Every thread should see the value correctly initialized.
                assert_eq!(*I, 42);
            }));
        }
        threads.into_iter().for_each(|t| t.join().unwrap());

        // The initialize_usize should only be called once
        assert_eq!(INITIALIZE_COUNT.load(Ordering::SeqCst), 1);
    }
}