//! Completion Queue

use std::fmt;
#[cfg(feature = "unstable")]
use std::mem::MaybeUninit;
use std::sync::atomic;

use crate::sys;
use crate::util::{unsync_load, Mmap};

pub(crate) struct Inner {
    head: *const atomic::AtomicU32,
    tail: *const atomic::AtomicU32,
    ring_mask: u32,
    ring_entries: u32,

    overflow: *const atomic::AtomicU32,

    cqes: *const sys::io_uring_cqe,

    #[allow(dead_code)]
    flags: *const atomic::AtomicU32,
}

/// An io_uring instance's completion queue. This stores all the I/O operations that have completed.
pub struct CompletionQueue<'a> {
    head: u32,
    tail: u32,
    queue: &'a Inner,
}

/// An entry in the completion queue, representing a complete I/O operation.
#[repr(transparent)]
#[derive(Clone)]
pub struct Entry(pub(crate) sys::io_uring_cqe);

impl Inner {
    #[rustfmt::skip]
    pub(crate) unsafe fn new(cq_mmap: &Mmap, p: &sys::io_uring_params) -> Self {
        let head         = cq_mmap.offset(p.cq_off.head         ) as *const atomic::AtomicU32;
        let tail         = cq_mmap.offset(p.cq_off.tail         ) as *const atomic::AtomicU32;
        let ring_mask    = cq_mmap.offset(p.cq_off.ring_mask    ).cast::<u32>().read();
        let ring_entries = cq_mmap.offset(p.cq_off.ring_entries ).cast::<u32>().read();
        let overflow     = cq_mmap.offset(p.cq_off.overflow     ) as *const atomic::AtomicU32;
        let cqes         = cq_mmap.offset(p.cq_off.cqes         ) as *const sys::io_uring_cqe;
        let flags        = cq_mmap.offset(p.cq_off.flags        ) as *const atomic::AtomicU32;

        Self {
            head,
            tail,
            ring_mask,
            ring_entries,
            overflow,
            cqes,
            flags,
        }
    }

    #[inline]
    pub(crate) unsafe fn borrow_shared(&self) -> CompletionQueue<'_> {
        CompletionQueue {
            head: unsync_load(self.head),
            tail: (*self.tail).load(atomic::Ordering::Acquire),
            queue: self,
        }
    }

    #[inline]
    pub(crate) fn borrow(&mut self) -> CompletionQueue<'_> {
        unsafe { self.borrow_shared() }
    }
}

impl CompletionQueue<'_> {
    /// Synchronize this type with the real completion queue.
    ///
    /// This will flush any entries consumed in this iterator and will make available new entries
    /// in the queue if the kernel has produced some entries in the meantime.
    #[inline]
    pub fn sync(&mut self) {
        unsafe {
            (*self.queue.head).store(self.head, atomic::Ordering::Release);
            self.tail = (*self.queue.tail).load(atomic::Ordering::Acquire);
        }
    }

    /// If queue is full and [`is_feature_nodrop`](crate::Parameters::is_feature_nodrop) is not set,
    /// new events may be dropped. This records the number of dropped events.
    pub fn overflow(&self) -> u32 {
        unsafe { (*self.queue.overflow).load(atomic::Ordering::Acquire) }
    }

    /// Whether eventfd notifications are disabled when a request is completed and queued to the CQ
    /// ring. This library currently does not provide a way to set it, so this will always be
    /// `false`.
    ///
    /// Requires the `unstable` feature.
    #[cfg(feature = "unstable")]
    pub fn eventfd_disabled(&self) -> bool {
        unsafe {
            (*self.queue.flags).load(atomic::Ordering::Acquire) & sys::IORING_CQ_EVENTFD_DISABLED
                != 0
        }
    }

    /// Get the total number of entries in the completion queue ring buffer.
    #[inline]
    pub fn capacity(&self) -> usize {
        self.queue.ring_entries as usize
    }

    /// Returns `true` if there are no completion queue events to be processed.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns `true` if the completion queue is at maximum capacity. If
    /// [`is_feature_nodrop`](crate::Parameters::is_feature_nodrop) is not set, this will cause any
    /// new completion queue events to be dropped by the kernel.
    #[inline]
    pub fn is_full(&self) -> bool {
        self.len() == self.capacity()
    }

    #[cfg(feature = "unstable")]
    #[inline]
    pub fn fill<'a>(&mut self, entries: &'a mut [MaybeUninit<Entry>]) -> &'a mut [Entry] {
        let len = std::cmp::min(self.len(), entries.len());

        for entry in &mut entries[..len] {
            *entry = MaybeUninit::new(Entry(unsafe {
                *self
                    .queue
                    .cqes
                    .add((self.head & self.queue.ring_mask) as usize)
            }));
            self.head = self.head.wrapping_add(1);
        }

        unsafe { std::slice::from_raw_parts_mut(entries as *mut _ as *mut Entry, len) }
    }
}

impl Drop for CompletionQueue<'_> {
    #[inline]
    fn drop(&mut self) {
        unsafe { &*self.queue.head }.store(self.head, atomic::Ordering::Release);
    }
}

impl Iterator for CompletionQueue<'_> {
    type Item = Entry;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        if self.head != self.tail {
            let entry = unsafe {
                *self
                    .queue
                    .cqes
                    .add((self.head & self.queue.ring_mask) as usize)
            };
            self.head = self.head.wrapping_add(1);
            Some(Entry(entry))
        } else {
            None
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.len(), Some(self.len()))
    }
}

impl ExactSizeIterator for CompletionQueue<'_> {
    #[inline]
    fn len(&self) -> usize {
        self.tail.wrapping_sub(self.head) as usize
    }
}

impl Entry {
    /// The operation-specific result code. For example, for a [`Read`](crate::opcode::Read)
    /// operation this is equivalent to the return value of the `read(2)` system call.
    #[inline]
    pub fn result(&self) -> i32 {
        self.0.res
    }

    /// The user data of the request, as set by
    /// [`Entry::user_data`](crate::squeue::Entry::user_data) on the submission queue event.
    #[inline]
    pub fn user_data(&self) -> u64 {
        self.0.user_data
    }

    /// Metadata related to the operation.
    ///
    /// This is currently used for:
    /// - Storing the selected buffer ID, if one was selected. See
    /// [`BUFFER_SELECT`](crate::squeue::Flags::BUFFER_SELECT) for more info.
    #[inline]
    pub fn flags(&self) -> u32 {
        self.0.flags
    }
}

impl fmt::Debug for Entry {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Entry")
            .field("result", &self.0.res)
            .field("user_data", &self.0.user_data)
            .field("flags", &self.0.flags)
            .finish()
    }
}

#[cfg(feature = "unstable")]
pub fn buffer_select(flags: u32) -> Option<u16> {
    if flags & sys::IORING_CQE_F_BUFFER != 0 {
        let id = flags >> sys::IORING_CQE_BUFFER_SHIFT;

        // FIXME
        //
        // Should we return u16? maybe kernel will change value of `IORING_CQE_BUFFER_SHIFT` in future.
        Some(id as u16)
    } else {
        None
    }
}