//! Submission Queue

use std::sync::atomic;
use bitflags::bitflags;
use crate::util::{ Mmap, unsync_load };
use crate::sys;


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

    pub(crate) sqes: *mut sys::io_uring_sqe
}

pub struct AvailableQueue<'a> {
    head: u32,
    tail: u32,
    ring_mask: u32,
    ring_entries: u32,
    queue: &'a mut SubmissionQueue
}

/// Submission Entry
#[repr(transparent)]
#[derive(Clone)]
pub struct Entry(pub(crate) sys::io_uring_sqe);

bitflags!{
    /// Submission flags
    pub struct Flags: u8 {
        /// When this flag is specified,
        /// `fd` is an index into the files array registered with the io_uring instance.
        #[doc(hidden)]
        const FIXED_FILE = sys::IOSQE_FIXED_FILE as _;

        /// When this flag is specified,
        /// the SQE will not be started before previously submitted SQEs have completed,
        /// and new SQEs will not be started before this one completes.
        const IO_DRAIN = sys::IOSQE_IO_DRAIN as _;

        /// When this flag is specified,
        /// it forms a link with the next SQE in the submission ring.
        /// That next SQE will not be started before this one completes.
        const IO_LINK = sys::IOSQE_IO_LINK as _;

        /// Like [IO_LINK], but it doesn’t sever regardless of the completion result.
        const IO_HARDLINK = sys::IOSQE_IO_HARDLINK as _;
    }
}

impl SubmissionQueue {
    pub(crate) unsafe fn new(sq_mmap: &Mmap, sqe_mmap: &Mmap, p: &sys::io_uring_params) -> SubmissionQueue {
        mmap_offset!{
            let head            = sq_mmap + p.sq_off.head           => *const atomic::AtomicU32;
            let tail            = sq_mmap + p.sq_off.tail           => *const atomic::AtomicU32;
            let ring_mask       = sq_mmap + p.sq_off.ring_mask      => *const u32;
            let ring_entries    = sq_mmap + p.sq_off.ring_entries   => *const u32;
            let flags           = sq_mmap + p.sq_off.flags          => *const atomic::AtomicU32;
            let dropped         = sq_mmap + p.sq_off.dropped        => *const atomic::AtomicU32;
            let array           = sq_mmap + p.sq_off.array          => *mut u32;

            let sqes            = sqe_mmap + 0                      => *mut sys::io_uring_sqe;
        }

        // To keep it simple, map it directly to `sqes`.
        for i in 0..*ring_entries {
            array.add(i as usize).write_volatile(i);
        }

        SubmissionQueue {
            head, tail,
            ring_mask, ring_entries,
            flags, dropped,
            sqes
        }
    }

    pub fn need_wakeup(&self) -> bool {
        unsafe {
            (*self.flags).load(atomic::Ordering::Acquire) & sys::IORING_SQ_NEED_WAKEUP
                != 0
        }
    }

    pub fn dropped(&self) -> u32 {
        unsafe {
            (*self.dropped).load(atomic::Ordering::Acquire)
        }
    }

    #[inline]
    pub fn capacity(&self) -> usize {
        unsafe {
            self.ring_entries.read() as usize
        }
    }

    #[inline]
    pub fn len(&self) -> usize {
        unsafe {
            let head = (*self.head).load(atomic::Ordering::Acquire);
            let tail = unsync_load(self.tail);

            tail.wrapping_sub(head) as usize
        }
    }

    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    #[inline]
    pub fn is_full(&self) -> bool {
        self.len() == self.capacity()
    }

    /// Get currently available submission queue
    pub fn available(&mut self) -> AvailableQueue<'_> {
        unsafe {
            AvailableQueue {
                head: (*self.head).load(atomic::Ordering::Acquire),
                tail: unsync_load(self.tail),
                ring_mask: self.ring_mask.read(),
                ring_entries: self.ring_entries.read(),
                queue: self
            }
        }
    }
}

impl AvailableQueue<'_> {
    /// Sync queue
    pub fn sync(&mut self) {
        unsafe {
            (*self.queue.tail).store(self.tail, atomic::Ordering::Release);
            self.head = (*self.queue.head).load(atomic::Ordering::Acquire);
        }
    }

    #[inline]
    pub fn capacity(&self) -> usize {
        self.ring_entries as usize
    }

    #[inline]
    pub fn len(&self) -> usize {
        self.tail.wrapping_sub(self.head) as usize
    }

    #[inline]
    pub fn is_empty(&self) -> bool {
        self.head == self.tail
    }

    #[inline]
    pub fn is_full(&self) -> bool {
        self.tail.wrapping_sub(self.head) == self.ring_entries
    }

    /// Attempts to push an [Entry] into the queue.
    /// If the queue is full, the element is returned back as an error.
    ///
    /// # Safety
    ///
    /// Developers must ensure that parameters of the [Entry] (such as buffer) are valid,
    /// otherwise it may cause memory problems.
    pub unsafe fn push(&mut self, Entry(entry): Entry) -> Result<(), Entry> {
        if !self.is_full() {
            *self.queue.sqes.add((self.tail & self.ring_mask) as usize)
                = entry;
            self.tail = self.tail.wrapping_add(1);
            Ok(())
        } else {
            Err(Entry(entry))
        }
    }
}

impl Drop for AvailableQueue<'_> {
    fn drop(&mut self) {
        unsafe {
            (*self.queue.tail).store(self.tail, atomic::Ordering::Release);
        }
    }
}

impl Entry {
    /// Set [Submission flags](Flags)
    pub fn flags(mut self, flags: Flags) -> Entry {
        self.0.flags |= flags.bits();
        self
    }

    /// The `user_data` is an application-supplied value that will be copied into the completion queue
    /// entry (see below).
    pub fn user_data(mut self, user_data: u64) -> Entry {
        self.0.user_data = user_data;
        self
    }
}