//! virtio-console: a second terminal, `hvc0` ("hypervisor console", the
//! name Linux gives it). QEMU connects it to anything a *chardev* can be:
//!
//!   -device virtio-serial-pci -device virtconsole,chardev=hvc
//!   -chardev socket,id=hvc,path=target/hvc0.sock,server=on,wait=off
//!       (then on the host:  socat - UNIX-CONNECT:target/hvc0.sock)
//!   -chardev file,id=hvc,path=out.txt,input-path=in.txt   (tests)
//!
//! Two queues: 0 = receive (host -> us), 1 = transmit (us -> host). (With
//! the MULTIPORT feature there'd be more ports and a control queue; we
//! don't ask for it, so it's exactly one port, which QEMU opens for us.)
//!
//! * **Receiving**: we keep 8 empty 512-byte buffers in the receive queue.
//!   When the host sends bytes the device fills one, puts it on the used
//!   ring and raises an interrupt; we copy the bytes into INPUT and put the
//!   buffer straight back. Readers of /dev/hvc0 wait on INPUT.
//! * **Sending**: 8 buffers too. A write copies into a free one and kicks
//!   the device. If all 8 are still in flight (host not reading), we wait a
//!   few milliseconds and then *drop* the output: kernel logging must never
//!   hang because nobody is listening.
//!
//! Every kernel log line (`serial_print!`) also goes here -- except while
//! a program has /dev/hvc0 open (e.g. a shell runs on it via `getty`), so
//! that session isn't interleaved with log noise.

use super::{Transport, Virtqueue};
use crate::memory;
use crate::pci::PciDevice;
use crate::thread::WaitQueue;
use alloc::{format, string::String, vec::Vec};
use conquer_once::spin::OnceCell;
use core::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use crossbeam_queue::ArrayQueue;
use spin::Mutex;
use x86_64::{PhysAddr, instructions::interrupts::without_interrupts};

const SLOTS: u16 = 8; // buffers per direction
const SLOT_SIZE: usize = 512; // 8 x 512 = one page per direction

struct Hvc {
    transport: Transport,
    rx: Virtqueue,
    tx: Virtqueue,
    rx_phys: u64,
    tx_phys: u64,
}

static HVC: Mutex<Option<Hvc>> = Mutex::new(None);
static PRESENT: AtomicBool = AtomicBool::new(false);
/// Bytes received from the host, not read yet.
static INPUT: OnceCell<ArrayQueue<u8>> = OnceCell::uninit();
static INPUT_READY: WaitQueue = WaitQueue::new();
/// How many open /dev/hvc0 files there are (kernel log mirroring pauses
/// while there are any).
pub static OPEN_COUNT: AtomicUsize = AtomicUsize::new(0);
/// Bytes we had to drop because the host wasn't reading.
pub static DROPPED: AtomicUsize = AtomicUsize::new(0);

fn input() -> &'static ArrayQueue<u8> {
    INPUT.get_or_init(|| ArrayQueue::new(4096))
}

/// virtio-serial: transitional id 0x1003, modern 0x1043.
pub fn is_virtio_console(d: &PciDevice) -> bool {
    d.vendor_id == super::VENDOR && matches!(d.device_id, 0x1003 | 0x1043)
}

pub fn present() -> bool {
    PRESENT.load(Ordering::Relaxed)
}

fn slot_ptr(base_phys: u64, slot: u16) -> *mut u8 {
    memory::phys_to_virt(PhysAddr::new(base_phys + slot as u64 * SLOT_SIZE as u64)).as_mut_ptr()
}

pub fn init(devices: &[PciDevice]) -> Option<String> {
    let pci = devices.iter().find(|d| is_virtio_console(d))?;
    let setup = || -> Option<Hvc> {
        let transport = Transport::new(pci)?;
        transport.begin_init(0)?;
        // Receive interrupts on; transmit is polled (we reap finished
        // buffers whenever we write).
        let mut rx = transport.setup_queue(0, SLOTS, true)?;
        let tx = transport.setup_queue(1, SLOTS, false)?;
        let rx_phys = memory::alloc_contiguous_frames(1)?.as_u64();
        let tx_phys = memory::alloc_contiguous_frames(1)?.as_u64();
        while let Some(slot) = rx.next_free() {
            rx.push(&[(rx_phys + slot as u64 * SLOT_SIZE as u64, SLOT_SIZE as u32, true)])?;
        }
        transport.driver_ok();
        rx.notify();
        Some(Hvc { transport, rx, tx, rx_phys, tx_phys })
    };
    let Some(hvc) = setup() else {
        return Some(String::from("hvc0: virtio-console found but setup failed"));
    };
    input();
    without_interrupts(|| *HVC.lock() = Some(hvc));
    PRESENT.store(true, Ordering::Relaxed);
    let irq = pci.interrupt_line();
    crate::interrupts::enable_pci_irq(irq);
    Some(format!("hvc0: virtio-console at {:02x}:{:02x}.{}, IRQ {}", pci.bus, pci.device, pci.function, irq))
}

/// Move received bytes from the used ring into INPUT and recycle the
/// buffers. Called from the interrupt handler, and by readers (in case the
/// interrupt line isn't one we handle).
fn poll_rx(hvc: &mut Hvc) -> bool {
    let mut got = false;
    while let Some((slot, len)) = hvc.rx.pop_used() {
        let data = unsafe { core::slice::from_raw_parts(slot_ptr(hvc.rx_phys, slot), (len as usize).min(SLOT_SIZE)) };
        for &b in data {
            let _ = input().push(b); // full: drop (4 KiB nobody reads)
        }
        got |= len > 0;
        let _ = hvc.rx.push(&[(hvc.rx_phys + slot as u64 * SLOT_SIZE as u64, SLOT_SIZE as u32, true)]);
    }
    if got {
        hvc.rx.notify();
    }
    got
}

pub fn handle_irq() {
    let mut got = false;
    if let Some(mut guard) = HVC.try_lock() {
        if let Some(hvc) = guard.as_mut() {
            hvc.transport.read_isr();
            got = poll_rx(hvc);
        }
    }
    if got {
        INPUT_READY.wake_all();
    }
}

/// Send bytes to the host. Never blocks for long: drops what doesn't fit
/// within ~10 ms. Returns how many bytes were sent.
pub fn write(bytes: &[u8]) -> usize {
    if !present() {
        return 0;
    }
    without_interrupts(|| {
        let mut guard = HVC.lock();
        let Some(hvc) = guard.as_mut() else { return 0 };
        let mut sent = 0;
        for chunk in bytes.chunks(SLOT_SIZE) {
            // Reap finished sends; wait a little if none is free.
            let deadline = crate::time::now_us() + 10_000;
            let slot = loop {
                while hvc.tx.pop_used().is_some() {}
                if let Some(slot) = hvc.tx.next_free() {
                    break Some(slot);
                }
                if crate::time::now_us() > deadline {
                    break None;
                }
                core::hint::spin_loop();
            };
            let Some(slot) = slot else {
                DROPPED.fetch_add(bytes.len() - sent, Ordering::Relaxed);
                break;
            };
            unsafe { core::ptr::copy_nonoverlapping(chunk.as_ptr(), slot_ptr(hvc.tx_phys, slot), chunk.len()) };
            hvc.tx.push(&[(hvc.tx_phys + slot as u64 * SLOT_SIZE as u64, chunk.len() as u32, false)]);
            hvc.tx.notify();
            sent += chunk.len();
        }
        sent
    })
}

/// Kernel log mirror (called by serial.rs for every `serial_print!`).
pub fn log(args: core::fmt::Arguments) {
    if !present() || OPEN_COUNT.load(Ordering::Relaxed) > 0 {
        return;
    }
    /// Collect the formatted text in a small stack buffer (no heap needed).
    struct Buf {
        data: [u8; 256],
        len: usize,
    }
    impl core::fmt::Write for Buf {
        fn write_str(&mut self, s: &str) -> core::fmt::Result {
            for &b in s.as_bytes() {
                if self.len == self.data.len() {
                    write(&self.data);
                    self.len = 0;
                }
                self.data[self.len] = b;
                self.len += 1;
            }
            Ok(())
        }
    }
    let mut buf = Buf { data: [0; 256], len: 0 };
    let _ = core::fmt::Write::write_fmt(&mut buf, args);
    write(&buf.data[..buf.len]);
}

/// Read received bytes into `buf`; blocks until at least one is there
/// (or a signal arrives: ERESTARTSYS).
pub fn read(buf: &mut [u8]) -> Result<usize, crate::errno::Errno> {
    if buf.is_empty() {
        return Ok(0);
    }
    loop {
        let take = |buf: &mut [u8]| {
            let mut n = 0;
            while n < buf.len() {
                match input().pop() {
                    Some(b) => { buf[n] = b; n += 1; }
                    None => break,
                }
            }
            n
        };
        without_interrupts(|| {
            if let Some(hvc) = HVC.lock().as_mut() {
                poll_rx(hvc);
            }
        });
        let n = take(buf);
        if n > 0 {
            return Ok(n);
        }
        // Sleep until the interrupt handler has data (or 1 tick, then poll).
        let deadline = crate::interrupts::ticks() + 1;
        INPUT_READY.wait_until_deadline_intr(deadline, || (!input().is_empty()).then_some(()))?;
    }
}

/// Bytes waiting in the input buffer (tests).
pub fn pending_input() -> usize {
    input().len()
}

/// Read all buffered input without blocking (tests).
pub fn drain_input() -> Vec<u8> {
    without_interrupts(|| {
        if let Some(hvc) = HVC.lock().as_mut() {
            poll_rx(hvc);
        }
    });
    core::iter::from_fn(|| input().pop()).collect()
}