scroll-ring 0.1.3

use core::cmp::min;
use core::num::Wrapping;
use portable_atomic::{AtomicU32, Ordering};
use ringbuf::Rb;

use crate::util::write_from_slices;

/// A very simple implementation of the infinilog buffer.
///
/// See module level documentation on its general properties.
#[derive(Default)]
pub struct Buffer<const N: usize> {
    ring: try_lock::TryLock<CountedRing<N>>,
    /// Bytes lost due to parallel operations
    lost: AtomicU32,
}

#[derive(Default)]
struct CountedRing<const N: usize> {
    buf: ringbuf::ring_buffer::LocalRb<u8, [core::mem::MaybeUninit<u8>; N]>,
    total_written: Wrapping<u32>,
}

/// Error type for read_earliest, indicating that not only could no data be read, but not even a cursor
/// position could be returned.
#[derive(Debug, PartialEq)]
pub struct BufferUnavailable;

/// Error type for read.
///
/// Note that "that is the latest cursor position, nobody wrote anything more" is not an error but
/// expressed as a 0-length result.
#[derive(Debug, PartialEq)]
pub enum ReadErr {
    /// No information about the buffer is available at this time
    BufferUnavailable,
    /// The indicated cursor position has either not been reached yet, or the data has been
    /// overwritten already
    DataUnavailable,
}

impl<const N: usize> Buffer<N> {
    /// Read the earliest available data into outbuf. On success, returns the cursor position
    /// corresponding to the start of outbuf, and the number of bytes available (which is the
    /// number of bytes copied into outbuf unless it exceeds its size).
    pub fn read_earliest(
        &self,
        outbuf: &mut [u8],
    ) -> Result<(Wrapping<u32>, usize), BufferUnavailable> {
        let Some(ring) = self.ring.try_lock() else {
            return Err(BufferUnavailable);
        };

        let (first, second) = ring.buf.as_slices();
        // Write as much data from first and second into outbuf
        write_from_slices(outbuf, first, second);

        Ok((
            ring.total_written - Wrapping(ring.buf.len() as _),
            ring.buf.len(),
        ))
    }

    pub fn read_from_cursor(
        &self,
        cursor: Wrapping<u32>,
        outbuf: &mut [u8],
    ) -> Result<usize, ReadErr> {
        let Some(ring) = self.ring.try_lock() else {
            return Err(ReadErr::BufferUnavailable);
        };

        let (mut first, mut second) = ring.buf.as_slices();
        let cursor_in_firstsecond = cursor - ring.total_written + Wrapping(ring.buf.len() as _);
        // With this coordinate transformation, we don't have to distinguish "before" and "after"
        // (which we can't do anyway, effectively) -- it's just "we have it" or "we don't have it"
        // any more.
        let mut cursor_in_firstsecond = cursor_in_firstsecond.0 as usize;
        if cursor_in_firstsecond > first.len() + second.len() {
            return Err(ReadErr::DataUnavailable);
        }

        let remove_from_first = min(first.len(), cursor_in_firstsecond);
        cursor_in_firstsecond -= remove_from_first;
        first = &first[remove_from_first..];
        let remove_from_second = min(second.len(), cursor_in_firstsecond);
        cursor_in_firstsecond -= remove_from_second;
        second = &second[remove_from_second..];
        debug_assert!(cursor_in_firstsecond == 0);

        let len_from_cursor = first.len() + second.len();
        write_from_slices(outbuf, first, second);

        Ok(len_from_cursor)
    }

    pub fn write(&self, data: &[u8]) {
        if let Some(mut ring) = self.ring.try_lock() {
            let lost_before_us = self.lost.swap(0, Ordering::Relaxed);

            if lost_before_us != 0 {
                ring.total_written += Wrapping(lost_before_us);
                ring.buf.clear();
            }

            ring.buf.push_slice_overwrite(data);
            ring.total_written += Wrapping(data.len() as _);
        } else {
            /* as _: deliberately truncating -- our logical address space only is that big */
            self.lost.fetch_add(data.len() as _, Ordering::Relaxed);
        }
    }
}

#[test]
fn test_buffer_readwrite() {
    const N: usize = 1024;

    let b: Buffer<N> = Default::default();
    let mut outbuf = [0; 4];
    assert_eq!(b.read_earliest(&mut outbuf), Ok((Wrapping(0), 0)));
    assert_eq!(b.read_from_cursor(Wrapping(0), &mut outbuf), Ok(0));
    assert_eq!(
        b.read_from_cursor(Wrapping(10), &mut outbuf),
        Err(ReadErr::DataUnavailable)
    );

    b.write(b"Hello");
    assert_eq!(b.read_earliest(&mut outbuf), Ok((Wrapping(0), 5)));
    assert_eq!(&outbuf, "Hell".as_bytes());
    outbuf[0] = 0;
    assert_eq!(b.read_earliest(&mut outbuf), Ok((Wrapping(0), 5)));
    assert_eq!(&outbuf, "Hell".as_bytes());
    outbuf[0] = 0;

    assert_eq!(b.read_from_cursor(Wrapping(2), &mut outbuf), Ok(3));
    assert_eq!(&outbuf[..3], "llo".as_bytes());

    b.write(b" World!");
    const HWLEN: usize = "Hello World!".len();
    outbuf[0] = 0;
    assert_eq!(b.read_earliest(&mut outbuf), Ok((Wrapping(0), HWLEN)));
    assert_eq!(&outbuf, "Hell".as_bytes());

    let erase_h = [0; N - HWLEN + 1];
    b.write(&erase_h);
    assert_eq!(b.read_earliest(&mut outbuf), Ok((Wrapping(1), N)));
    assert_eq!(&outbuf, "ello".as_bytes());
}

#[test]
/// Test collisions in the buffer
///
/// We can't do that easily in threads because we can't force them to turn into a collision, but
/// with internal access we can block access while we're writing.
fn test_buffer_collisions() {
    const N: usize = 1024;

    let b: Buffer<N> = Default::default();
    let mut outbuf = [0; 4];

    const OFFSET: usize = N - 6; // The overflow would straddle the boundary
    b.write(&[0; OFFSET]);

    b.write(b"1234");
    let locked = b.ring.try_lock();
    b.write(b"5678");
    drop(locked);

    // We don't guarantee this -- the read could just as well say that we're at 8 and no data is
    // available
    assert_eq!(b.read_earliest(&mut outbuf), Ok((Wrapping(0), OFFSET + 4)));
    assert_eq!(
        b.read_from_cursor(Wrapping(OFFSET as _), &mut outbuf),
        Ok(4)
    );
    assert_eq!(&outbuf, b"1234");

    b.write(b"90ab");
    assert_eq!(
        b.read_earliest(&mut outbuf),
        Ok((Wrapping(OFFSET as u32 + 8), 4))
    );
    assert_eq!(&outbuf, b"90ab");
}