lowlet 0.1.2

Low-latency IPC library using shared memory and lock-free structures
Documentation 
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use std::ptr::NonNull;
use std::sync::atomic::{AtomicUsize, Ordering};

use crate::err::{Error, Result};

pub const CACHE_LINE: usize = 64;

#[repr(C, align(64))]
pub struct BumpAllocator {
    base: NonNull<u8>,
    capacity: usize,
    offset: AtomicUsize,
    _pad: [u8; CACHE_LINE - 24],
}

unsafe impl Send for BumpAllocator {}
unsafe impl Sync for BumpAllocator {}

impl BumpAllocator {
    pub unsafe fn init(ptr: *mut u8, capacity: usize) -> Result<&'static Self> {
        if ptr.is_null() || capacity < std::mem::size_of::<Self>() * 2 {
            return Err(Error::OutOfMemory);
        }

        if !(ptr as usize).is_multiple_of(CACHE_LINE) {
            return Err(Error::Alignment);
        }

        #[allow(clippy::cast_ptr_alignment)]
        let alloc = ptr.cast::<Self>();
        let usable_capacity = capacity - std::mem::size_of::<Self>();
        let data_start = NonNull::new_unchecked(ptr.add(std::mem::size_of::<Self>()));

        std::ptr::write(
            alloc,
            Self {
                base: data_start,
                capacity: usable_capacity,
                offset: AtomicUsize::new(0),
                _pad: [0; CACHE_LINE - 24],
            },
        );

        Ok(&*alloc)
    }

    pub unsafe fn from_existing(ptr: *mut u8) -> Result<&'static Self> {
        if ptr.is_null() {
            return Err(Error::Alignment);
        }

        #[allow(clippy::cast_ptr_alignment)]
        Ok(&*ptr.cast::<Self>())
    }

    pub fn alloc<T>(&self) -> Result<NonNull<T>> {
        let size = std::mem::size_of::<T>();
        let align = std::mem::align_of::<T>().max(CACHE_LINE);

        self.alloc_raw(size, align).map(NonNull::cast)
    }

    pub fn alloc_slice<T>(&self, count: usize) -> Result<NonNull<T>> {
        let size = std::mem::size_of::<T>()
            .checked_mul(count)
            .ok_or(Error::OutOfMemory)?;
        let align = std::mem::align_of::<T>().max(CACHE_LINE);

        self.alloc_raw(size, align).map(NonNull::cast)
    }

    fn alloc_raw(&self, size: usize, align: usize) -> Result<NonNull<u8>> {
        loop {
            let current = self.offset.load(Ordering::Relaxed);
            let base = self.base.as_ptr();

            let ptr = unsafe { base.add(current) };
            let padding = ptr.align_offset(align);

            let next = current
                .checked_add(padding)
                .and_then(|n| n.checked_add(size))
                .ok_or(Error::OutOfMemory)?;

            if next > self.capacity {
                return Err(Error::OutOfMemory);
            }

            if self
                .offset
                .compare_exchange_weak(current, next, Ordering::AcqRel, Ordering::Relaxed)
                .is_ok()
            {
                return Ok(unsafe { NonNull::new_unchecked(ptr.add(padding)) });
            }

            std::hint::spin_loop();
        }
    }

    pub unsafe fn reset(&self) {
        self.offset.store(0, Ordering::Release);
    }

    #[inline]
    pub fn used(&self) -> usize {
        self.offset.load(Ordering::Acquire)
    }

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

    #[inline]
    pub fn available(&self) -> usize {
        self.capacity.saturating_sub(self.used())
    }
}