asterinas / xarray

An XArray struct implemented by Rust.

XArray

XArray is an abstract data type functioning like an expansive array of items where each item must be an 8-byte object, such as Arc<T> or Box<T>. User-stored pointers must have a minimum alignment of 4 bytes. XArray facilitates efficient sequential access to adjacent entries, supporting multiple concurrent reads and exclusively allowing one write operation at a time.

Features

• Cursors: Provide cursors for precise and efficient iteration over the array. Cursors have both immutable and mutable versions. One can hold multiple immutable cursors or hold a mutable cursor exclusively at a time.
• Marking: Provide ability to mark entries and the XArray itself for easy state tracking.
• Generics: Generic implementation that can work with any entry type that fits the use case.
• Copy-on-Write (COW): Efficient cloning of XArrays with shared structure until mutation.

Installation

Add this to your Cargo.toml:

[dependencies]
xarray = "0.1.0"

Usage

This crate is developed in no_std environment, but std users can still use this crate with --feature="std":

The following section covers how to interact with XArray including creating an XArray, using cursors, marking, cloning, and more.

---

Creating an XArray:

// In std environment
extern crate alloc;

use alloc::sync::Arc;
use xarray::XArray;

// Create a new XArray instance
let mut xarray: XArray<Arc<i32>> = XArray::new();

• Users should declare the type of items (Arc) stored in the XArray, and the item type should implement ItemEntry trait.
• We implement ItemEntry for alloc::sync::Arc and alloc::sync::Box by default, hence std users can use them directly.

Using Cursor

extern crate alloc;

use alloc::sync::Arc;
use xarray::XArray;

let mut xarray_arc: XArray<Arc<i32>> = XArray::new();

let mut cursor = xarray_arc.cursor_mut(0);
// Store the Arc at the index range 0~10000.
for i in 0..10000 {
    let value = Arc::new(i * 2);
    cursor.store(value);
    cursor.next();
}

cursor.reset_to(0);
for i in 0..10000 {
    let value = cursor.load().unwrap();
    assert!(*value.as_ref() == i * 2);
    cursor.next();
}

Using Marks

Here is an example of using marks for the stored pages in the XArray, where PageMark represents the states of each individual Page:

extern crate alloc;

use alloc::sync::Arc;
use xarray::{XArray, XMark, StdMutex};

#[derive(Clone, Copy)]

enum PageMark {
    DirtyPage 
    ...
}

impl From<PageState> for XMark {
    fn from(mark: PageState) -> Self {
        match mark {
            PageState::DirtyPage => Self::Mark0,
            ...
        }
    }
}

let mut pages: XArray<Page, StdMutex, PageState> = XArray::new();

let mut cursor = pages.cursor_mut(1000);
cursor.store(Page::alloc_zero());
// Mark the Page as DirtyPage.
cursor.set_mark(PageState::DirtyPage).unwrap();
assert!(cursor.is_marked(PageState::DirtyPage));

• Items and the XArray can have up to three distinct marks by default, with each mark independently maintained.
• Users need to use a struct to represent the marks that need to be used. For the situation where multiple marks are required, these marks are typically encapsulated within an enumeration class.
• If users want to use a struct M for marks, they should implement From<M> trait for XMark and declare M in the generics list of XArray.

Copy-On-Write (COW) Clone

use std::sync::Arc;
use xarray::{XArray};

let mut xarray: XArray<Arc<i32>> = XArray::new();

// Store values
let value = Arc::new(10);
xarray.store(1, value.clone());
assert_eq!(*xarray.load(1).unwrap().as_ref(), 10);

// Clone the XArray
let mut xarray_clone = xarray.clone();
assert_eq!(*xarray_clone.load(1).unwrap().as_ref(), 10);

// Store a new value in the clone
let new_value = Arc::new(100);
xarray_clone.store(1, new_value);

// The original XArray is unaffected by changes in the clone
assert_eq!(*xarray.load(1).unwrap().as_ref(), 10);
assert_eq!(*xarray_clone.load(1).unwrap().as_ref(), 100);

Iteration

use std::sync::Arc;
use xarray::XArray;

let mut xarray: XArray<Arc<i32>> = XArray::new();

// Store item to even index in the range 100~200.
for i in 100..200 {
    if i % 2 == 0 {
        let value = Arc::new(i * 2);
        cursor.store(value);
    }
    cursor.next();
}

// Iterate at the range 100~200.
let mut count = 0;
for item in xarray.range(100..200) {
    count += 1;
}
assert_eq!(count == 50);

License

xarray's Issues

Incorrect transmute

The transmute function in the following code is incorrect.

impl<'a, T> Deref for BoxRef<'a, T> {
    type Target = Box<T>;

    fn deref(&self) -> &Self::Target {
        // SAFETY: A `Box<T>` is guaranteed to be represented by a single pointer [1] and a shared
        // reference to the `Box<T>` during the lifetime `'a` can be created according to the
        // safety requirements of `ItemEntry::raw_as_ref`.
        //
        // [1]: https://doc.rust-lang.org/std/boxed/#memory-layout
        unsafe { core::mem::transmute(&self.inner) }
    }
}

The method should take self.inner, instead of &self.inner.