#cursor #mark #version #array #immutability #copy-on-write #operation

nightly no-std xarray

A Rust version of the XArray with copy-on-write capabilities

2 releases

0.1.1 Jul 8, 2024
0.1.0 Jun 3, 2024

#110 in Memory management

Download history 147/week @ 2024-06-03 3/week @ 2024-06-10 9/week @ 2024-07-01 159/week @ 2024-07-08

169 downloads per month
Used in ostd

MPL-2.0 license

1.5K SLoC


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.


  • 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.


Add this to your Cargo.toml:

xarray = "0.1.0"


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);

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

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 {

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);
// Mark the Page as 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);


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);

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


XArray primarily use the Mozilla Public License (MPL), Version 2.0.