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#![no_std]
//! `snmalloc-rs` provides a wrapper for [`microsoft/snmalloc`](https://github.com/microsoft/snmalloc) to make it usable as a global allocator for rust.
//! snmalloc is a research allocator. Its key design features are:
//! - Memory that is freed by the same thread that allocated it does not require any synchronising operations.
//! - Freeing memory in a different thread to initially allocated it, does not take any locks and instead uses a novel message passing scheme to return the memory to the original allocator, where it is recycled.
//! - The allocator uses large ranges of pages to reduce the amount of meta-data required.
//! 
//! The benchmark is available at the [paper](https://github.com/microsoft/snmalloc/blob/master/snmalloc.pdf) of `snmalloc`
//! There are three features defined in this crate:
//! - `debug`: Enable the `Debug` mode in `snmalloc`.
//! - `1mib`: Use the `1mib` chunk configuration.
//! - `cache-friendly`: Make the allocator more cache friendly (setting `CACHE_FRIENDLY_OFFSET` to `64` in building the library).
//! 
//! The whole library supports `no_std`.
//! 
//! To use `snmalloc-rs` add it as a dependency:
//! ```toml
//! # Cargo.toml
//! [dependencies]
//! snmalloc-rs = "0.1.0"
//! ```
//! 
//! To set `SnMalloc` as the global allocator add this to your project:
//! ```rust
//! #[global_allocator]
//! static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
//! ```
extern crate snmalloc_sys as ffi;

use core::alloc::{GlobalAlloc, Layout};

pub struct SnMalloc;

unsafe impl GlobalAlloc for SnMalloc {
    /// Allocate the memory with the given alignment and size. 
    /// On success, it returns a pointer pointing to the required memory address. 
    /// On failure, it returns a null pointer.
    /// The client must assure the following things:
    /// - `alignment` is greater than zero
    /// - Other constrains are the same as the rust standard library.
    /// The program may be forced to abort if the constrains are not full-filled.
    #[inline(always)]
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        ffi::rust_alloc(layout.align(), layout.size()) as _
    }

    /// De-allocate the memory at the given address with the given alignment and size. 
    /// The client must assure the following things:
    /// - the memory is acquired using the same allocator and the pointer points to the start position.
    /// - Other constrains are the same as the rust standard library.
    /// The program may be forced to abort if the constrains are not full-filled.
    #[inline(always)]
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        ffi::rust_dealloc(ptr as _, layout.align(), layout.size());
    }

    /// Re-allocate the memory at the given address with the given alignment and size. 
    /// On success, it returns a pointer pointing to the required memory address. 
    /// The memory content within the `new_size` will remains the same as previous.
    /// On failure, it returns a null pointer. In this situation, the previous memory is not returned to the allocator.
    /// The client must assure the following things:
    /// - the memory is acquired using the same allocator and the pointer points to the start position
    /// - `alignment` fulfills all the requirements as `rust_alloc`
    /// - Other constrains are the same as the rust standard library.
    /// The program may be forced to abort if the constrains are not full-filled.
    #[inline(always)]
    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
        ffi::rust_realloc(ptr as _, layout.align(), layout.size(), new_size) as _
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_frees_allocated_memory() {
        unsafe {
            let layout = Layout::from_size_align(8, 8).unwrap();
            let alloc = SnMalloc;

            let ptr = alloc.alloc(layout.clone());
            alloc.dealloc(ptr, layout);
        }
    }

    #[test]
    fn it_frees_zero_allocated_memory() {
        unsafe {
            let layout = Layout::from_size_align(8, 8).unwrap();
            let alloc = SnMalloc;

            let ptr = alloc.alloc_zeroed(layout.clone());
            alloc.dealloc(ptr, layout);
        }
    }

    #[test]
    fn it_frees_reallocated_memory() {
        unsafe {
            let layout = Layout::from_size_align(8, 8).unwrap();
            let alloc = SnMalloc;

            let ptr = alloc.alloc(layout.clone());
            let ptr = alloc.realloc(ptr, layout.clone(), 16);
            alloc.dealloc(ptr, layout);
        }
    }
}