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// // Copyright 2018 yvt, all rights reserved. // // Licensed under the MIT license <LICENSE-MIT or // http://opensource.org/licenses/MIT>. This file may // not be copied, modified,or distributed except // according to those terms. // //! A dynamic external memory allocator implementing the functionality of a //! [circular buffer]. //! //! [circular buffer]: https://en.wikipedia.org/wiki/Circular_buffer //! //! The calling program is responsible for tracking which allocated part is //! currently the frontmost/backmost region of a [`Ring`]. When deallocating //! a region, it must appropriately call `dealloc_front` or `dealloc_back` //! depending on the position of the region within a `Ring`. //! //! # Examples //! //! ``` //! use xalloc::{Ring, RingRegion}; //! let mut ring: Ring<u32> = Ring::new(10); //! //! // Allocate regions //! // [ ] //! let alloc1: (RingRegion<u32>, u32) = ring.alloc_back(4).unwrap(); //! // [[ 1 ] ] //! let alloc2: (RingRegion<u32>, u32) = ring.alloc_back(4).unwrap(); //! // [[ 1 ][ 2 ] ] //! let (region1, offset1) = alloc1; //! let (region2, offset2) = alloc2; //! println!("allocated #1: {:?}", (®ion1, offset1)); //! println!("allocated #2: {:?}", (®ion2, offset2)); //! //! // Deallocate regions //! // [[ 1 ][ 2 ] ] //! ring.dealloc_front(region1); //! // [ [ 2 ] ] //! ring.dealloc_front(region2); //! // [ ] //! ``` use num::{One, Zero}; use int::{round_down, round_up, BinaryUInteger}; /// A dynamic external memory allocator providing the functionality of a /// [circular buffer]. /// /// [circular buffer]: https://en.wikipedia.org/wiki/Circular_buffer /// /// See [the module-level documentation] for more. /// /// [the module-level documentation]: index.html /// /// ## Type parameters /// /// - `T` is an integer type used to represent region sizes. You usually use /// `u32` or `u64` for this. /// #[derive(Debug)] pub struct Ring<T> { size: T, /// The starting location of the allocated region. Must be less than `size`. start: T, /// The ending location of the allocated region. Must be less than `size`. end: T, /// Indicates whether this `Ring` is empty or not. empty: bool, } /// A handle type to a region allocated in a [`Ring`]. /// /// `RingRegion` returned by a `Ring` only can be used with the /// same `Ring`. #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)] pub struct RingRegion<T> { start: T, end: T, } impl<T: BinaryUInteger> Ring<T> { /// Construct a `RingRegion`. /// /// `size` must be smaller than `T::max_value() >> 1` (this is a precaution /// taken not to cause unintentional overflows). pub fn new(size: T) -> Self { assert!(size < T::max_value() >> 1); Self { size: size.clone(), start: Zero::zero(), end: Zero::zero(), empty: true, } } /// Return `true` if `Ring` has no allocated regions. pub fn is_empty(&self) -> bool { self.empty } /// Return `true` if `Ring` has no free space. pub fn is_full(&self) -> bool { self.start == self.end } /// Allocate a region of the size `size` to the back of the allocated /// region. /// /// Returns a handle of the allocated region and its offset if the /// allocation succeeds. Returns `None` otherwise. /// /// `size` must not be zero. pub fn alloc_back(&mut self, size: T) -> Option<(RingRegion<T>, T)> { self.alloc_back_aligned(size, One::one()) } /// Allocate a region of the size `size` to the front of the allocated /// region. /// /// Returns a handle of the allocated region and its offset if the /// allocation succeeds. Returns `None` otherwise. /// /// `size` must not be zero. pub fn alloc_front(&mut self, size: T) -> Option<(RingRegion<T>, T)> { self.alloc_front_aligned(size, One::one()) } /// Allocate a region of the size `size` with a given alignment requirement /// to the back of the allocated region. /// /// Returns a handle of the allocated region and its offset if the /// allocation succeeds. Returns `None` otherwise. /// /// - `align` must be a power of two. /// - `size` must not be zero. pub fn alloc_back_aligned(&mut self, size: T, align: T) -> Option<(RingRegion<T>, T)> { assert_ne!(size, Zero::zero()); assert!(align.is_power_of_two()); if self.empty { self.alloc_empty(size) } else if size >= self.size { None } else { let mut new_wrapped = self.end <= self.start; let mut new_end = round_up(&self.end, &align); if new_end.clone() + size.clone() > self.size && !new_wrapped { new_end = Zero::zero(); new_wrapped = true; } if new_wrapped && new_end.clone() + size.clone() > self.start { return None; } let offset = new_end.clone(); new_end += size; if new_end == self.size { new_end = Zero::zero(); } let region = RingRegion { start: self.end.clone(), end: new_end.clone(), }; self.end = new_end; Some((region, offset)) } } /// Allocate a region of the size `size` with a given alignment requirement /// to the front of the allocated region. /// /// Returns a handle of the allocated region and its offset if the /// allocation succeeds. Returns `None` otherwise. /// /// - `align` must be a power of two. /// - `size` must not be zero. pub fn alloc_front_aligned(&mut self, size: T, align: T) -> Option<(RingRegion<T>, T)> { assert_ne!(size, Zero::zero()); assert!(align.is_power_of_two()); if self.empty { self.alloc_empty(size) } else if size >= self.size { None } else { // 0 1align 2align // | | size | // |===================|=====| | // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ enlarged_size // ^^^^^^^^^^^^^^^ pad let enlarged_size = round_up(&size, &align); let pad = enlarged_size.clone() - size.clone(); let mut new_wrapped = self.end <= self.start; let mut new_start = round_down(&(self.start.clone() + pad.clone()), &align); if new_start < enlarged_size.clone() && !new_wrapped { new_start = round_down(&(self.size.clone() + pad.clone()), &align); new_wrapped = true; } if new_wrapped && self.end.clone() + enlarged_size.clone() > new_start { return None; } new_start -= enlarged_size; let offset = new_start.clone(); let region = RingRegion { start: new_start.clone(), end: self.start.clone(), }; self.start = new_start; Some((region, offset)) } } fn alloc_empty(&mut self, size: T) -> Option<(RingRegion<T>, T)> { debug_assert!(self.empty); if size <= self.size { self.start = Zero::zero(); self.end = if size == self.size { Zero::zero() } else { size.clone() }; self.empty = false; Some(( RingRegion { start: Zero::zero(), end: size, }, Zero::zero(), )) } else { None } } /// Deallocate frontmost (first) regions until `r` becomes the new frontmost /// region. `r` is not removed. /// /// `r` must be in `Ring`. /// Otherwise, `Ring` might enter an inconsistent state and/or panic, but /// does not cause an undefined behavior. pub fn dealloc_front_until(&mut self, r: RingRegion<T>) { assert!(!self.empty, "empty"); self.start = r.start; } /// Deallocate backmost (last) regions until `r` becomes the new backmost /// region. `r` is not removed. /// /// `r` must be in `Ring`. /// Otherwise, `Ring` might enter an inconsistent state and/or panic, but /// does not cause an undefined behavior. pub fn dealloc_back_until(&mut self, r: RingRegion<T>) { assert!(!self.empty, "empty"); self.end = r.end; } /// Deallocate the frontmost (first) region. /// /// `r` must be the current frontmost region of `Ring`. /// Otherwise, `Ring` might enter an inconsistent state and/or panic, but /// does not cause an undefined behavior. pub fn dealloc_front(&mut self, r: RingRegion<T>) { assert!(!self.empty, "empty"); assert!(self.start == r.start, "not front"); self.start = r.end; if self.start == self.end { self.empty = true; } } /// Deallocate the backmost (last) region. /// /// `r` must be the current backmost region of `Ring`. /// Otherwise, `Ring` might enter an inconsistent state and/or panic, but /// does not cause an undefined behavior. pub fn dealloc_back(&mut self, r: RingRegion<T>) { assert!(!self.empty, "empty"); assert!(self.end == r.end, "not back"); self.end = r.start; if self.start == self.end { self.empty = true; } } }