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// Copyright 2018 Theodore Cipicchio // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! `Scratchpad` type implementation. use core::fmt; use super::{ Buffer, Error, ErrorKind, MarkerBack, MarkerFront, StaticBuffer, Tracking, }; use core::cell::{RefCell, UnsafeCell}; use core::mem::{size_of, uninitialized}; /// Front and back stacks for `Marker` tracking (used internally). pub(crate) struct MarkerStacks<TrackingT> where TrackingT: Tracking, { /// Stack data. pub(crate) data: TrackingT, /// Front stack offset. pub(crate) front: usize, /// Back stack offset. pub(crate) back: usize, } impl<TrackingT> fmt::Debug for MarkerStacks<TrackingT> where TrackingT: Tracking, { #[inline] fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "MarkerStacks {{ ... }}") } } /// Stack-like dynamic memory pool with double-ended allocation support. /// /// `Scratchpad` manages dynamic allocations from a fixed-size region of /// memory in a stack-like manner. Allocations can be made simultaneously from /// either the "front" or "back" of the scratchpad by setting a [`Marker`] /// using either [`mark_front()`][`mark_front()`] (returning a /// [`MarkerFront`]) or [`mark_back()`][`mark_back()`] (returning a /// [`MarkerBack`]). Multiple markers can be set, but only the most-recently /// set marker of a given type that is still active can be used to allocate /// objects. /// /// Individual allocations can be made from the marker, but no memory is /// actually freed back into the pool until the marker is dropped, where all /// the memory allocated through the marker is released at once. If the marker /// is not the most-recently set active marker of its type, its memory will /// simply be flagged as unused until all markers of the same type that were /// created after it are also dropped, at which point the memory will be once /// again made available for allocations. /// /// `Scratchpad`, [`Marker`] implementations, and [`Allocation`] all make use /// of static lifetimes to ensure that an object cannot be used after the /// object from which it was created is dropped (an allocation cannot outlive /// its marker, and a marker cannot outlive its scratchpad). /// /// *See also the [crate-level documentation](index.html) for more detailed /// information about how `Scratchpad` works and can be used.* /// /// [`Allocation`]: struct.Allocation.html /// [`mark_back()`]: #method.mark_back /// [`mark_front()`]: #method.mark_front /// [`Marker`]: trait.Marker.html /// [`MarkerBack`]: struct.MarkerBack.html /// [`MarkerFront`]: struct.MarkerFront.html pub struct Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { /// Buffer from which allocations are made. pub(crate) buffer: UnsafeCell<BufferT>, /// Dual stack containing the offsets of each active marker. If a marker /// not at the end of one of the stacks is freed, its offset is set to /// `core::usize::MAX` to indicate it is no longer active until the /// allocations that came after it (in the same stack) have also been /// freed. pub(crate) markers: RefCell<MarkerStacks<TrackingT>>, } impl<BufferT, TrackingT> Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { /// Creates a new scratchpad instance. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::Scratchpad; /// use std::mem::uninitialized; /// /// # fn main() { /// // Creates a scratchpad that can hold up to 256 bytes of data and up /// // to 4 allocation markers. The initial contents of each buffer are /// // ignored, so we can provide uninitialized data in order to reduce /// // the runtime overhead of creating a scratchpad. /// let scratchpad = unsafe { Scratchpad::new( /// uninitialized::<array_type_for_bytes!(u64, 256)>(), /// uninitialized::<array_type_for_markers!(usize, 4)>(), /// ) }; /// # } /// ``` #[inline(always)] #[cfg(feature = "unstable")] pub const fn new(buffer: BufferT, tracking: TrackingT) -> Self { Scratchpad { buffer: UnsafeCell::new(buffer), markers: RefCell::new(MarkerStacks { data: tracking, front: 0, back: ::core::usize::MAX, // Lazy initialization. }), } } /// Creates a new scratchpad instance. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::Scratchpad; /// use std::mem::uninitialized; /// /// # fn main() { /// // Creates a scratchpad that can hold up to 256 bytes of data and up /// // to 4 allocation markers. The initial contents of each buffer are /// // ignored, so we can provide uninitialized data in order to reduce /// // the runtime overhead of creating a scratchpad. /// let scratchpad = unsafe { Scratchpad::new( /// uninitialized::<array_type_for_bytes!(u64, 256)>(), /// uninitialized::<array_type_for_markers!(usize, 4)>(), /// ) }; /// # } /// ``` #[inline(always)] #[cfg(not(feature = "unstable"))] pub fn new(buffer: BufferT, tracking: TrackingT) -> Self { Scratchpad { buffer: UnsafeCell::new(buffer), markers: RefCell::new(MarkerStacks { back: tracking.capacity(), data: tracking, front: 0, }), } } } impl<BufferT, TrackingT> Scratchpad<BufferT, TrackingT> where BufferT: StaticBuffer, TrackingT: Tracking + StaticBuffer, { /// Creates a new instance for scratchpad types backed entirely by static /// arrays without initializing array memory. /// /// Since static array [`Buffer`] and [`Tracking`] types are owned by the /// scratchpad, and their sizes are known ahead of time to the scratchpad /// type, scratchpads using only static arrays for storage can be created /// without having to provide any parameters. /// /// Note that this cannot be `const` as there is no support in Rust for /// creating uninitialized values in `const` code. [`Scratchpad::new()`] /// must be used with the `unstable` crate feature enabled if `const` code /// is required. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate scratchpad; /// use scratchpad::Scratchpad; /// /// # fn main() { /// // Creates a scratchpad that can hold up to 256 bytes of data and up /// // to 4 allocation markers. /// let scratchpad = Scratchpad::< /// array_type_for_bytes!(u64, 256), /// array_type_for_markers!(usize, 4), /// >::static_new(); /// # } /// ``` /// /// [`Buffer`]: trait.Buffer.html /// [`Tracking`]: trait.Tracking.html /// [`Scratchpad::new()`]: #method.new #[inline(always)] pub fn static_new() -> Self { Scratchpad { buffer: unsafe { uninitialized() }, markers: RefCell::new(MarkerStacks { data: unsafe { uninitialized() }, front: 0, back: size_of::<TrackingT>() / size_of::<usize>(), }), } } } impl<BufferT, TrackingT> Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { /// Creates a marker at the front of the allocation buffer for subsequent /// allocations. /// /// # Examples /// /// ``` /// use scratchpad::Scratchpad; /// /// let scratchpad = Scratchpad::<[u64; 1], [usize; 1]>::new([0], [0]); /// /// let marker = scratchpad.mark_front().unwrap(); /// // `marker` can now be used for allocations... /// ``` pub fn mark_front<'scratchpad>( &'scratchpad self, ) -> Result<MarkerFront<'scratchpad, BufferT, TrackingT>, Error<()>> { let mut markers = self.markers.borrow_mut(); // `markers.back` is lazy-initialized when the "unstable" feature is // enabled so that `Scratchpad::new()` can be a `const` function. #[cfg(feature = "unstable")] { if markers.back == ::core::usize::MAX { markers.back = markers.data.capacity(); } } let index = markers.front; if index == markers.back { return Err(Error::new(ErrorKind::MarkerLimit, ())); } let buffer_offset = if index == 0 { 0 } else { markers.data.get(index - 1) }; markers.data.set(index, buffer_offset); markers.front = index + 1; Ok(MarkerFront { scratchpad: self, index, }) } /// Creates a marker at the back of the allocation buffer for subsequent /// allocations. /// /// # Examples /// /// ``` /// use scratchpad::Scratchpad; /// /// let scratchpad = Scratchpad::<[u64; 1], [usize; 1]>::new([0], [0]); /// /// let marker = scratchpad.mark_back().unwrap(); /// // `marker` can now be used for allocations... /// ``` pub fn mark_back<'scratchpad>( &'scratchpad self, ) -> Result<MarkerBack<'scratchpad, BufferT, TrackingT>, Error<()>> { let mut markers = self.markers.borrow_mut(); // `markers.back` is lazy-initialized when the "unstable" feature is // enabled so that `Scratchpad::new()` can be a `const` function. #[cfg(feature = "unstable")] { if markers.back == ::core::usize::MAX { markers.back = markers.data.capacity(); } } let mut index = markers.back; if index == markers.front { return Err(Error::new(ErrorKind::MarkerLimit, ())); } let buffer_offset = if index == markers.data.capacity() { unsafe { (*self.buffer.get()).as_bytes().len() } } else { markers.data.get(index) }; index -= 1; markers.data.set(index, buffer_offset); markers.back = index; Ok(MarkerBack { scratchpad: self, index, }) } } impl<BufferT, TrackingT> fmt::Debug for Scratchpad<BufferT, TrackingT> where BufferT: Buffer, TrackingT: Tracking, { #[inline] fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!( f, "Scratchpad {{ buffer.len = {}, markers: {:?} }}", unsafe { &*self.buffer.get() }.as_bytes().len(), self.markers.borrow(), ) } }