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// -*- coding: utf-8 -*-
// ------------------------------------------------------------------------------------------------
// Copyright © 2021, stack-graphs authors.
// Licensed under either of Apache License, Version 2.0, or MIT license, at your option.
// Please see the LICENSE-APACHE or LICENSE-MIT files in this distribution for license details.
// ------------------------------------------------------------------------------------------------
//! Cache-friendly arena allocation for stack graph data.
//!
//! A stack graph is composed of instances of many different data types, and to store the graph
//! structure itself, we need cyclic or self-referential data types. The typical way to achieve
//! this in Rust is to use [arena allocation][], where all of the instances of a particular type
//! are stored in a single vector. You then use indexes into this vector to store references to a
//! data instance. Because indexes are just numbers, you don't run afoul of borrow checker. And
//! because all instances live together in a continguous region of memory, your data access
//! patterns are very cache-friendly.
//!
//! This module implements a simple arena allocation scheme for stack graphs. An
//! [`Arena<T>`][`Arena`] is an arena that holds all of the instances of type `T` for a stack
//! graph. A [`Handle<T>`][`Handle`] holds the index of a particular instance of `T` in its arena.
//! All of our stack graph data types then use handles to refer to other parts of the stack graph.
//!
//! Note that our arena implementation does not support deletion! Any content that you add to a
//! [`StackGraph`][] will live as long as the stack graph itself does. The entire region of memory
//! for each arena will be freed in a single operation when the stack graph is dropped.
//!
//! [arena allocation]: https://en.wikipedia.org/wiki/Region-based_memory_management
//! [`Arena`]: struct.Arena.html
//! [`Handle`]: struct.Handle.html
//! [`StackGraph`]: ../graph/struct.StackGraph.html
use std::cell::Cell;
use std::fmt::Debug;
use std::hash::Hash;
use std::hash::Hasher;
use std::marker::PhantomData;
use std::mem::MaybeUninit;
use std::num::NonZeroU32;
use std::ops::Index;
use std::ops::IndexMut;
use bitvec::vec::BitVec;
use controlled_option::Niche;
use crate::utils::cmp_option;
use crate::utils::equals_option;
//-------------------------------------------------------------------------------------------------
// Arenas and handles
/// A handle to an instance of type `T` that was allocated from an [`Arena`][].
///
/// #### Safety
///
/// Because of the type parameter `T`, the compiler can ensure that you don't use a handle for one
/// type to index into an arena of another type. However, if you have multiple arenas for the
/// _same type_, we do not do anything to ensure that you only use a handle with the corresponding
/// arena.
#[repr(transparent)]
pub struct Handle<T> {
index: NonZeroU32,
_phantom: PhantomData<T>,
}
impl<T> Handle<T> {
pub(crate) fn new(index: NonZeroU32) -> Handle<T> {
Handle {
index,
_phantom: PhantomData,
}
}
#[inline(always)]
pub fn as_u32(self) -> u32 {
self.index.get()
}
#[inline(always)]
pub fn as_usize(self) -> usize {
self.index.get() as usize
}
}
impl<T> Niche for Handle<T> {
type Output = u32;
#[inline]
fn none() -> Self::Output {
0
}
#[inline]
fn is_none(value: &Self::Output) -> bool {
*value == 0
}
#[inline]
fn into_some(value: Self) -> Self::Output {
value.index.get()
}
#[inline]
fn from_some(value: Self::Output) -> Self {
Self::new(unsafe { NonZeroU32::new_unchecked(value) })
}
}
// Normally we would #[derive] all of these traits, but the auto-derived implementations all
// require that T implement the trait as well. We don't store any real instances of T inside of
// Handle, so our implementations do _not_ require that.
impl<T> Clone for Handle<T> {
fn clone(&self) -> Handle<T> {
Handle::new(self.index)
}
}
impl<T> Copy for Handle<T> {}
impl<T> Debug for Handle<T> {
fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
f.debug_struct("Handle")
.field("index", &self.index)
.finish()
}
}
impl<T> Eq for Handle<T> {}
impl<T> Hash for Handle<T> {
fn hash<H: Hasher>(&self, state: &mut H) {
self.index.hash(state);
}
}
impl<T> Ord for Handle<T> {
fn cmp(&self, other: &Self) -> std::cmp::Ordering {
self.index.cmp(&other.index)
}
}
impl<T> PartialEq for Handle<T> {
fn eq(&self, other: &Self) -> bool {
self.index == other.index
}
}
impl<T> PartialOrd for Handle<T> {
fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
self.index.partial_cmp(&other.index)
}
}
// Handles are always Send and Sync, even if the underlying types are not. After all, a handle is
// just a number! And you _also_ need access to the Arena (which _won't_ be Send/Sync if T isn't)
// to dereference the handle.
unsafe impl<T> Send for Handle<T> {}
unsafe impl<T> Sync for Handle<T> {}
/// Manages the life cycle of instances of type `T`. You can allocate new instances of `T` from
/// the arena. All of the instances managed by this arena will be dropped as a single operation
/// when the arena itself is dropped.
pub struct Arena<T> {
items: Vec<MaybeUninit<T>>,
}
impl<T> Drop for Arena<T> {
fn drop(&mut self) {
unsafe {
let items = std::mem::transmute::<_, &mut [T]>(&mut self.items[1..]) as *mut [T];
items.drop_in_place();
}
}
}
impl<T> Arena<T> {
/// Creates a new arena.
pub fn new() -> Arena<T> {
Arena {
items: vec![MaybeUninit::uninit()],
}
}
/// Clear the arena, keeping underlying allocated capacity. After this, all previous handles into
/// the arena are invalid.
#[inline(always)]
pub fn clear(&mut self) {
self.items.truncate(1);
}
/// Adds a new instance to this arena, returning a stable handle to it.
///
/// Note that we do not deduplicate instances of `T` in any way. If you add two instances that
/// have the same content, you will get distinct handles for each one.
pub fn add(&mut self, item: T) -> Handle<T> {
let index = self.items.len() as u32;
self.items.push(MaybeUninit::new(item));
Handle::new(unsafe { NonZeroU32::new_unchecked(index) })
}
/// Dereferences a handle to an instance owned by this arena, returning a reference to it.
pub fn get(&self, handle: Handle<T>) -> &T {
unsafe { std::mem::transmute(&self.items[handle.as_usize()]) }
}
///
/// Dereferences a handle to an instance owned by this arena, returning a mutable reference to
/// it.
pub fn get_mut(&mut self, handle: Handle<T>) -> &mut T {
unsafe { std::mem::transmute(&mut self.items[handle.as_usize()]) }
}
/// Returns an iterator of all of the handles in this arena. (Note that this iterator does not
/// retain a reference to the arena!)
pub fn iter_handles(&self) -> impl Iterator<Item = Handle<T>> {
(1..self.items.len())
.into_iter()
.map(|index| Handle::new(unsafe { NonZeroU32::new_unchecked(index as u32) }))
}
/// Returns a pointer to this arena's storage.
pub(crate) fn as_ptr(&self) -> *const T {
self.items.as_ptr() as *const T
}
/// Returns the number of instances stored in this arena.
#[inline(always)]
pub fn len(&self) -> usize {
self.items.len()
}
}
//-------------------------------------------------------------------------------------------------
// Supplemental arenas
/// A supplemental arena lets you store additional data about some data type that is itself stored
/// in an [`Arena`][].
///
/// We implement `Index` and `IndexMut` for a more ergonomic syntax. Please note that when
/// indexing in an _immutable_ context, we **_panic_** if you try to access data for a handle that
/// doesn't exist in the arena. (Use the [`get`][] method if you don't know whether the value
/// exists or not.) In a _mutable_ context, we automatically create a `Default` instance of the
/// type if there isn't already an instance for that handle in the arena.
///
/// ```
/// # use stack_graphs::arena::Arena;
/// # use stack_graphs::arena::SupplementalArena;
/// // We need an Arena to create handles.
/// let mut arena = Arena::<u32>::new();
/// let handle = arena.add(1);
///
/// let mut supplemental = SupplementalArena::<u32, String>::new();
///
/// // But indexing will panic if the element doesn't already exist.
/// // assert_eq!(supplemental[handle].as_str(), "");
///
/// // The `get` method is always safe, since it returns an Option.
/// assert_eq!(supplemental.get(handle), None);
///
/// // Once we've added the element to the supplemental arena, indexing
/// // won't panic anymore.
/// supplemental[handle] = "hello".to_string();
/// assert_eq!(supplemental[handle].as_str(), "hello");
/// ```
///
/// [`Arena`]: struct.Arena.html
/// [`get`]: #method.get
pub struct SupplementalArena<H, T> {
items: Vec<MaybeUninit<T>>,
_phantom: PhantomData<H>,
}
impl<H, T> Drop for SupplementalArena<H, T> {
fn drop(&mut self) {
unsafe {
let items = std::mem::transmute::<_, &mut [T]>(&mut self.items[1..]) as *mut [T];
items.drop_in_place();
}
}
}
impl<H, T> SupplementalArena<H, T> {
/// Creates a new, empty supplemental arena.
pub fn new() -> SupplementalArena<H, T> {
SupplementalArena {
items: vec![MaybeUninit::uninit()],
_phantom: PhantomData,
}
}
/// Clear the supplemantal arena, keeping underlying allocated capacity. After this,
/// all previous handles into the arena are invalid.
#[inline(always)]
pub fn clear(&mut self) {
self.items.truncate(1);
}
/// Creates a new, empty supplemental arena, preallocating enough space to store supplemental
/// data for all of the instances that have already been allocated in a (regular) arena.
pub fn with_capacity(arena: &Arena<H>) -> SupplementalArena<H, T> {
let mut items = Vec::with_capacity(arena.items.len());
items[0] = MaybeUninit::uninit();
SupplementalArena {
items,
_phantom: PhantomData,
}
}
/// Returns the item belonging to a particular handle, if it exists.
pub fn get(&self, handle: Handle<H>) -> Option<&T> {
self.items
.get(handle.as_usize())
.map(|x| unsafe { &*(x.as_ptr()) })
}
/// Returns a mutable reference to the item belonging to a particular handle, if it exists.
pub fn get_mut(&mut self, handle: Handle<H>) -> Option<&mut T> {
self.items
.get_mut(handle.as_usize())
.map(|x| unsafe { &mut *(x.as_mut_ptr()) })
}
/// Returns a pointer to this arena's storage.
pub(crate) fn as_ptr(&self) -> *const T {
self.items.as_ptr() as *const T
}
/// Returns the number of instances stored in this arena.
#[inline(always)]
pub fn len(&self) -> usize {
self.items.len()
}
/// Iterate over the items in this arena.
pub(crate) fn iter(&self) -> impl Iterator<Item = (Handle<T>, &T)> {
self.items
.iter()
.enumerate()
.skip(1)
.map(|(i, x)| (Handle::from_some(i as u32), unsafe { &*(x.as_ptr()) }))
}
}
impl<H, T> SupplementalArena<H, T>
where
T: Default,
{
/// Returns a mutable reference to the item belonging to a particular handle, creating it first
/// (using the type's `Default` implementation) if it doesn't already exist.
pub fn get_mut_or_default(&mut self, handle: Handle<H>) -> &mut T {
let index = handle.as_usize();
if self.items.len() <= index {
self.items
.resize_with(index + 1, || MaybeUninit::new(T::default()));
}
unsafe { std::mem::transmute(&mut self.items[handle.as_usize()]) }
}
}
impl<H, T> Default for SupplementalArena<H, T> {
fn default() -> SupplementalArena<H, T> {
SupplementalArena::new()
}
}
impl<H, T> Index<Handle<H>> for SupplementalArena<H, T> {
type Output = T;
fn index(&self, handle: Handle<H>) -> &T {
unsafe { std::mem::transmute(&self.items[handle.as_usize()]) }
}
}
impl<H, T> IndexMut<Handle<H>> for SupplementalArena<H, T>
where
T: Default,
{
fn index_mut(&mut self, handle: Handle<H>) -> &mut T {
self.get_mut_or_default(handle)
}
}
//-------------------------------------------------------------------------------------------------
// Handle sets
/// Contains a set of handles, encoded efficiently using a bit set.
#[repr(C)]
pub struct HandleSet<T> {
elements: BitVec<u32, bitvec::order::Lsb0>,
_phantom: PhantomData<T>,
}
impl<T> HandleSet<T> {
/// Creates a new, empty handle set.
pub fn new() -> HandleSet<T> {
HandleSet::default()
}
/// Removes all elements from this handle set.
pub fn clear(&mut self) {
self.elements.clear();
}
/// Returns whether this set contains a particular handle.
pub fn contains(&self, handle: Handle<T>) -> bool {
let index = handle.as_usize();
self.elements.get(index).map(|bit| *bit).unwrap_or(false)
}
/// Adds a handle to this set.
pub fn add(&mut self, handle: Handle<T>) {
let index = handle.as_usize();
if self.elements.len() <= index {
self.elements.resize(index + 1, false);
}
let mut bit = unsafe { self.elements.get_unchecked_mut(index) };
*bit = true;
}
/// Removes a handle from this set.
pub fn remove(&mut self, handle: Handle<T>) {
let index = handle.as_usize();
if let Some(mut bit) = self.elements.get_mut(index) {
*bit = false;
}
}
/// Returns an iterator of all of the handles in this set.
pub fn iter(&self) -> impl Iterator<Item = Handle<T>> + '_ {
self.elements
.iter_ones()
.map(|index| Handle::new(unsafe { NonZeroU32::new_unchecked(index as u32) }))
}
/// Returns a pointer to this set's storage.
pub(crate) fn as_ptr(&self) -> *const u32 {
self.elements.as_bitptr().pointer()
}
/// Returns the number of instances stored in this arena.
#[inline(always)]
pub(crate) fn len(&self) -> usize {
self.elements.as_raw_slice().len()
}
}
impl<T> Default for HandleSet<T> {
fn default() -> HandleSet<T> {
HandleSet {
elements: BitVec::default(),
_phantom: PhantomData,
}
}
}
//-------------------------------------------------------------------------------------------------
// Arena-allocated lists
/// An arena-allocated singly-linked list.
///
/// Linked lists are often a poor choice because they aren't very cache-friendly. However, this
/// linked list implementation _should_ be cache-friendly, since the individual cells are allocated
/// out of an arena.
#[repr(C)]
#[derive(Niche)]
pub struct List<T> {
// The value of this handle will be EMPTY_LIST_HANDLE if the list is empty. For an
// Option<List<T>>, the value will be zero (via the Option<NonZero> optimization) if the list
// is None.
#[niche]
cells: Handle<ListCell<T>>,
}
#[doc(hidden)]
#[repr(C)]
pub struct ListCell<T> {
head: T,
// The value of this handle will be EMPTY_LIST_HANDLE if this is the last element of the list.
tail: Handle<ListCell<T>>,
}
const EMPTY_LIST_HANDLE: NonZeroU32 = unsafe { NonZeroU32::new_unchecked(u32::MAX) };
// An arena that's used to manage `List<T>` instances.
//
// (Note that the arena doesn't store `List<T>` itself; it stores the `ListCell<T>`s that the lists
// are made of.)
pub type ListArena<T> = Arena<ListCell<T>>;
impl<T> List<T> {
/// Creates a new `ListArena` that will manage lists of this type.
pub fn new_arena() -> ListArena<T> {
ListArena::new()
}
/// Returns whether this list is empty.
#[inline(always)]
pub fn is_empty(&self) -> bool {
self.cells.index == EMPTY_LIST_HANDLE
}
/// Returns an empty list.
pub fn empty() -> List<T> {
List {
cells: Handle::new(EMPTY_LIST_HANDLE),
}
}
pub fn from_handle(handle: Handle<ListCell<T>>) -> List<T> {
List { cells: handle }
}
/// Returns a handle to the head of the list.
pub fn handle(&self) -> Handle<ListCell<T>> {
self.cells
}
/// Pushes a new element onto the front of this list.
pub fn push_front(&mut self, arena: &mut ListArena<T>, head: T) {
self.cells = arena.add(ListCell {
head,
tail: self.cells,
});
}
/// Removes and returns the element at the front of this list. If the list is empty, returns
/// `None`.
pub fn pop_front<'a>(&mut self, arena: &'a ListArena<T>) -> Option<&'a T> {
if self.is_empty() {
return None;
}
let cell = arena.get(self.cells);
self.cells = cell.tail;
Some(&cell.head)
}
/// Returns an iterator over the elements of this list.
pub fn iter<'a>(mut self, arena: &'a ListArena<T>) -> impl Iterator<Item = &'a T> + 'a {
std::iter::from_fn(move || self.pop_front(arena))
}
}
impl<T> List<T> {
pub fn equals_with<F>(mut self, arena: &ListArena<T>, mut other: List<T>, mut eq: F) -> bool
where
F: FnMut(&T, &T) -> bool,
{
loop {
if self.cells == other.cells {
return true;
}
if !equals_option(self.pop_front(arena), other.pop_front(arena), &mut eq) {
return false;
}
}
}
pub fn cmp_with<F>(
mut self,
arena: &ListArena<T>,
mut other: List<T>,
mut cmp: F,
) -> std::cmp::Ordering
where
F: FnMut(&T, &T) -> std::cmp::Ordering,
{
use std::cmp::Ordering;
loop {
if self.cells == other.cells {
return Ordering::Equal;
}
match cmp_option(self.pop_front(arena), other.pop_front(arena), &mut cmp) {
Ordering::Equal => (),
result @ _ => return result,
}
}
}
}
impl<T> List<T>
where
T: Eq,
{
pub fn equals(self, arena: &ListArena<T>, other: List<T>) -> bool {
self.equals_with(arena, other, |a, b| *a == *b)
}
}
impl<T> List<T>
where
T: Ord,
{
pub fn cmp(self, arena: &ListArena<T>, other: List<T>) -> std::cmp::Ordering {
self.cmp_with(arena, other, |a, b| a.cmp(b))
}
}
// Normally we would #[derive] all of these traits, but the auto-derived implementations all
// require that T implement the trait as well. We don't store any real instances of T inside of
// List, so our implementations do _not_ require that.
impl<T> Clone for List<T> {
fn clone(&self) -> List<T> {
List { cells: self.cells }
}
}
impl<T> Copy for List<T> {}
//-------------------------------------------------------------------------------------------------
// Reversible arena-allocated list
/// An arena-allocated list that can be reversed.
///
/// Well, that is, you can reverse a [`List`][] just fine by yourself. This type takes care of
/// doing that for you, and importantly, _saves the result_ so that if you only have to compute the
/// reversal once even if you need to access it multiple times.
///
/// [`List`]: struct.List.html
#[repr(C)]
#[derive(Niche)]
pub struct ReversibleList<T> {
#[niche]
cells: Handle<ReversibleListCell<T>>,
}
#[repr(C)]
#[doc(hidden)]
pub struct ReversibleListCell<T> {
head: T,
tail: Handle<ReversibleListCell<T>>,
reversed: Cell<Option<Handle<ReversibleListCell<T>>>>,
}
// An arena that's used to manage `ReversibleList<T>` instances.
//
// (Note that the arena doesn't store `ReversibleList<T>` itself; it stores the
// `ReversibleListCell<T>`s that the lists are made of.)
pub type ReversibleListArena<T> = Arena<ReversibleListCell<T>>;
impl<T> ReversibleList<T> {
/// Creates a new `ReversibleListArena` that will manage lists of this type.
pub fn new_arena() -> ReversibleListArena<T> {
ReversibleListArena::new()
}
/// Returns whether this list is empty.
#[inline(always)]
pub fn is_empty(&self) -> bool {
ReversibleListCell::is_empty_handle(self.cells)
}
/// Returns an empty list.
pub fn empty() -> ReversibleList<T> {
ReversibleList {
cells: ReversibleListCell::empty_handle(),
}
}
/// Returns whether we have already calculated the reversal of this list.
pub fn have_reversal(&self, arena: &ReversibleListArena<T>) -> bool {
if self.is_empty() {
// The empty list is already reversed.
return true;
}
arena.get(self.cells).reversed.get().is_some()
}
/// Pushes a new element onto the front of this list.
pub fn push_front(&mut self, arena: &mut ReversibleListArena<T>, head: T) {
self.cells = arena.add(ReversibleListCell::new(head, self.cells, None));
}
/// Removes and returns the element at the front of this list. If the list is empty, returns
/// `None`.
pub fn pop_front<'a>(&mut self, arena: &'a ReversibleListArena<T>) -> Option<&'a T> {
if self.is_empty() {
return None;
}
let cell = arena.get(self.cells);
self.cells = cell.tail;
Some(&cell.head)
}
/// Returns an iterator over the elements of this list.
pub fn iter<'a>(
mut self,
arena: &'a ReversibleListArena<T>,
) -> impl Iterator<Item = &'a T> + 'a {
std::iter::from_fn(move || self.pop_front(arena))
}
}
impl<T> ReversibleList<T>
where
T: Clone,
{
/// Reverses the list. Since we're already caching everything in an arena, we make sure to
/// only calculate the reversal once, returning it as-is if you call this function multiple
/// times.
pub fn reverse(&mut self, arena: &mut ReversibleListArena<T>) {
if self.is_empty() {
return;
}
self.ensure_reversal_available(arena);
self.cells = arena.get(self.cells).reversed.get().unwrap();
}
/// Ensures that the reversal of this list is available. It can be useful to precalculate this
/// when you have mutable access to the arena, so that you can then reverse and un-reverse the
/// list later when you only have shared access to it.
pub fn ensure_reversal_available(&mut self, arena: &mut ReversibleListArena<T>) {
// First check to see if the list has already been reversed.
if self.is_empty() {
// The empty list is already reversed.
return;
}
if arena.get(self.cells).reversed.get().is_some() {
return;
}
// If not, reverse the list and cache the result.
let new_reversed = ReversibleListCell::reverse(self.cells, arena);
arena.get(self.cells).reversed.set(Some(new_reversed));
}
}
impl<T> ReversibleList<T> {
/// Reverses the list, assuming that the reversal has already been computed. If it hasn't we
/// return an error.
pub fn reverse_reused(&mut self, arena: &ReversibleListArena<T>) -> Result<(), ()> {
if self.is_empty() {
// The empty list is already reversed.
return Ok(());
}
self.cells = arena.get(self.cells).reversed.get().ok_or(())?;
Ok(())
}
}
impl<T> ReversibleListCell<T> {
fn new(
head: T,
tail: Handle<ReversibleListCell<T>>,
reversed: Option<Handle<ReversibleListCell<T>>>,
) -> ReversibleListCell<T> {
ReversibleListCell {
head,
tail,
reversed: Cell::new(reversed),
}
}
fn empty_handle() -> Handle<ReversibleListCell<T>> {
Handle::new(EMPTY_LIST_HANDLE)
}
fn is_empty_handle(handle: Handle<ReversibleListCell<T>>) -> bool {
handle.index == EMPTY_LIST_HANDLE
}
}
impl<T> ReversibleListCell<T>
where
T: Clone,
{
fn reverse(
forwards: Handle<ReversibleListCell<T>>,
arena: &mut ReversibleListArena<T>,
) -> Handle<ReversibleListCell<T>> {
let mut reversed = ReversibleListCell::empty_handle();
let mut current = forwards;
while !ReversibleListCell::is_empty_handle(current) {
let cell = arena.get(current);
let head = cell.head.clone();
current = cell.tail;
reversed = arena.add(Self::new(
head,
reversed,
// The reversal of the reversal that we just calculated is our original list! Go
// ahead and cache that away preemptively.
if ReversibleListCell::is_empty_handle(current) {
Some(forwards)
} else {
None
},
));
}
reversed
}
}
impl<T> ReversibleList<T> {
pub fn equals_with<F>(
mut self,
arena: &ReversibleListArena<T>,
mut other: ReversibleList<T>,
mut eq: F,
) -> bool
where
F: FnMut(&T, &T) -> bool,
{
loop {
if self.cells == other.cells {
return true;
}
if !equals_option(self.pop_front(arena), other.pop_front(arena), &mut eq) {
return false;
}
}
}
pub fn cmp_with<F>(
mut self,
arena: &ReversibleListArena<T>,
mut other: ReversibleList<T>,
mut cmp: F,
) -> std::cmp::Ordering
where
F: FnMut(&T, &T) -> std::cmp::Ordering,
{
use std::cmp::Ordering;
loop {
if self.cells == other.cells {
return Ordering::Equal;
}
match cmp_option(self.pop_front(arena), other.pop_front(arena), &mut cmp) {
Ordering::Equal => (),
result @ _ => return result,
}
}
}
}
impl<T> ReversibleList<T>
where
T: Eq,
{
pub fn equals(self, arena: &ReversibleListArena<T>, other: ReversibleList<T>) -> bool {
self.equals_with(arena, other, |a, b| *a == *b)
}
}
impl<T> ReversibleList<T>
where
T: Ord,
{
pub fn cmp(
self,
arena: &ReversibleListArena<T>,
other: ReversibleList<T>,
) -> std::cmp::Ordering {
self.cmp_with(arena, other, |a, b| a.cmp(b))
}
}
// Normally we would #[derive] all of these traits, but the auto-derived implementations all
// require that T implement the trait as well. We don't store any real instances of T inside of
// ReversibleList, so our implementations do _not_ require that.
impl<T> Clone for ReversibleList<T> {
fn clone(&self) -> ReversibleList<T> {
ReversibleList { cells: self.cells }
}
}
impl<T> Copy for ReversibleList<T> {}
//-------------------------------------------------------------------------------------------------
// Arena-allocated deque
/// An arena-allocated deque.
///
/// Under the covers, this is implemented as a [`List`][]. Because these lists are singly-linked,
/// we can only add elements to, and remove them from, one side of the list.
///
/// To handle this, each deque stores its contents either _forwards_ or _backwards_. We
/// automatically shift between these two representations as needed, depending on the requirements
/// of each method.
///
/// Note that we cache the result of reversing the list, so it should be quick to switch back and
/// forth between representations _as long as you have not added any additional elements to the
/// deque_! If performance is critical, you should ensure that you don't call methods in a pattern
/// that causes the deque to reverse itself each time you add an element.
///
/// [`List`]: struct.List.html
#[repr(C)]
#[derive(Niche)]
pub struct Deque<T> {
#[niche]
list: ReversibleList<T>,
direction: DequeDirection,
}
#[repr(C)]
#[derive(Clone, Copy, Eq, Hash, Ord, PartialEq, PartialOrd)]
enum DequeDirection {
Forwards,
Backwards,
}
impl std::ops::Not for DequeDirection {
type Output = DequeDirection;
fn not(self) -> DequeDirection {
match self {
DequeDirection::Forwards => DequeDirection::Backwards,
DequeDirection::Backwards => DequeDirection::Forwards,
}
}
}
// An arena that's used to manage `Deque<T>` instances.
pub type DequeArena<T> = ReversibleListArena<T>;
impl<T> Deque<T> {
/// Creates a new `DequeArena` that will manage deques of this type.
pub fn new_arena() -> DequeArena<T> {
ReversibleList::new_arena()
}
/// Returns whether this deque is empty.
#[inline(always)]
pub fn is_empty(&self) -> bool {
self.list.is_empty()
}
/// Returns an empty deque.
pub fn empty() -> Deque<T> {
Deque {
list: ReversibleList::empty(),
// A philosophical question for you: is the empty list forwards or backwards? It
// doesn't really matter which one we choose here; if we immediately start pushing onto
// the back, we'll "reverse" the current list before proceeding, but reversing the
// empty list is a no-op.
direction: DequeDirection::Forwards,
}
}
/// Returns whether we have already calculated the reversal of this deque.
pub fn have_reversal(&self, arena: &DequeArena<T>) -> bool {
self.list.have_reversal(arena)
}
fn is_backwards(&self) -> bool {
matches!(self.direction, DequeDirection::Backwards)
}
fn is_forwards(&self) -> bool {
matches!(self.direction, DequeDirection::Forwards)
}
/// Returns an iterator over the contents of this deque, with no guarantee about the ordering of
/// the elements. (By not caring about the ordering of the elements, you can call this method
/// regardless of which direction the deque's elements are currently stored. And that, in
/// turn, means that we only need shared access to the arena, and not mutable access to it.)
pub fn iter_unordered<'a>(&self, arena: &'a DequeArena<T>) -> impl Iterator<Item = &'a T> + 'a {
self.list.iter(arena)
}
}
impl<T> Deque<T>
where
T: Clone,
{
/// Ensures that this deque has computed its backwards-facing list of elements.
pub fn ensure_backwards(&mut self, arena: &mut DequeArena<T>) {
if self.is_backwards() {
return;
}
self.list.reverse(arena);
self.direction = DequeDirection::Backwards;
}
/// Ensures that this deque has computed its forwards-facing list of elements.
pub fn ensure_forwards(&mut self, arena: &mut DequeArena<T>) {
if self.is_forwards() {
return;
}
self.list.reverse(arena);
self.direction = DequeDirection::Forwards;
}
/// Pushes a new element onto the front of this deque.
pub fn push_front(&mut self, arena: &mut DequeArena<T>, element: T) {
self.ensure_forwards(arena);
self.list.push_front(arena, element);
}
/// Pushes a new element onto the back of this deque.
pub fn push_back(&mut self, arena: &mut DequeArena<T>, element: T) {
self.ensure_backwards(arena);
self.list.push_front(arena, element);
}
/// Removes and returns the element from the front of this deque. If the deque is empty,
/// returns `None`.
pub fn pop_front<'a>(&mut self, arena: &'a mut DequeArena<T>) -> Option<&'a T> {
self.ensure_forwards(arena);
self.list.pop_front(arena)
}
/// Removes and returns the element from the back of this deque. If the deque is empty,