//! Small lists of entity references.
use crate::packed_option::ReservedValue;
use crate::EntityRef;
use alloc::vec::Vec;
use core::marker::PhantomData;
use core::mem;
/// A small list of entity references allocated from a pool.
///
/// An `EntityList<T>` type provides similar functionality to `Vec<T>`, but with some important
/// differences in the implementation:
///
/// 1. Memory is allocated from a `ListPool<T>` instead of the global heap.
/// 2. The footprint of an entity list is 4 bytes, compared with the 24 bytes for `Vec<T>`.
/// 3. An entity list doesn't implement `Drop`, leaving it to the pool to manage memory.
///
/// The list pool is intended to be used as a LIFO allocator. After building up a larger data
/// structure with many list references, the whole thing can be discarded quickly by clearing the
/// pool.
///
/// # Safety
///
/// Entity lists are not as safe to use as `Vec<T>`, but they never jeopardize Rust's memory safety
/// guarantees. These are the problems to be aware of:
///
/// - If you lose track of an entity list, its memory won't be recycled until the pool is cleared.
/// This can cause the pool to grow very large with leaked lists.
/// - If entity lists are used after their pool is cleared, they may contain garbage data, and
/// modifying them may corrupt other lists in the pool.
/// - If an entity list is used with two different pool instances, both pools are likely to become
/// corrupted.
///
/// Entity lists can be cloned, but that operation should only be used as part of cloning the whole
/// function they belong to. *Cloning an entity list does not allocate new memory for the clone*.
/// It creates an alias of the same memory.
///
/// Entity lists cannot be hashed and compared for equality because it's not possible to compare the
/// contents of the list without the pool reference.
///
/// # Implementation
///
/// The `EntityList` itself is designed to have the smallest possible footprint. This is important
/// because it is used inside very compact data structures like `InstructionData`. The list
/// contains only a 32-bit index into the pool's memory vector, pointing to the first element of
/// the list.
///
/// The pool is just a single `Vec<T>` containing all of the allocated lists. Each list is
/// represented as three contiguous parts:
///
/// 1. The number of elements in the list.
/// 2. The list elements.
/// 3. Excess capacity elements.
///
/// The total size of the three parts is always a power of two, and the excess capacity is always
/// as small as possible. This means that shrinking a list may cause the excess capacity to shrink
/// if a smaller power-of-two size becomes available.
///
/// Both growing and shrinking a list may cause it to be reallocated in the pool vector.
///
/// The index stored in an `EntityList` points to part 2, the list elements. The value 0 is
/// reserved for the empty list which isn't allocated in the vector.
#[derive(Clone, Debug)]
pub struct EntityList<T: EntityRef + ReservedValue> {
index: u32,
unused: PhantomData<T>,
}
/// Create an empty list.
impl<T: EntityRef + ReservedValue> Default for EntityList<T> {
fn default() -> Self {
Self {
index: 0,
unused: PhantomData,
}
}
}
/// A memory pool for storing lists of `T`.
#[derive(Clone, Debug)]
pub struct ListPool<T: EntityRef + ReservedValue> {
// The main array containing the lists.
data: Vec<T>,
// Heads of the free lists, one for each size class.
free: Vec<usize>,
}
/// Lists are allocated in sizes that are powers of two, starting from 4.
/// Each power of two is assigned a size class number, so the size is `4 << SizeClass`.
type SizeClass = u8;
/// Get the size of a given size class. The size includes the length field, so the maximum list
/// length is one less than the class size.
fn sclass_size(sclass: SizeClass) -> usize {
4 << sclass
}
/// Get the size class to use for a given list length.
/// This always leaves room for the length element in addition to the list elements.
fn sclass_for_length(len: usize) -> SizeClass {
30 - (len as u32 | 3).leading_zeros() as SizeClass
}
/// Is `len` the minimum length in its size class?
fn is_sclass_min_length(len: usize) -> bool {
len > 3 && len.is_power_of_two()
}
impl<T: EntityRef + ReservedValue> ListPool<T> {
/// Create a new list pool.
pub fn new() -> Self {
Self {
data: Vec::new(),
free: Vec::new(),
}
}
/// Clear the pool, forgetting about all lists that use it.
///
/// This invalidates any existing entity lists that used this pool to allocate memory.
///
/// The pool's memory is not released to the operating system, but kept around for faster
/// allocation in the future.
pub fn clear(&mut self) {
self.data.clear();
self.free.clear();
}
/// Read the length of a list field, if it exists.
fn len_of(&self, list: &EntityList<T>) -> Option<usize> {
let idx = list.index as usize;
// `idx` points at the list elements. The list length is encoded in the element immediately
// before the list elements.
//
// The `wrapping_sub` handles the special case 0, which is the empty list. This way, the
// cost of the bounds check that we have to pay anyway is co-opted to handle the special
// case of the empty list.
self.data.get(idx.wrapping_sub(1)).map(|len| len.index())
}
/// Allocate a storage block with a size given by `sclass`.
///
/// Returns the first index of an available segment of `self.data` containing
/// `sclass_size(sclass)` elements. The allocated memory is filled with reserved
/// values.
fn alloc(&mut self, sclass: SizeClass) -> usize {
// First try the free list for this size class.
match self.free.get(sclass as usize).cloned() {
Some(head) if head > 0 => {
// The free list pointers are offset by 1, using 0 to terminate the list.
// A block on the free list has two entries: `[ 0, next ]`.
// The 0 is where the length field would be stored for a block in use.
// The free list heads and the next pointer point at the `next` field.
self.free[sclass as usize] = self.data[head].index();
head - 1
}
_ => {
// Nothing on the free list. Allocate more memory.
let offset = self.data.len();
self.data
.resize(offset + sclass_size(sclass), T::reserved_value());
offset
}
}
}
/// Free a storage block with a size given by `sclass`.
///
/// This must be a block that was previously allocated by `alloc()` with the same size class.
fn free(&mut self, block: usize, sclass: SizeClass) {
let sclass = sclass as usize;
// Make sure we have a free-list head for `sclass`.
if self.free.len() <= sclass {
self.free.resize(sclass + 1, 0);
}
// Make sure the length field is cleared.
self.data[block] = T::new(0);
// Insert the block on the free list which is a single linked list.
self.data[block + 1] = T::new(self.free[sclass]);
self.free[sclass] = block + 1
}
/// Returns two mutable slices representing the two requested blocks.
///
/// The two returned slices can be longer than the blocks. Each block is located at the front
/// of the respective slice.
fn mut_slices(&mut self, block0: usize, block1: usize) -> (&mut [T], &mut [T]) {
if block0 < block1 {
let (s0, s1) = self.data.split_at_mut(block1);
(&mut s0[block0..], s1)
} else {
let (s1, s0) = self.data.split_at_mut(block0);
(s0, &mut s1[block1..])
}
}
/// Reallocate a block to a different size class.
///
/// Copy `elems_to_copy` elements from the old to the new block.
fn realloc(
&mut self,
block: usize,
from_sclass: SizeClass,
to_sclass: SizeClass,
elems_to_copy: usize,
) -> usize {
debug_assert!(elems_to_copy <= sclass_size(from_sclass));
debug_assert!(elems_to_copy <= sclass_size(to_sclass));
let new_block = self.alloc(to_sclass);
if elems_to_copy > 0 {
let (old, new) = self.mut_slices(block, new_block);
(&mut new[0..elems_to_copy]).copy_from_slice(&old[0..elems_to_copy]);
}
self.free(block, from_sclass);
new_block
}
}
impl<T: EntityRef + ReservedValue> EntityList<T> {
/// Create a new empty list.
pub fn new() -> Self {
Default::default()
}
/// Create a new list with the contents initialized from a slice.
pub fn from_slice(slice: &[T], pool: &mut ListPool<T>) -> Self {
let len = slice.len();
if len == 0 {
return Self::new();
}
let block = pool.alloc(sclass_for_length(len));
pool.data[block] = T::new(len);
pool.data[block + 1..=block + len].copy_from_slice(slice);
Self {
index: (block + 1) as u32,
unused: PhantomData,
}
}
/// Returns `true` if the list has a length of 0.
pub fn is_empty(&self) -> bool {
// 0 is a magic value for the empty list. Any list in the pool array must have a positive
// length.
self.index == 0
}
/// Get the number of elements in the list.
pub fn len(&self, pool: &ListPool<T>) -> usize {
// Both the empty list and any invalidated old lists will return `None`.
pool.len_of(self).unwrap_or(0)
}
/// Returns `true` if the list is valid
pub fn is_valid(&self, pool: &ListPool<T>) -> bool {
// We consider an empty list to be valid
self.is_empty() || pool.len_of(self) != None
}
/// Get the list as a slice.
pub fn as_slice<'a>(&'a self, pool: &'a ListPool<T>) -> &'a [T] {
let idx = self.index as usize;
match pool.len_of(self) {
None => &[],
Some(len) => &pool.data[idx..idx + len],
}
}
/// Get a single element from the list.
pub fn get(&self, index: usize, pool: &ListPool<T>) -> Option<T> {
self.as_slice(pool).get(index).cloned()
}
/// Get the first element from the list.
pub fn first(&self, pool: &ListPool<T>) -> Option<T> {
if self.is_empty() {
None
} else {
Some(pool.data[self.index as usize])
}
}
/// Get the list as a mutable slice.
pub fn as_mut_slice<'a>(&'a mut self, pool: &'a mut ListPool<T>) -> &'a mut [T] {
let idx = self.index as usize;
match pool.len_of(self) {
None => &mut [],
Some(len) => &mut pool.data[idx..idx + len],
}
}
/// Get a mutable reference to a single element from the list.
pub fn get_mut<'a>(&'a mut self, index: usize, pool: &'a mut ListPool<T>) -> Option<&'a mut T> {
self.as_mut_slice(pool).get_mut(index)
}
/// Removes all elements from the list.
///
/// The memory used by the list is put back in the pool.
pub fn clear(&mut self, pool: &mut ListPool<T>) {
let idx = self.index as usize;
match pool.len_of(self) {
None => debug_assert_eq!(idx, 0, "Invalid pool"),
Some(len) => pool.free(idx - 1, sclass_for_length(len)),
}
// Switch back to the empty list representation which has no storage.
self.index = 0;
}
/// Take all elements from this list and return them as a new list. Leave this list empty.
///
/// This is the equivalent of `Option::take()`.
pub fn take(&mut self) -> Self {
mem::replace(self, Default::default())
}
/// Appends an element to the back of the list.
/// Returns the index where the element was inserted.
pub fn push(&mut self, element: T, pool: &mut ListPool<T>) -> usize {
let idx = self.index as usize;
match pool.len_of(self) {
None => {
// This is an empty list. Allocate a block and set length=1.
debug_assert_eq!(idx, 0, "Invalid pool");
let block = pool.alloc(sclass_for_length(1));
pool.data[block] = T::new(1);
pool.data[block + 1] = element;
self.index = (block + 1) as u32;
0
}
Some(len) => {
// Do we need to reallocate?
let new_len = len + 1;
let block;
if is_sclass_min_length(new_len) {
// Reallocate, preserving length + all old elements.
let sclass = sclass_for_length(len);
block = pool.realloc(idx - 1, sclass, sclass + 1, len + 1);
self.index = (block + 1) as u32;
} else {
block = idx - 1;
}
pool.data[block + new_len] = element;
pool.data[block] = T::new(new_len);
len
}
}
}
/// Grow list by adding `count` reserved-value elements at the end.
///
/// Returns a mutable slice representing the whole list.
fn grow<'a>(&'a mut self, count: usize, pool: &'a mut ListPool<T>) -> &'a mut [T] {
let idx = self.index as usize;
let new_len;
let block;
match pool.len_of(self) {
None => {
// This is an empty list. Allocate a block.
debug_assert_eq!(idx, 0, "Invalid pool");
if count == 0 {
return &mut [];
}
new_len = count;
block = pool.alloc(sclass_for_length(new_len));
self.index = (block + 1) as u32;
}
Some(len) => {
// Do we need to reallocate?
let sclass = sclass_for_length(len);
new_len = len + count;
let new_sclass = sclass_for_length(new_len);
if new_sclass != sclass {
block = pool.realloc(idx - 1, sclass, new_sclass, len + 1);
self.index = (block + 1) as u32;
} else {
block = idx - 1;
}
}
}
pool.data[block] = T::new(new_len);
&mut pool.data[block + 1..block + 1 + new_len]
}
/// Appends multiple elements to the back of the list.
pub fn extend<I>(&mut self, elements: I, pool: &mut ListPool<T>)
where
I: IntoIterator<Item = T>,
{
// TODO: use `size_hint()` to reduce reallocations.
for x in elements {
self.push(x, pool);
}
}
/// Inserts an element as position `index` in the list, shifting all elements after it to the
/// right.
pub fn insert(&mut self, index: usize, element: T, pool: &mut ListPool<T>) {
// Increase size by 1.
self.push(element, pool);
// Move tail elements.
let seq = self.as_mut_slice(pool);
if index < seq.len() {
let tail = &mut seq[index..];
for i in (1..tail.len()).rev() {
tail[i] = tail[i - 1];
}
tail[0] = element;
} else {
debug_assert_eq!(index, seq.len());
}
}
/// Removes the element at position `index` from the list. Potentially linear complexity.
pub fn remove(&mut self, index: usize, pool: &mut ListPool<T>) {
let len;
{
let seq = self.as_mut_slice(pool);
len = seq.len();
debug_assert!(index < len);
// Copy elements down.
for i in index..len - 1 {
seq[i] = seq[i + 1];
}
}
// Check if we deleted the last element.
if len == 1 {
self.clear(pool);
return;
}
// Do we need to reallocate to a smaller size class?
let mut block = self.index as usize - 1;
if is_sclass_min_length(len) {
let sclass = sclass_for_length(len);
block = pool.realloc(block, sclass, sclass - 1, len);
self.index = (block + 1) as u32;
}
// Finally adjust the length.
pool.data[block] = T::new(len - 1);
}
/// Removes the element at `index` in constant time by switching it with the last element of
/// the list.
pub fn swap_remove(&mut self, index: usize, pool: &mut ListPool<T>) {
let len = self.len(pool);
debug_assert!(index < len);
if index == len - 1 {
self.remove(index, pool);
} else {
{
let seq = self.as_mut_slice(pool);
seq.swap(index, len - 1);
}
self.remove(len - 1, pool);
}
}
/// Grow the list by inserting `count` elements at `index`.
///
/// The new elements are not initialized, they will contain whatever happened to be in memory.
/// Since the memory comes from the pool, this will be either zero entity references or
/// whatever where in a previously deallocated list.
pub fn grow_at(&mut self, index: usize, count: usize, pool: &mut ListPool<T>) {
let data = self.grow(count, pool);
// Copy elements after `index` up.
for i in (index + count..data.len()).rev() {
data[i] = data[i - count];
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use super::{sclass_for_length, sclass_size};
use crate::EntityRef;
/// An opaque reference to an instruction in a function.
#[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct Inst(u32);
entity_impl!(Inst, "inst");
#[test]
fn size_classes() {
assert_eq!(sclass_size(0), 4);
assert_eq!(sclass_for_length(0), 0);
assert_eq!(sclass_for_length(1), 0);
assert_eq!(sclass_for_length(2), 0);
assert_eq!(sclass_for_length(3), 0);
assert_eq!(sclass_for_length(4), 1);
assert_eq!(sclass_for_length(7), 1);
assert_eq!(sclass_for_length(8), 2);
assert_eq!(sclass_size(1), 8);
for l in 0..300 {
assert!(sclass_size(sclass_for_length(l)) >= l + 1);
}
}
#[test]
fn block_allocator() {
let mut pool = ListPool::<Inst>::new();
let b1 = pool.alloc(0);
let b2 = pool.alloc(1);
let b3 = pool.alloc(0);
assert_ne!(b1, b2);
assert_ne!(b1, b3);
assert_ne!(b2, b3);
pool.free(b2, 1);
let b2a = pool.alloc(1);
let b2b = pool.alloc(1);
assert_ne!(b2a, b2b);
// One of these should reuse the freed block.
assert!(b2a == b2 || b2b == b2);
// Check the free lists for a size class smaller than the largest seen so far.
pool.free(b1, 0);
pool.free(b3, 0);
let b1a = pool.alloc(0);
let b3a = pool.alloc(0);
assert_ne!(b1a, b3a);
assert!(b1a == b1 || b1a == b3);
assert!(b3a == b1 || b3a == b3);
}
#[test]
fn empty_list() {
let pool = &mut ListPool::<Inst>::new();
let mut list = EntityList::<Inst>::default();
{
let ilist = &list;
assert!(ilist.is_empty());
assert_eq!(ilist.len(pool), 0);
assert_eq!(ilist.as_slice(pool), &[]);
assert_eq!(ilist.get(0, pool), None);
assert_eq!(ilist.get(100, pool), None);
}
assert_eq!(list.as_mut_slice(pool), &[]);
assert_eq!(list.get_mut(0, pool), None);
assert_eq!(list.get_mut(100, pool), None);
list.clear(pool);
assert!(list.is_empty());
assert_eq!(list.len(pool), 0);
assert_eq!(list.as_slice(pool), &[]);
assert_eq!(list.first(pool), None);
}
#[test]
fn from_slice() {
let pool = &mut ListPool::<Inst>::new();
let list = EntityList::<Inst>::from_slice(&[Inst(0), Inst(1)], pool);
assert!(!list.is_empty());
assert_eq!(list.len(pool), 2);
assert_eq!(list.as_slice(pool), &[Inst(0), Inst(1)]);
assert_eq!(list.get(0, pool), Some(Inst(0)));
assert_eq!(list.get(100, pool), None);
let list = EntityList::<Inst>::from_slice(&[], pool);
assert!(list.is_empty());
assert_eq!(list.len(pool), 0);
assert_eq!(list.as_slice(pool), &[]);
assert_eq!(list.get(0, pool), None);
assert_eq!(list.get(100, pool), None);
}
#[test]
fn push() {
let pool = &mut ListPool::<Inst>::new();
let mut list = EntityList::<Inst>::default();
let i1 = Inst::new(1);
let i2 = Inst::new(2);
let i3 = Inst::new(3);
let i4 = Inst::new(4);
assert_eq!(list.push(i1, pool), 0);
assert_eq!(list.len(pool), 1);
assert!(!list.is_empty());
assert_eq!(list.as_slice(pool), &[i1]);
assert_eq!(list.first(pool), Some(i1));
assert_eq!(list.get(0, pool), Some(i1));
assert_eq!(list.get(1, pool), None);
assert_eq!(list.push(i2, pool), 1);
assert_eq!(list.len(pool), 2);
assert!(!list.is_empty());
assert_eq!(list.as_slice(pool), &[i1, i2]);
assert_eq!(list.first(pool), Some(i1));
assert_eq!(list.get(0, pool), Some(i1));
assert_eq!(list.get(1, pool), Some(i2));
assert_eq!(list.get(2, pool), None);
assert_eq!(list.push(i3, pool), 2);
assert_eq!(list.len(pool), 3);
assert!(!list.is_empty());
assert_eq!(list.as_slice(pool), &[i1, i2, i3]);
assert_eq!(list.first(pool), Some(i1));
assert_eq!(list.get(0, pool), Some(i1));
assert_eq!(list.get(1, pool), Some(i2));
assert_eq!(list.get(2, pool), Some(i3));
assert_eq!(list.get(3, pool), None);
// This triggers a reallocation.
assert_eq!(list.push(i4, pool), 3);
assert_eq!(list.len(pool), 4);
assert!(!list.is_empty());
assert_eq!(list.as_slice(pool), &[i1, i2, i3, i4]);
assert_eq!(list.first(pool), Some(i1));
assert_eq!(list.get(0, pool), Some(i1));
assert_eq!(list.get(1, pool), Some(i2));
assert_eq!(list.get(2, pool), Some(i3));
assert_eq!(list.get(3, pool), Some(i4));
assert_eq!(list.get(4, pool), None);
list.extend([i1, i1, i2, i2, i3, i3, i4, i4].iter().cloned(), pool);
assert_eq!(list.len(pool), 12);
assert_eq!(
list.as_slice(pool),
&[i1, i2, i3, i4, i1, i1, i2, i2, i3, i3, i4, i4]
);
}
#[test]
fn insert_remove() {
let pool = &mut ListPool::<Inst>::new();
let mut list = EntityList::<Inst>::default();
let i1 = Inst::new(1);
let i2 = Inst::new(2);
let i3 = Inst::new(3);
let i4 = Inst::new(4);
list.insert(0, i4, pool);
assert_eq!(list.as_slice(pool), &[i4]);
list.insert(0, i3, pool);
assert_eq!(list.as_slice(pool), &[i3, i4]);
list.insert(2, i2, pool);
assert_eq!(list.as_slice(pool), &[i3, i4, i2]);
list.insert(2, i1, pool);
assert_eq!(list.as_slice(pool), &[i3, i4, i1, i2]);
list.remove(3, pool);
assert_eq!(list.as_slice(pool), &[i3, i4, i1]);
list.remove(2, pool);
assert_eq!(list.as_slice(pool), &[i3, i4]);
list.remove(0, pool);
assert_eq!(list.as_slice(pool), &[i4]);
list.remove(0, pool);
assert_eq!(list.as_slice(pool), &[]);
assert!(list.is_empty());
}
#[test]
fn growing() {
let pool = &mut ListPool::<Inst>::new();
let mut list = EntityList::<Inst>::default();
let i1 = Inst::new(1);
let i2 = Inst::new(2);
let i3 = Inst::new(3);
let i4 = Inst::new(4);
// This is not supposed to change the list.
list.grow_at(0, 0, pool);
assert_eq!(list.len(pool), 0);
assert!(list.is_empty());
list.grow_at(0, 2, pool);
assert_eq!(list.len(pool), 2);
list.as_mut_slice(pool).copy_from_slice(&[i2, i3]);
list.grow_at(1, 0, pool);
assert_eq!(list.as_slice(pool), &[i2, i3]);
list.grow_at(1, 1, pool);
list.as_mut_slice(pool)[1] = i1;
assert_eq!(list.as_slice(pool), &[i2, i1, i3]);
// Append nothing at the end.
list.grow_at(3, 0, pool);
assert_eq!(list.as_slice(pool), &[i2, i1, i3]);
// Append something at the end.
list.grow_at(3, 1, pool);
list.as_mut_slice(pool)[3] = i4;
assert_eq!(list.as_slice(pool), &[i2, i1, i3, i4]);
}
}