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//! An incremental buffer extension implementation.
mod partial;
pub use partial::{IncrementalIterator, Partial};
pub trait OrderedCollection {
/// Append an item to the collection.
fn append(&mut self, item: usize);
/// Get an exclusive reference to the last element in the collection.
///
/// ## Safety
/// Must be valid if and only if there was a previous call to `append`.
unsafe fn last_appended(&mut self) -> &mut usize;
/// Get a slice corresponding to the current items.
#[cfg(test)]
fn slice(&self) -> &[usize];
}
impl OrderedCollection for &'_ mut Vec<usize> {
fn append(&mut self, item: usize) {
self.push(item);
}
unsafe fn last_appended(&mut self) -> &mut usize {
// SAFETY: `append` was previously called.
unsafe { self.last_mut().unwrap_unchecked() }
}
#[cfg(test)]
fn slice(&self) -> &[usize] {
self
}
}
impl OrderedCollection for Vec<usize> {
fn append(&mut self, item: usize) {
self.push(item);
}
unsafe fn last_appended(&mut self) -> &mut usize {
// SAFETY: `append` was previously called.
unsafe { self.last_mut().unwrap_unchecked() }
}
#[cfg(test)]
fn slice(&self) -> &[usize] {
self
}
}
pub trait ExtendIncremental {
/// Extend the internal collection, ensuring not to add more than `limit_size`
/// to the buffer in total, and not step the underlying iterator more than `limit_steps` times.
///
/// Returns the total of the elements added to the buffer.
fn extend_bounded(&mut self, limit_size: u16, limit_steps: usize) -> u16;
/// Extend the internal collection, ensuring not to add more than `limit_size` to the
/// buffer in total.
///
/// Returns the total of the elements added to the buffer.
fn extend_unbounded(&mut self, limit_size: u16) -> u16;
}
/// Incremental collection of an [`Iterator`] of [`usize`] into a vector.
///
/// See the [`extended_bounded`](Self::extend_bounded) method for more detail.
pub struct Incremental<C: OrderedCollection, I: Iterator<Item = usize>> {
/// The internal vector.
vec: C,
/// The internal iterator.
sizes: IncrementalIterator<I>,
}
impl<C: OrderedCollection, I: Iterator<Item = usize>> ExtendIncremental for Incremental<C, I> {
#[inline]
fn extend_bounded(&mut self, limit_size: u16, limit_steps: usize) -> u16 {
self.extend_impl(limit_size, limit_steps)
}
fn extend_unbounded(&mut self, limit_size: u16) -> u16 {
self.extend_impl(limit_size, ())
}
}
impl<C: OrderedCollection, I: Iterator<Item = usize>> Incremental<C, I> {
/// Initialize an [`IncrementalExtension`] targeting the given vector with the
/// provided iterator.
///
/// New elements will be appended to the vector.
pub fn new(vec: C, sizes: I) -> Self {
Self {
vec,
sizes: IncrementalIterator::new(sizes),
}
}
#[cfg(test)]
pub fn view(&self) -> &[usize] {
self.vec.slice()
}
#[inline]
fn extend_impl<D: Decrement>(&mut self, limit_size: u16, limit_steps: D) -> u16 {
// SAFETY: extend_impl_inverted returns a value less than `limit_size`.
unsafe { limit_size.unchecked_sub(self.extend_impl_inverted(limit_size, limit_steps)) }
}
/// The actual implementation of the 'reversed' version, which returns the number of remaining
/// elements to be added.
#[inline]
fn extend_impl_inverted<D: Decrement>(
&mut self,
mut remaining: u16,
mut limit_steps: D,
) -> u16 {
while remaining > 0 {
if limit_steps.is_finished() && !self.sizes.is_incomplete() {
return remaining;
}
match self.sizes.next_partial(remaining) {
Some(Partial { new, size }) => {
unsafe {
// SAFETY: `next_partial` returns a `size` which is at most `limit_size`.
remaining = remaining.unchecked_sub(size);
if new {
// SAFETY: we can only be in this branch if the guard call to
// `self.sizes.next_is_not_new()` returned false, in which
// if `limit_steps.is_finished()`, we would have returned earlier.
limit_steps.decr();
self.vec.append(size as usize);
} else {
// SAFETY: there must have been a previous call to `self.vec.append`
// since the first item returned by an `IncrementalIterator` is
// guaranteed to be new.
let buf_last = self.vec.last_appended();
// SAFETY: the underlying iterator yields `usize`, so the size of each
// element in total cannot exceed a `usize`.
*buf_last = buf_last.unchecked_add(size as usize);
}
}
}
None => {
return remaining;
}
}
}
0
}
}
/// An internal trait for a counter which can be decreased until it is finished.
///
/// The implementation for [`usize`] represents a 'bounded' counter, and the implementation for
/// `()` represents an 'unbounded' counter.
trait Decrement {
/// Whether or not we have finished decrementing this value.
fn is_finished(&self) -> bool;
/// Decrement the value.
///
/// # Safety
/// Can only be called if `is_finished` returned false.
unsafe fn decr(&mut self);
}
impl Decrement for () {
#[inline]
fn is_finished(&self) -> bool {
false
}
#[inline]
unsafe fn decr(&mut self) {}
}
impl Decrement for usize {
#[inline]
fn is_finished(&self) -> bool {
*self == 0
}
#[inline]
unsafe fn decr(&mut self) {
// SAFETY: only called if `is_finished` returned false, in which case `self >= 1`.
unsafe { *self = self.unchecked_sub(1) }
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_incremental() {
let mut vec = Vec::new();
let mut incr = Incremental::new(&mut vec, [1, 6, 2, 3, 5, 3, 5].into_iter());
assert_eq!(incr.extend_bounded(5, 2), 5);
assert_eq!(incr.view(), &[1, 4]);
assert_eq!(incr.extend_bounded(2, 1), 2);
assert_eq!(incr.view(), &[1, 6]);
assert_eq!(incr.extend_bounded(1, 1), 1);
assert_eq!(incr.view(), &[1, 6, 1]);
assert_eq!(incr.extend_bounded(0, 1), 0);
assert_eq!(incr.view(), &[1, 6, 1]);
assert_eq!(incr.extend_bounded(10, 1), 4);
assert_eq!(incr.view(), &[1, 6, 2, 3]);
assert_eq!(incr.extend_bounded(2, 3), 2);
assert_eq!(incr.view(), &[1, 6, 2, 3, 2]);
assert_eq!(incr.extend_bounded(1, 2), 1);
assert_eq!(incr.view(), &[1, 6, 2, 3, 3]);
assert_eq!(incr.extend_bounded(1, 4), 1);
assert_eq!(incr.view(), &[1, 6, 2, 3, 4]);
assert_eq!(incr.extend_bounded(0, 0), 0);
assert_eq!(incr.view(), &[1, 6, 2, 3, 4]);
assert_eq!(incr.extend_bounded(100, 4), 9);
assert_eq!(incr.view(), &[1, 6, 2, 3, 5, 3, 5]);
}
}