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use std::{
fmt::Debug,
iter::Sum,
ops::{Add, AddAssign, Range, Sub},
};
#[derive(Debug)]
pub struct RangeAllocator<T> {
/// The range this allocator covers.
initial_range: Range<T>,
/// A Vec of ranges in this heap which are unused.
/// Must be ordered with ascending range start to permit short circuiting allocation.
/// No two ranges in this vec may overlap.
free_ranges: Vec<Range<T>>,
}
#[derive(Clone, Debug, PartialEq)]
pub struct RangeAllocationError<T> {
pub fragmented_free_length: T,
}
impl<T> RangeAllocator<T>
where
T: Clone + Copy + Add<Output = T> + AddAssign + Sub<Output = T> + Eq + PartialOrd + Debug,
{
pub fn new(range: Range<T>) -> Self {
RangeAllocator {
initial_range: range.clone(),
free_ranges: vec![range],
}
}
pub fn initial_range(&self) -> &Range<T> {
&self.initial_range
}
pub fn grow_to(&mut self, new_end: T) {
if let Some(last_range) = self.free_ranges.last_mut() {
last_range.end = new_end;
} else {
self.free_ranges.push(self.initial_range.end..new_end);
}
self.initial_range.end = new_end;
}
pub fn allocate_range(&mut self, length: T) -> Result<Range<T>, RangeAllocationError<T>> {
assert_ne!(length + length, length);
let mut best_fit: Option<(usize, Range<T>)> = None;
// This is actually correct. With the trait bound as it is, we have
// no way to summon a value of 0 directly, so we make one by subtracting
// something from itself. Once the trait bound can be changed, this can
// be fixed.
#[allow(clippy::eq_op)]
let mut fragmented_free_length = length - length;
for (index, range) in self.free_ranges.iter().cloned().enumerate() {
let range_length = range.end - range.start;
fragmented_free_length += range_length;
if range_length < length {
continue;
} else if range_length == length {
// Found a perfect fit, so stop looking.
best_fit = Some((index, range));
break;
}
best_fit = Some(match best_fit {
Some((best_index, best_range)) => {
// Find best fit for this allocation to reduce memory fragmentation.
if range_length < best_range.end - best_range.start {
(index, range)
} else {
(best_index, best_range.clone())
}
}
None => (index, range),
});
}
match best_fit {
Some((index, range)) => {
if range.end - range.start == length {
self.free_ranges.remove(index);
} else {
self.free_ranges[index].start += length;
}
Ok(range.start..(range.start + length))
}
None => Err(RangeAllocationError {
fragmented_free_length,
}),
}
}
pub fn free_range(&mut self, range: Range<T>) {
assert!(self.initial_range.start <= range.start && range.end <= self.initial_range.end);
assert!(range.start < range.end);
// Get insertion position.
let i = self
.free_ranges
.iter()
.position(|r| r.start > range.start)
.unwrap_or(self.free_ranges.len());
// Try merging with neighboring ranges in the free list.
// Before: |left|-(range)-|right|
if i > 0 && range.start == self.free_ranges[i - 1].end {
// Merge with |left|.
self.free_ranges[i - 1].end =
if i < self.free_ranges.len() && range.end == self.free_ranges[i].start {
// Check for possible merge with |left| and |right|.
let right = self.free_ranges.remove(i);
right.end
} else {
range.end
};
return;
} else if i < self.free_ranges.len() && range.end == self.free_ranges[i].start {
// Merge with |right|.
self.free_ranges[i].start = if i > 0 && range.start == self.free_ranges[i - 1].end {
// Check for possible merge with |left| and |right|.
let left = self.free_ranges.remove(i - 1);
left.start
} else {
range.start
};
return;
}
// Debug checks
assert!(
(i == 0 || self.free_ranges[i - 1].end < range.start)
&& (i >= self.free_ranges.len() || range.end < self.free_ranges[i].start)
);
self.free_ranges.insert(i, range);
}
/// Returns an iterator over allocated non-empty ranges
pub fn allocated_ranges(&self) -> impl Iterator<Item = Range<T>> + '_ {
let first = match self.free_ranges.first() {
Some(Range { ref start, .. }) if *start > self.initial_range.start => {
Some(self.initial_range.start..*start)
}
None => Some(self.initial_range.clone()),
_ => None,
};
let last = match self.free_ranges.last() {
Some(Range { end, .. }) if *end < self.initial_range.end => {
Some(*end..self.initial_range.end)
}
_ => None,
};
let mid = self
.free_ranges
.iter()
.zip(self.free_ranges.iter().skip(1))
.map(|(ra, rb)| ra.end..rb.start);
first.into_iter().chain(mid).chain(last)
}
pub fn reset(&mut self) {
self.free_ranges.clear();
self.free_ranges.push(self.initial_range.clone());
}
pub fn is_empty(&self) -> bool {
self.free_ranges.len() == 1 && self.free_ranges[0] == self.initial_range
}
}
impl<T: Copy + Sub<Output = T> + Sum> RangeAllocator<T> {
pub fn total_available(&self) -> T {
self.free_ranges
.iter()
.map(|range| range.end - range.start)
.sum()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_basic_allocation() {
let mut alloc = RangeAllocator::new(0..10);
// Test if an allocation works
assert_eq!(alloc.allocate_range(4), Ok(0..4));
assert!(alloc.allocated_ranges().eq(std::iter::once(0..4)));
// Free the prior allocation
alloc.free_range(0..4);
// Make sure the free actually worked
assert_eq!(alloc.free_ranges, vec![0..10]);
assert!(alloc.allocated_ranges().eq(std::iter::empty()));
}
#[test]
fn test_out_of_space() {
let mut alloc = RangeAllocator::new(0..10);
// Test if the allocator runs out of space correctly
assert_eq!(alloc.allocate_range(10), Ok(0..10));
assert!(alloc.allocated_ranges().eq(std::iter::once(0..10)));
assert!(alloc.allocate_range(4).is_err());
alloc.free_range(0..10);
}
#[test]
fn test_grow() {
let mut alloc = RangeAllocator::new(0..11);
// Test if the allocator runs out of space correctly
assert_eq!(alloc.allocate_range(10), Ok(0..10));
assert!(alloc.allocated_ranges().eq(std::iter::once(0..10)));
assert!(alloc.allocate_range(4).is_err());
alloc.grow_to(20);
assert_eq!(alloc.allocate_range(4), Ok(10..14));
alloc.free_range(0..14);
}
#[test]
fn test_dont_use_block_that_is_too_small() {
let mut alloc = RangeAllocator::new(0..10);
// Allocate three blocks then free the middle one and check for correct state
assert_eq!(alloc.allocate_range(3), Ok(0..3));
assert_eq!(alloc.allocate_range(3), Ok(3..6));
assert_eq!(alloc.allocate_range(3), Ok(6..9));
alloc.free_range(3..6);
assert_eq!(alloc.free_ranges, vec![3..6, 9..10]);
assert_eq!(
alloc.allocated_ranges().collect::<Vec<Range<i32>>>(),
vec![0..3, 6..9]
);
// Now request space that the middle block can fill, but the end one can't.
assert_eq!(alloc.allocate_range(3), Ok(3..6));
}
#[test]
fn test_free_blocks_in_middle() {
let mut alloc = RangeAllocator::new(0..100);
// Allocate many blocks then free every other block.
assert_eq!(alloc.allocate_range(10), Ok(0..10));
assert_eq!(alloc.allocate_range(10), Ok(10..20));
assert_eq!(alloc.allocate_range(10), Ok(20..30));
assert_eq!(alloc.allocate_range(10), Ok(30..40));
assert_eq!(alloc.allocate_range(10), Ok(40..50));
assert_eq!(alloc.allocate_range(10), Ok(50..60));
assert_eq!(alloc.allocate_range(10), Ok(60..70));
assert_eq!(alloc.allocate_range(10), Ok(70..80));
assert_eq!(alloc.allocate_range(10), Ok(80..90));
assert_eq!(alloc.allocate_range(10), Ok(90..100));
assert_eq!(alloc.free_ranges, vec![]);
assert!(alloc.allocated_ranges().eq(std::iter::once(0..100)));
alloc.free_range(10..20);
alloc.free_range(30..40);
alloc.free_range(50..60);
alloc.free_range(70..80);
alloc.free_range(90..100);
// Check that the right blocks were freed.
assert_eq!(
alloc.free_ranges,
vec![10..20, 30..40, 50..60, 70..80, 90..100]
);
assert_eq!(
alloc.allocated_ranges().collect::<Vec<Range<i32>>>(),
vec![0..10, 20..30, 40..50, 60..70, 80..90]
);
// Fragment the memory on purpose a bit.
assert_eq!(alloc.allocate_range(6), Ok(10..16));
assert_eq!(alloc.allocate_range(6), Ok(30..36));
assert_eq!(alloc.allocate_range(6), Ok(50..56));
assert_eq!(alloc.allocate_range(6), Ok(70..76));
assert_eq!(alloc.allocate_range(6), Ok(90..96));
// Check for fragmentation.
assert_eq!(
alloc.free_ranges,
vec![16..20, 36..40, 56..60, 76..80, 96..100]
);
assert_eq!(
alloc.allocated_ranges().collect::<Vec<Range<i32>>>(),
vec![0..16, 20..36, 40..56, 60..76, 80..96]
);
// Fill up the fragmentation
assert_eq!(alloc.allocate_range(4), Ok(16..20));
assert_eq!(alloc.allocate_range(4), Ok(36..40));
assert_eq!(alloc.allocate_range(4), Ok(56..60));
assert_eq!(alloc.allocate_range(4), Ok(76..80));
assert_eq!(alloc.allocate_range(4), Ok(96..100));
// Check that nothing is free.
assert_eq!(alloc.free_ranges, vec![]);
assert!(alloc.allocated_ranges().eq(std::iter::once(0..100)));
}
#[test]
fn test_ignore_block_if_another_fits_better() {
let mut alloc = RangeAllocator::new(0..10);
// Allocate blocks such that the only free spaces available are 3..6 and 9..10
// in order to prepare for the next test.
assert_eq!(alloc.allocate_range(3), Ok(0..3));
assert_eq!(alloc.allocate_range(3), Ok(3..6));
assert_eq!(alloc.allocate_range(3), Ok(6..9));
alloc.free_range(3..6);
assert_eq!(alloc.free_ranges, vec![3..6, 9..10]);
assert_eq!(
alloc.allocated_ranges().collect::<Vec<Range<i32>>>(),
vec![0..3, 6..9]
);
// Now request space that can be filled by 3..6 but should be filled by 9..10
// because 9..10 is a perfect fit.
assert_eq!(alloc.allocate_range(1), Ok(9..10));
}
#[test]
fn test_merge_neighbors() {
let mut alloc = RangeAllocator::new(0..9);
assert_eq!(alloc.allocate_range(3), Ok(0..3));
assert_eq!(alloc.allocate_range(3), Ok(3..6));
assert_eq!(alloc.allocate_range(3), Ok(6..9));
alloc.free_range(0..3);
alloc.free_range(6..9);
alloc.free_range(3..6);
assert_eq!(alloc.free_ranges, vec![0..9]);
assert!(alloc.allocated_ranges().eq(std::iter::empty()));
}
}