[emschwartz]

Closes #136.

RoaringBitmap::shrink_to_fit shrinks the container vector and every array and run store to its length. Bitmap stores are fixed-size boxes, so there is nothing to shrink there. RoaringTreemap::shrink_to_fit calls it on each bitmap and leaves empty partitions alone, since removing them is optimize's job.

The use case is the one @fjmaco measured on the issue: intersection and difference results keep the capacity of their inputs, so a 55,887-value intersection of two ~970K-value bitmaps holds 1,889 KiB of heap where a clone of it holds 119 KiB. Today that memory only comes back by rebuilding the bitmap, for example with *bitmap = bitmap.clone(). CRoaring has roaring_bitmap_shrink_to_fit for the same reason. This version returns () rather than CRoaring's count of freed bytes, to behave like Vec::shrink_to_fit, which is what a Rust caller will expect from the name. fjmaco offered on the issue to write this too. Happy to close this if fjmaco's version is underway.

Values and store kinds are unchanged. The one visible side effect is that statistics().n_bytes_array_containers goes down, since it is computed from capacity.

Tested with doctests.

Authored with Claude (claude-opus-5-5).

[Kerollmops]

Hey @emschwartz 👋

Thanks for the changes! The code looks good but I would rather prefer removing the ugly "hidden" methods and extensive tests for a simple addition like that and make sure we keep the codebase small and maintanable.

Have a nice day 🌵

[Kerollmops]

Suggested change

	#[cfg(test)]
	pub fn capacity(&self) -> usize {
	self.0.capacity()
	}

[Kerollmops]

Suggested change

	#[cfg(test)]
	impl RoaringBitmap {
	pub(crate) fn has_spare_capacity(&self) -> bool {
	use crate::bitmap::store::Store;
	self.containers.capacity() > self.containers.len()
	|| self.containers.iter().any(|container| match &container.store {
	Store::Array(array) => array.capacity() > array.len() as usize,
	Store::Bitmap(_) => false,
	Store::Run(runs) => runs.capacity() > runs.run_amount() as usize,
	})
	}
	}

[Kerollmops]

Suggested change

	#[test]
	fn shrink_to_fit() {
	// Key 3 is in both bitmaps but not in their intersection
	let mut lhs = RoaringBitmap::from_iter(
	(0..2000)
	.step_by(2)
	.chain((0x1_0000..0x2_0000).step_by(2))
	.chain(0x2_0000..0x2_1000)
	.chain([0x3_0000]),
	);
	let mut rhs = RoaringBitmap::from_iter(
	(1980..4000)
	.step_by(2)
	.chain((0x1_0000..0x2_0000).step_by(2))
	.chain((0x2_0000..0x2_0300).filter(|x| x % 0x100 < 0x10))
	.chain([0x3_0001]),
	);
	lhs.optimize();
	rhs.optimize();
	let mut bitmap = &lhs & &rhs;
	let stores: Vec<_> = bitmap.containers.iter().map(|c| &c.store).collect();
	let [Store::Array(array), Store::Bitmap(_), Store::Run(runs)] = stores[..] else {
	panic!("expected an array, a bitmap and a run container");
	};
	assert!(bitmap.containers.capacity() > bitmap.containers.len());
	assert!(array.capacity() > array.len() as usize);
	assert!(runs.capacity() > runs.run_amount() as usize);
	let before = bitmap.clone();
	bitmap.shrink_to_fit();
	assert!(!bitmap.has_spare_capacity());
	assert_eq!(bitmap, before);
	}
	#[test]
	fn shrink_to_fit_empty() {
	let mut bitmap = RoaringBitmap::from_iter([1, 0x1_0000]);
	bitmap.remove_range(..);
	assert!(bitmap.has_spare_capacity());
	bitmap.shrink_to_fit();
	assert!(!bitmap.has_spare_capacity());
	assert!(bitmap.is_empty());
	}

[Kerollmops]

Suggested change

	#[test]
	fn shrink_to_fit() {
	let lhs = RoaringTreemap::from_iter((0..2000).chain((1 << 32)..(1 << 32) + 2000));
	let rhs = RoaringTreemap::from_iter((1990..4000).chain((1 << 32) + 1990..(1 << 32) + 4000));
	let mut treemap = &lhs & &rhs;
	assert!(treemap.bitmaps().all(|(_, bitmap)| bitmap.has_spare_capacity()));
	let before = treemap.clone();
	treemap.shrink_to_fit();
	assert!(treemap.bitmaps().all(|(_, bitmap)| !bitmap.has_spare_capacity()));
	assert_eq!(treemap, before);
	}

[Kerollmops]

Suggested change

use crate::bitmap::store::Store;