add dense codec

[PSeitz]

100k elements
test null_index::dense::bench::bench_dense_codec_translate_dense_to_orig ... bench:   1,191,989 ns/iter (+/- 48,555)
test null_index::dense::bench::bench_dense_codec_translate_orig_to_dense ... bench:     286,443 ns/iter (+/- 2,509)

[PSeitz]

this method will eventually work on a &mut Vec<u32> probably

[codecov-commenter]

Codecov Report

Merging #1711 (789d29c) into main (96c93a6) will increase coverage by 0.03%.
The diff coverage is 99.30%.

@@            Coverage Diff             @@
##             main    #1711      +/-   ##
==========================================
+ Coverage   94.07%   94.10%   +0.03%     
==========================================
  Files         259      261       +2     
  Lines       49637    49925     +288     
==========================================
+ Hits        46694    46980     +286     
- Misses       2943     2945       +2     

Impacted Files	Coverage Δ
fastfield_codecs/src/lib.rs	98.89% <ø> (ø)
fastfield_codecs/src/null_index/dense.rs	99.29% <99.29%> (ø)
fastfield_codecs/src/null_index/mod.rs	100.00% <100.00%> (ø)
src/postings/stacker/expull.rs	98.57% <0.00%> (-0.48%)	⬇️
src/schema/schema.rs	98.91% <0.00%> (+0.13%)	⬆️

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[fulmicoton]

Unfortunately I didn't manage to find rust code to spew this...

pub fn count_ones(block: u64, pos_in_block: u32) -> u32 {
    unsafe { core::arch::x86_64::_bzhi_u64(block, pos_in_block) }.count_ones()
}

[fulmicoton]

nitpick:
I tend to prefer a for-loop.

Suggested change

	loop {
	for i in block_pos.. {
	}
	panic!("...

[fulmicoton]

Question:

Do we want to use a faster algorithm? Should we add a TODO in there?

[PSeitz]

I did extend the benchmark to see what happens if we do a full scan, but have very large steps, so that time is mostly spent in the linear search. Linear search seems to be fine, as it is a minor contributor to cost.

test null_index::dense::bench::bench_dense_codec_translate_dense_to_orig_90percent_filled_full_scan              ... bench:  25,282,406 ns/iter (+/- 13,598,775)
test null_index::dense::bench::bench_dense_codec_translate_dense_to_orig_90percent_filled_random_stride_100          ... bench:   1,083,598 ns/iter (+/- 322,955)
test null_index::dense::bench::bench_dense_codec_translate_dense_to_orig_90percent_filled_random_stride_50_000 ... bench:      15,187 ns/iter (+/- 4,774)

[fulmicoton]

Suggested change

pub use null_index::*;

That's actually nicer to not expose symbol here. The module is perfectly nice.

[PSeitz]

I added it to get temporarily rid of the annoying unused warnings

[shikhar]

I think you can place #[allow(dead_code)] on the mod null_index declaration instead :)