greg/sparsemap
1
0
Fork 0
Code Issues Pull requests 1 Packages Projects Releases Wiki Activity

spelling

Browse source
This commit is contained in:
Gregory Burd 2024-05-03 21:12:57 +00:00
parent 9dccdcbf76
commit 4e2b4bde26
1 changed files with 9 additions and 9 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

18
README.md
View file

@ -4,9 +4,9 @@ Bitsets, also called bitmaps, are commonly used as fast data structures.
Unfortunately, they can use too much memory. To compensate, we often use
compressed bitmaps.
`sparsemap` is a sparse, compressed bitmap. In best case, it can store 2048
bits in just 8 bytes. In worst case, it stores the 2048 bits uncompressed and
requires additional 8 bytes of overhead.
`sparsemap` is a sparse, compressed bitmap. In the best case, it can store 2048
bits in just 8 bytes. In the worst case, it stores the 2048 bits uncompressed and
requires an additional 8 bytes of overhead.
The "best" case happens when large consecutive sequences of the bits are
either set ("1") or not set ("0"). If your numbers are consecutive 64bit
@ -20,7 +20,7 @@ Each sm_bitvec_t has an additional descriptor (2 bits). A single word prepended
to each sm_bitvec_t describes its condition. The descriptor word and the
sm_bitvec_t's have the same size. The descriptor of a sm_bitvec_t
specifies whether the sm_bitvec_t consists only of set bits ("1"), unset
bits ("0") or has a mixed payload. In the first and second case the
bits ("0") or has a mixed payload. In the first and second cases, the
sm_bitvec_t is not stored.
An example shows a sequence of 4 x 16 bits (here, each sm_bitvec_t and the
@ -40,9 +40,9 @@ The actual memory sequence looks like this:
0000000011001110 0110010101111001
Instead of storing 8 Words (16 bytes), we only store 2 Words (2 bytes): one
for the descriptor, one for last sm_bitvec_t #7.
for the descriptor, and one for the last sm_bitvec_t #7.
The sparsemap stores a list of chunk maps, and for each chunk map it stores the
The sparsemap stores a list of chunk maps, and for each chunk map, it stores the
absolute address (i.e. if the user sets bit 0 and bit 10000, and the chunk map
capacity is 2048, the sparsemap creates two chunk maps; the first starts at
offset 0, the second starts at offset 8192).
@ -55,9 +55,9 @@ Review the `examples/*` and `tests/*` code.
## Final words
This bitmap has efficient compression when used on long sequences of set (or
unset) bits (i.e. with a word size of 64bit, and a payload of consecutive
unset) bits (i.e. with a word size of 64 bit and a payload of consecutive
numbers without gaps, the payload of 2048 x sizeof(uint64_t) = 16kb will occupy
only 8 bytes!
only 8 bytes!).
However, if the sequence is not consecutive and has gaps, it's possible that
the compression is inefficient, and the size (in the worst case) is identical
@ -68,6 +68,6 @@ include in `lib` the amalgamated (git `2dc8070`) and well-known
[Roaring Bitmaps](https://github.com/RoaringBitmap/CRoaring/tree/master) and
use it in the soak test to ensure our results are as accurate as theirs.
This library was originally created for [hamsterdb](http://hamsterdb.com) in
This library was created for [hamsterdb](http://hamsterdb.com) in
C++ and then translated to C and further improved by Greg Burd <greg@burd.me>
for use in LMDB and OpenLDAP.