diff --git a/README.md b/README.md index c4c3715..5a27ad3 100644 --- a/README.md +++ b/README.md @@ -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 for use in LMDB and OpenLDAP.