== 位图（Bitmap） ==
===Bit array ===
A bit array (also known as bit map , bit set, bit string, or bit vector) is an array data structure that compactly stores bits. It can be used to implement a simple set data structure.
https://en.wikipedia.org/wiki/Bit_array
* Population / Hamming weight
* Find first one
* Inversion
===位图压缩===
Verbatim Bitmap 带来的空间开销，尤其是bitmap很稀疏的情况下。为此，人们发明了Bitmap Compression机制。不仅可以降低空间开销，而且可以显著加快运算速度。