NOTE
Bitmap
Compact set membership and counting over dense integer domains using bits.
This is a historical learning note and may contain outdated or incomplete understanding.
A bitmap represents membership of integer positions with bits: bit i records whether value/condition i is present.
It is extremely compact for dense bounded domains and supports fast set operations such as intersection/union through machine-word bit operations.
For sparse huge domains, a plain bitmap can waste space; compressed bitmaps or hash/set structures may be better. Map arbitrary values to bit positions only when the mapping/range is controlled.