Man page 9 : bitmap_bitremap

bitmap_bitremap - Apply map defined by a pair of bitmaps to a single bit

int bitmap_bitremap(int  oldbit , const unsigned long *  old , const unsigned long *  new , int  bits );

oldbit
    bit position to be mapped

old
    defines domain of map

new
    defines range of map

bits
    number of bits in each of these bitmaps

Let old and new define a mapping of bit positions, such that whatever position is held by the n-th set bit in old is mapped to the n-th set bit in new. In the more general case, allowing for the possibility that the weight AqwAq of new is less than the weight of old, map the position of the n-th set bit in old to the position of the m-th set bit in new, where m == n % w.

The positions of unset bits in old are mapped to themselves (the identify map).

Apply the above specified mapping to bit position oldbit, returning the new bit position.

For example, lets say that old has bits 4 through 7 set, and new has bits 12 through 15 set. This defines the mapping of bit position 4 to 12, 5 to 13, 6 to 14 and 7 to 15, and of all other bit positions unchanged. So if say oldbit is 5, then this routine returns 13.