Question

I have a bit field consisting of 64 bits:

long bitfield = 0;

I can set the bit for a given index as follows:

void Set(long index)
{
   bitfield |= 1L << (int)(index % 64);
}

i.e. if the index is 0, 64, 128, … then bit 0 is set, if the index is 1, 65, 129, … then bit 1 is set, and so on.

Question: given an index and a count (or a lower and upper index), how can I set the bits for all indexes in this range without using a loop?

Answer 1

long SetRangeMask(int lower, int upper)     // 3..7
{
   if (! (lower <= upper)) throw new ArgumentException("...");

   int size = upper - lower + 1;            // 7 - 3 + 1 = 5
   if (size >= 64) return -1;
   long mask = (1 << size) - 1;             // #00100000 - 1  = #000011111
   return mask << lower | mask >> -lower;   // #00011111 << 3 = #011111000
}