Fastest way to calculate an X-bit bitmask?
I have been trying to solve this problem for a while, but couldn't with just integer arithmetic and bitwise operators. However, I think its possible and it should be fairly easy. What am I missing?
The problem: to get an integer value of arbitrary length (this is not relevant to the problem) with it's X least significant bits sets to 1 and the rest to 0. For example, given the number 31, I need to get an integer value which equals 0x7FFFFFFF (31 least significant bits are 1 and the rest zeros).
Of course, using a loop OR-ing a shifted 1 to an integer X times will do the 开发者_StackOverflow社区job. But that's not the solution I'm looking for. It should be more in the direction of (X << Y - 1)
, thus using no loops.
Try this: (1 << X) - 1
Try this:
uint.MaxValue >> (32 - something)
I think the following should work:
int mask = (int)Math.Pow(2, 31) - 1;
This is a single mathematical expression, but it isn't particularly efficient because calculating the power in this way is not really a good idea. However, since we're calculating a power of 2, we can do the same thing using shift:
int mask = (1 << 31) - 1;
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