Modify bits x...y in an integer

Problem:
given two integers and two bit positions. Set the first integer between the two bit positions to be that of the second integer.

Solution:
The trickiest part is to calculate the mask:

int replace_bits(int a, int b, int x, int y) 
{ 
    int mask = ((1 << (y - x + 1)) - 1) << x; 
    // Clear a and replace with that of b 
    return ((a & ~mask) | (b & mask)); 
}

Complexity:

time - O(1)
space - O(1)

Links and credits:
http://www.careercup.com/question?id=13532675