/**
* produces a set of ids (keys) that are evenly distributed around the id ring. One invocation
* produces the i-th member of a set of size num. The set is evenly distributed around the ring,
* with an offset given by this Id. The set is useful for constructing, for instance, Scribe trees
* with disjoint sets of interior nodes.
*
* @param num the number of Ids in the set (must be <= 2^b)
* @param b the routing base (as a power of 2)
* @param i the index of the requested member of the set (0<=i<num; the 0-th member is this)
* @return the resulting set member, or null in case of illegal arguments
*/
public Id getAlternateId(int num, int b, int i) {
if (num > (1 << b) || i < 0 || i >= num) {
return null;
}
Id res = new Id(Id);
int digit = res.getDigit(numDigits(b) - 1, b) + ((1 << b) / num) * i;
res.setDigit(numDigits(b) - 1, digit, b);
return build(res.Id);
}
