There are functions named ceiling and floor in Java’s TreeSet. However I didn’t find something like them in Scala’s TreeSet.

I use code like set.from(123).min to do such thing. However in this way a new set is created, thus the complexity is O(n) right? But actually I only need that value, the complexity should be O(logn)

Is there any function functioning like ceiling and floor?

I know that. But I want to get the max value below an arbitrary value x. Use plain max will only get the max value in the whole tree. So I have to use to method and then max.

Does it hurt performance? It is mentioned in here few years ago. Is it sill the same?

maxBy method is used for finding the first element which yields the largest value measured by function f.

And the f you gave is _ < x, which yields Boolean. Seems like Scala considers True is larger than False, since TreeSet(1, 3, 5).maxBy(_ < 5) returns 1, due to 1 and 3 both yield True.

maxBy is not the one I want in this case.

TreeSet(1, 3, 5).maxBy(x => if (x < 5) x else Int.MinValue) this will return the desired result 3. However, I think the complexity of it is O(n).

slice looks like the combination of from and to, which means it’s still the same thing. slice returns a new TreeSet, which means a new red-black tree will be created, thus the complexity of O(n) is inevitable.

The point is, in a BST the complexity should be O(logn) if you want to find a specific value. In Java, ceiling and floor are provided to do this.