Question

I have a big number of SortedSet<Long> structures:

1, 2, 5, 8, 10, 35, 77, ...
5, 9, 35, 50, 132, ...
2, 4, 8, 15, 17, 23, ...
... hundreds of thousands of such rows...

I need to find a number that goes after, say, 50. In this example (if there are just three sets) it is 77. The number of sets is huge – hundreds of thousands. What algorithm would you suggest?

Answer 1

If I understand correctly here is my idea:

Collection<SortedSet<Long>> sets = //...

long minAfter50 = Long.MAX_VALUE;
for (SortedSet<Long> set : sets) {
    final Long first = set.tailSet(51L).first();
    minAfter50 = Math.min(minAfter50, first);
}

Here is the idea:

• iterate over all input sets
• crop all values less then or equal 50
• take the first argument of the cropped set (guaranteed to be greater than 50)
• calcualte the smallest value out of those collected in previous step

UPDATE (based on @beerbajay comment): if SortedSet is actually a TreeSet, the following code might perform better. Also I am making sure that there is any value greater than 50 in each and every set:

long minAfter50 = Long.MAX_VALUE;
for (TreeSet<Long> set : sets) {
    final Long higher = set.higher(50L);
    if (higher != null && higher < minAfter50) {
        minAfter50 = higher;
    }
}