[ziyziy89]

FrequentLongsSketch.getInstance(String) builds the hash map from the length and numActive tokens in the string without checking either against the map. The probe loops in ReversePurgeLongHashMap (adjustOrPutValue, hashProbe) only stop at an empty slot, and the only guard is an assert, so once every slot is taken a probe for a new key spins forever. getInstance("1,10,3,0,3,0,3,2,1,1,2,1,3,1") never returns, and an empty image that claims a map length of 2 ("1,10,3,5,0,0,0,2") deserializes fine but hangs on the third distinct update(), because curMapCap is computed from the 8-slot minimum while the map only has 2 slots. With assertions off this is an unbounded CPU loop from a ~30 byte input in any service that deserializes sketch strings it receives.

Reject a map length below 1 << LG_MIN_MAP_SIZE and a numActive outside [0, capacity] in getInstance(String), next to the existing "Possible Corruption" checks. serializeToString() never produces either, so valid images are unaffected; the MemorySegment path already goes through update() and resizes/purges, so it does not need the same check. Added two tests to LongsSketchTest (with a timeout so a regression fails instead of hanging the suite); mvn clean verify passes.