fix(hll): update silently dropped after deserializing a compact List sketch#117
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notfilippo merged 1 commit intoapache:mainfrom Apr 22, 2026
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…sketch When an `HllSketch` in **List mode** is serialized it uses compact format: only the live coupons are written to disk, with no trailing `COUPON_EMPTY` sentinels. On deserialization, the container was allocated with exactly `coupon_count` slots, leaving the array fully packed. `list::update()` scans linearly for a `COUPON_EMPTY` (`0`) sentinel to find an insertion slot. Finding none, it fell through silently — discarding every value added after the round-trip. Always allocate `1 << lg_arr` slots (initialized to `COUPON_EMPTY`) in `List::deserialize()`, reading only `coupon_count` elements from the byte stream in compact mode. The trailing empty slots are then available as sentinels for subsequent `update()` calls. Fixes apache#115.
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When an
HllSketchin List mode is serialized it uses compact format: only the live coupons are written to disk, with no trailingCOUPON_EMPTYsentinels. On deserialization, the container was allocated with exactlycoupon_countslots, leaving the array fully packed.list::update()scans linearly for aCOUPON_EMPTY(0) sentinel to find an insertion slot. Finding none, it fell through silently — discarding every value added after the round-trip.Always allocate
1 << lg_arrslots (initialized toCOUPON_EMPTY) inList::deserialize(), reading onlycoupon_countelements from the byte stream in compact mode. The trailing empty slots are then available as sentinels for subsequentupdate()calls.Fixes #115.