There are only 2^48 possible carver arrangements, despite there being 2^64 seeds

The bug

The generation of carvers only appears to use the bottom 48 bits of the world seed, and ignores the top 2^16. As a result, the arrangement of carvers in one world will be shared by another 65,535 worlds, despite noise caves differing.

As it appears that Mojang intended to eliminate such similarities with the fixing of MC-236650, the fact that this remains is likely a bug.

How to reproduce

This can be seen by generating a world with a desired seed, locating a carver-based cave, then generating another world with an integer multiple of 281474976710656 added to the seed and flying to the same location. Assuming that the carver's generation has not been interrupted by another cave, not being underground or some other factor, the basic cave layout should appear identical.

The screenshots provided with this ticket use the seeds 1 and 281474976710657, with coordinates provided.

Expected results

The carvers would appear completely different for all 2^64 seeds.

Actual results

The carvers are a complete match for seeds constructed in this way.

How to fix

Carvers should be made to also respect the upper 16 bits in the world seed as to eliminate duplicate caves like these.

If the carver RNG is redone, it should also be reworked in the same way in Bedrock Edition as to fix MCPE-154323. Such a change could also fix MC-111378 and MCPE-95011.

Further notes

This likely affects world generation as far back as Beta 1.8, and possibly even earlier (in b1.8 to 1.6.4, it affected the entire world like with MC-236650, but from 1.7 onwards only affects carvers and other minor details): https://www.minecraftforum.net/forums/minecraft-java-edition/seeds/2229720-can-two-different-seeds-produce-identical-worlds