https://github.com/redis/redis/blob/unstable/src/hyperloglog.c https://github.com/redis/redis/blob/unstable/src/hyperloglog... seems to be the starting point, g

https://github.com/redis/redis/blob/unstable/src/hyperloglog.c https://github.com/redis/redis/blob/unstable/src/hyperloglog... seems to be the starting point, given a number of identical comments (complete with typos, e.g. "Estimate cardinality form register histogram"), function names, macros. Copyright isn't preserved, which is troubling in itself. There's a brief mention of Redis's HLL implementation in the README, which is a hint re: the code's origins. The first divergence I see is in hllSparseToDense, although it seems more like a tweak to the input/output (passing in o->ptr instead of o, returning hdr instead of C_OK / C_ERR), than an algorithmic difference. Line 550 contains a looser check: if (span == 0) return -1; omitting the check that p >= end. And... the only meaningful algorithmic change I found is in hllAdd, wherein we invalidate the cache if hllDenseAdd() returns 1. There might be something else, but a lot of the details look to be standard (e.g. impl of murmurhash64a).