1. Shall we check the seed when creating sketches? Currently, we check only when serde, thus if the sketch never serde, we don't have this guard. Not sure if check earilier helps.

I think we should check the seed when creating sketches. I think it's not convenient to change the seek after using the sketch, so it's better to guarantee that the seed hash is valid at begining.

U r right! Checking the seed when first entered makes the most sense.

Updated.

@leerho I'd appreciate it if you can share some history about:

Since the serialized seed_hash occurs in the middle of the final result, I'm curious about the format of earlier serialized sketches. Do they have a placeholder 2-byte buffer that is fulfilled in later version? If we panic always when the computed seed = 0, then all those old snapshots cannot be read.

What does an old serialized sketch without seed concept look like?