jaideeppyne commented on PR #517:
URL: https://github.com/apache/datasketches-cpp/pull/517#issuecomment-5472897508
Vouched, and I went and read it rather than taking my own word for it.
`EmptyCompactSketch` says it in the source:
```java
//For backward compatibility, a candidate long must have Flags= compact,
read-only,
// COMPACT-Family=3, SerVer=3, PreLongs=1, and be exactly 8 bytes long. The
seedHash is ignored.
// NOTE: The empty and ordered flags may or may not be set
private static final long EMPTY_SKETCH_MASK = 0X00_00_EB_00_00_FF_FF_FFL;
private static final long EMPTY_SKETCH_TEST = 0X00_00_0A_00_00_03_03_01L;
static final byte[] EMPTY_COMPACT_SKETCH_ARR = { 1, 3, 3, 0, 0, 0x1E, 0, 0 };
```
Two things line up with what you said. The serialized array carries `0, 0`
in the seed hash position, so Java writes zeros rather than a computed hash.
And the mask has `00_00` in its top two bytes, so those same two bytes are
excluded from the recognition test, which is what makes "the seedHash is
ignored" structural rather than just a convention. The `0xEB` in the flags byte
is the other half of the backward compatibility you mentioned, since it lets
the empty and ordered bits be either way.
On scope, this PR only does the read half. It stops `theta_a_not_b` throwing
on an empty B, which brings it in line with `theta_union_base::update`,
`theta_intersection_base::update` and `deserialize_v3/v4`, all three of which
already exempt empty sketches. It does not change what C++ writes:
`compact_theta_sketch_alloc` takes `seed_hash_(other.get_seed_hash())`, so an
empty compact sketch still serializes `compute_seed_hash(seed)` and that is the
remaining difference from Java, 6 of the 20 cases I diffed on #460.
Happy to do the write half too, emitting a zero seed hash for empty compact
sketches to match `EMPTY_COMPACT_SKETCH_ARR`. I did not put it here because it
changes bytes on the wire and felt like it deserved its own PR and your call on
release timing, rather than being folded into a one line read fix. Say which
you prefer and I will do it that way.
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