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fix(cmSketch test) and add better cmSketch test #325

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fix(cmSketch test) and add better cmSketch test #325

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Commits on Oct 14, 2024

  1. fix(cmSketch test) and add better cmSketch test 

    One Sketch test was intended to check that the seeds had caused each
sketch row to be unique but the way the test was iterating, the failure
wasn't going to be triggered.

    With this fix, the test passes seeming to indicate the seeds are
    working as intended on the row creation. But there is a problem.

A new Sketch test is added that goes to the trouble of sorting the
counters of each row to ensure that each row isn't just essentially a
copy of another row, where the only difference is which position the
counters occupy.

And the new Sketch test is performed twice, once with high-entropy
hashes, because they come from a PRNG, and once with low-entropy hashes,
which in many cases will be normal, because they are all small numbers,
good for indexing, but not good for getting a spread when simply applied
to a bitwise XOR operation.

    These new tests show a problem with the counter increment logic
    within the cmSketch.Increment method which was most likely
    introduced by commit f823dc4.

    A subsequent commit addresses the problems surfaced. But as the
    discussion from issue dgraph-io#108 shows (discussion later moved to
    https://discuss.dgraph.io/t/cmsketch-not-benefitting-from-four-rows/8712 ),
    other ideas for incrementing the counters were considered
    by previous authors as well.

Fix existing cmSketch test and add improved cmSketch test

(Marked as draft because this introduces a test that fails for now. I can
commit a fix to the cmSketch increment method to get the new test to
pass - if a maintainer agrees there is a problem to be fixed. See dgraph-io#108.
I tried a few years ago.)

One Sketch test was intended to check that the seeds had caused each
sketch row to be unique but the way the test was iterating, the failure
wasn't going to be triggered.

    With this fix, the test passes seeming to indicate the seeds are
    working as intended on the row creation. But there is a problem with
    the actual increment method. A new Sketch test is added that goes
    further and sorts the counters of each row to ensure that each row
    isn't just essentially a copy of another row, where the only
    difference is which position the counters occupy.

And the new Sketch test is performed twice, once with high-entropy
hashes, because they come from a PRNG, and once with low-entropy hashes,
which in many cases will be normal, because they are all small numbers,
good for indexing, but not good for getting a spread when simply applied
with a bitwise XOR operation.

    These new tests show a problem with the counter increment logic
    within the cmSketch.Increment method which was most likely
    introduced by commit f823dc4.

    A subsequent commit addresses the problems surfaced. But as the
    discussion from issue dgraph-io#108 shows (discussion later moved to
    https://discuss.dgraph.io/t/cmsketch-not-benefitting-from-four-rows/8712),
    other ideas for incrementing the counters were considered by
    previous authors of the original Java code as well.
    @FrankReh @mangalaman93
    FrankReh authored and mangalaman93 committed Oct 14, 2024
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