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Calculate byte core set for data sample:
- sort buckets' numbers in decreasing order
- count how many values cover 90% of the sample

If the core set size is low (<=25%), data are easily compressible.
If the core set size is high (>=80%), data are not compressible.
Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: David Sterba <dsterba@suse.com>