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doc: clarify CRollingBloomFilter size estimate

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Anthony Towns 2020-06-01 14:18:32 +10:00 • committed by Elliott Jin
parent 652c45fdbb
commit d9141a0002
1 changed files with 12 additions and 1 deletions
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13
src/bloom.h
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@ -94,7 +94,18 @@ public:
* insert()'ed ... but may also return true for items that were not inserted. * insert()'ed ... but may also return true for items that were not inserted.
* *
* It needs around 1.8 bytes per element per factor 0.1 of false positive rate. * It needs around 1.8 bytes per element per factor 0.1 of false positive rate.
* (More accurately: 3/(log(256)*log(2)) * log(1/fpRate) * nElements bytes) * For example, if we want 1000 elements, we'd need:
* - ~1800 bytes for a false positive rate of 0.1
* - ~3600 bytes for a false positive rate of 0.01
* - ~5400 bytes for a false positive rate of 0.001
*
* If we make these simplifying assumptions:
* - logFpRate / log(0.5) doesn't get rounded or clamped in the nHashFuncs calculation
* - nElements is even, so that nEntriesPerGeneration == nElements / 2
*
* Then we get a more accurate estimate for filter bytes:
*
* 3/(log(256)*log(2)) * log(1/fpRate) * nElements
*/ */
class CRollingBloomFilter class CRollingBloomFilter
{ {