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Elaborate bucket size math

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sipa authored and TheBlueMatt committed Jun 17, 2016
1 parent 0d4cb48 commit ccd06b94f69c3e7758c35ac4bcd36d0e9450e158
Showing with 10 additions and 4 deletions.
  1. +10 −4 src/blockencodings.cpp
@@ -85,10 +85,16 @@ ReadStatus PartiallyDownloadedBlock::InitData(const CBlockHeaderAndShortTxIDs& c
while (txn_available[i + index_offset])
index_offset++;
shorttxids[cmpctblock.shorttxids[i]] = i + index_offset;
// Bucket selection is a simple Binomial distribution. If we assume blocks of
// 10,000 transactions, allowing up to 12 elements per bucket should only fail
// once every ~1.3 million blocks and once every 74,000 blocks in a worst-case
// 16,000-transaction block.
// To determine the chance that the number of entries in a bucket exceeds N,
// we use the fact that the number of elements in a single bucket is
// binomially distributed (with n = the number of shorttxids S, and p =
// 1 / the number of buckets), that in the worst case the number of buckets is
// equal to S (due to std::unordered_map having a default load factor of 1.0),
// and that the chance for any bucket to exceed N elements is at most
// buckets * (the chance that any given bucket is above N elements).
// Thus: P(max_elements_per_bucket > N) <= S * (1 - cdf(binomial(n=S,p=1/S), N)).
// If we assume blocks of up to 16000, allowing 12 elements per bucket should
// only fail once per ~1 million block transfers (per peer and connection).
if (shorttxids.bucket_size(shorttxids.bucket(cmpctblock.shorttxids[i])) > 12)
return READ_STATUS_FAILED;
}

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