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Document our Bloom filter parameter choices.

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svn:r17785
This commit is contained in:
Nick Mathewson 2008-12-26 17:35:18 +00:00
parent 0f9f45ff33
commit 73e1a1d26e
1 changed files with 10 additions and 0 deletions
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10
src/common/container.c
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@ -1233,6 +1233,16 @@ IMPLEMENT_ORDER_FUNC(find_nth_long, long)
digestset_t * digestset_t *
digestset_new(int max_elements) digestset_new(int max_elements)
{ {
/* The probability of false positivies is about P=(1 - exp(-kn/m))^k, where k
* is the number of hash functions per entry, m is the bits in the array,
* and n is the number of elements inserted. For us, k==4, n<=max_elements,
* and m==n_bits= approximately max_elements*32. This gives
* P<(1-exp(-4*n/(32*n)))^4 == (1-exp(1/-8))^4 == .00019
*
* It would be more optimal in space vs false positives to get this false
* positive rate by going for k==13, and m==18.5n, but we also want to
* conserve CPU, and k==13 is pretty big.
*/
int n_bits = 1u << (tor_log2(max_elements)+5); int n_bits = 1u << (tor_log2(max_elements)+5);
digestset_t *r = tor_malloc(sizeof(digestset_t)); digestset_t *r = tor_malloc(sizeof(digestset_t));
r->mask = n_bits - 1; r->mask = n_bits - 1;