[tor-bugs] #6232 [Analysis]: Make entropy-over-time graph
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Thu Jul 5 08:54:31 UTC 2012
#6232: Make entropy-over-time graph
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Reporter: arma | Owner:
Type: enhancement | Status: needs_review
Priority: normal | Milestone:
Component: Analysis | Version:
Keywords: | Parent:
Points: | Actualpoints:
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Comment(by phw):
A clarification for the math which changed between the initial code and my
update:
Assume the early days of Tor, where we have only a few relays in the
consensus. In fact, it's 7 relays with the following bandwidth:
`[1, 1, 2, 2, 2, 3, 4]`
The old code determined the total bandwidth (15, the sum of all bandwidths
in the list) and calculated:
`1/15 * log2(1/15) + 1/15 * log2(1/15) + 2/15 * log2(2/15) + 2/15 *
log2(2/15) + 2/15 * log2(2/15) + 3/15 * log2(3/15) + 4/15 * log2(4/15)`
The problem is the probabilities. E.g., for the value '1', we expect a
probability of 2/7 (There are two instances of '1' in all 7 values) and
not 1/15.
The uploaded version fixes that. It builds a hash table of the form: `{
bandwidth_value => observed occurrences }`. Then, it iterates over the
hash table, adds up the result and we have the entropy.
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/6232#comment:9>
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