On Apr 12, 2010, at 9:44 PM, Maciej Stachowiak wrote:


On Apr 12, 2010, at 8:15 PM, Timothy Hatcher wrote:

Only the first 5-7 characters are needed to identify a single commit (enough of the hash prefix to be unique). So REGRESSION (96c3b0) vs REGRESSION(r12345).

How long will 7 characters last before you hit the birthday paradox?

And to answer my own question (because I'm a nerd), a 7 hex digit hash prefix has a 50% chance of at least one pair of revisions colliding when there are at least 19290 revs. So we're already past the birthday paradox threshold to be using only 7 digits (assuming the hashes are uniformly distributed).

This answer brought to you by Wikipedia and Wolfram|Alpha, favorites of pedantic nerds everywhere:

http://en.wikipedia.org/wiki/Birthday_problem#Cast_as_a_collision_problem
http://www.wolframalpha.com/input/?i=sqrt%282*2%5E%284*7%29*ln%281%2F%281-.5%29%29%29

Regards,
Maciej

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