# HG changeset patch
# User mpm@selenic.com
# Date 1118898700 28800
# Node ID 67081329d49a5f03175616abf7856ebfa45029f0
# Parent  4b0f562c61f4b2b23396088c8ee68247cb9b3e62
Change the size of the short hash representation

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Hash: SHA1

Change the size of the short hash representation

First note that this number doesn't really matter, as we always check
for ambiguous short hash ids.

Here's the math on collision probability:

>>> import math
>>> def p(f, n): return 1 - (1 / math.exp(n**2/(2*f)))
...
>>> p(2**32, 30000.0)
0.09947179164613551   # with 30000 changesets (BKCVS), we have a 9% chance
>>> p(2**32, 65000.0)
0.38850881217977273   # and with a full import from BK, we'd have a 39% chance
>>> p(2**40, 1e6)
0.36539171908447321   # we'd like to be "safe" for 1M csets, so 40 isn't enough
>>> p(2**48, 1e6)
0.001774780051374103  # But 48 looks good
>>> p(2**48, 1e7)
0.16275260939624481
>>> p(2**48, 5e6)
0.043437281083569146
>>> p(2**48, 2e6)
0.0070802434913129764
>>> p(2**48, 3e6)
0.01586009440574343

manifest hash: 24d9f928a463f46708b0e11fb781d5a241851424
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diff -r 4b0f562c61f4 -r 67081329d49a mercurial/revlog.py
--- a/mercurial/revlog.py	Wed Jun 15 19:58:48 2005 -0800
+++ b/mercurial/revlog.py	Wed Jun 15 21:11:40 2005 -0800
@@ -13,7 +13,7 @@
 
 def hex(node): return binascii.hexlify(node)
 def bin(node): return binascii.unhexlify(node)
-def short(node): return hex(node[:4])
+def short(node): return hex(node[:6])
 
 def compress(text):
     if not text: return text