-#!/usr/bin/env python
-import sys
-from bup import options, git, _hashsplit
-from bup.helpers import *
+#!/bin/sh
+"""": # -*-python-*-
+bup_python="$(dirname "$0")/bup-python" || exit $?
+exec "$bup_python" "$0" ${1+"$@"}
+"""
+# end of bup preamble
+
+from __future__ import absolute_import, print_function
+import sys, struct, math
+
+from bup import options, git, _helpers
+from bup.helpers import log
+POPULATION_OF_EARTH=6.7e9 # as of September, 2010
optspec = """
bup margin
+--
+predict Guess object offsets and report the maximum deviation
+ignore-midx Don't use midx files; use only plain pack idx files.
"""
-o = options.Options('bup margin', optspec)
+o = options.Options(optspec)
(opt, flags, extra) = o.parse(sys.argv[1:])
if extra:
o.fatal("no arguments expected")
git.check_repo_or_die()
-#git.ignore_midx = 1
-
-mi = git.PackIdxList(git.repo('objects/pack'))
-last = '\0'*20
-longmatch = 0
-for i in mi:
- if i == last:
- continue
- #assert(str(i) >= last)
- pm = _hashsplit.bitmatch(last, i)
- longmatch = max(longmatch, pm)
- last = i
-print longmatch
+
+mi = git.PackIdxList(git.repo('objects/pack'), ignore_midx=opt.ignore_midx)
+
+def do_predict(ix):
+ total = len(ix)
+ maxdiff = 0
+ for count,i in enumerate(ix):
+ prefix = struct.unpack('!Q', i[:8])[0]
+ expected = prefix * total / (1<<64)
+ diff = count - expected
+ maxdiff = max(maxdiff, abs(diff))
+ print('%d of %d (%.3f%%) ' % (maxdiff, len(ix), maxdiff*100.0/len(ix)))
+ sys.stdout.flush()
+ assert(count+1 == len(ix))
+
+if opt.predict:
+ if opt.ignore_midx:
+ for pack in mi.packs:
+ do_predict(pack)
+ else:
+ do_predict(mi)
+else:
+ # default mode: find longest matching prefix
+ last = '\0'*20
+ longmatch = 0
+ for i in mi:
+ if i == last:
+ continue
+ #assert(str(i) >= last)
+ pm = _helpers.bitmatch(last, i)
+ longmatch = max(longmatch, pm)
+ last = i
+ print(longmatch)
+ log('%d matching prefix bits\n' % longmatch)
+ doublings = math.log(len(mi), 2)
+ bpd = longmatch / doublings
+ log('%.2f bits per doubling\n' % bpd)
+ remain = 160 - longmatch
+ rdoublings = remain / bpd
+ log('%d bits (%.2f doublings) remaining\n' % (remain, rdoublings))
+ larger = 2**rdoublings
+ log('%g times larger is possible\n' % larger)
+ perperson = larger/POPULATION_OF_EARTH
+ log('\nEveryone on earth could have %d data sets like yours, all in one\n'
+ 'repository, and we would expect 1 object collision.\n'
+ % int(perperson))