Lib/cmpcache.py

   1 # Module 'cmpcache'
   2 #
   3 # Efficiently compare files, boolean outcome only (equal / not equal).
   4 #
   5 # Tricks (used in this order):
   6 #       - Use the statcache module to avoid statting files more than once
   7 #       - Files with identical type, size & mtime are assumed to be clones
   8 #       - Files with different type or size cannot be identical
   9 #       - We keep a cache of outcomes of earlier comparisons
  10 #       - We don't fork a process to run 'cmp' but read the files ourselves
  11
  12 import os
  13 from stat import *
  14 import statcache
  15
  16
  17 # The cache.
  18 #
  19 cache = {}
  20
  21
  22 # Compare two files, use the cache if possible.
  23 # May raise os.error if a stat or open of either fails.
  24 #
  25 def cmp(f1, f2):
  26         # Return 1 for identical files, 0 for different.
  27         # Raise exceptions if either file could not be statted, read, etc.
  28         s1, s2 = sig(statcache.stat(f1)), sig(statcache.stat(f2))
  29         if not S_ISREG(s1[0]) or not S_ISREG(s2[0]):
  30                 # Either is a not a plain file -- always report as different
  31                 return 0
  32         if s1 == s2:
  33                 # type, size & mtime match -- report same
  34                 return 1
  35         if s1[:2] <> s2[:2]: # Types or sizes differ, don't bother
  36                 # types or sizes differ -- report different
  37                 return 0
  38         # same type and size -- look in the cache
  39         key = f1 + ' ' + f2
  40         if cache.has_key(key):
  41                 cs1, cs2, outcome = cache[key]
  42                 # cache hit
  43                 if s1 == cs1 and s2 == cs2:
  44                         # cached signatures match
  45                         return outcome
  46                 # stale cached signature(s)
  47         # really compare
  48         outcome = do_cmp(f1, f2)
  49         cache[key] = s1, s2, outcome
  50         return outcome
  51
  52 # Return signature (i.e., type, size, mtime) from raw stat data.
  53 #
  54 def sig(st):
  55         return S_IFMT(st[ST_MODE]), st[ST_SIZE], st[ST_MTIME]
  56
  57 # Compare two files, really.
  58 #
  59 def do_cmp(f1, f2):
  60         #print '    cmp', f1, f2 # XXX remove when debugged
  61         bufsize = 8*1024 # Could be tuned
  62         fp1 = open(f1, 'r')
  63         fp2 = open(f2, 'r')
  64         while 1:
  65                 b1 = fp1.read(bufsize)
  66                 b2 = fp2.read(bufsize)
  67                 if b1 <> b2: return 0
  68                 if not b1: return 1