Lib/cmp.py

   1 # Module 'cmp'
   2
   3 # Efficiently compare files, boolean outcome only (equal / not equal).
   4
   5 # Tricks (used in this order):
   6 #       - Files with identical type, size & mtime are assumed to be clones
   7 #       - Files with different type or size cannot be identical
   8 #       - We keep a cache of outcomes of earlier comparisons
   9 #       - We don't fork a process to run 'cmp' but read the files ourselves
  10
  11 import os
  12
  13 cache = {}
  14
  15 def cmp(f1, f2): # Compare two files, use the cache if possible.
  16         # Return 1 for identical files, 0 for different.
  17         # Raise exceptions if either file could not be statted, read, etc.
  18         s1, s2 = sig(os.stat(f1)), sig(os.stat(f2))
  19         if s1[0] <> 8 or s2[0] <> 8:
  20                 # Either is a not a plain file -- always report as different
  21                 return 0
  22         if s1 == s2:
  23                 # type, size & mtime match -- report same
  24                 return 1
  25         if s1[:2] <> s2[:2]: # Types or sizes differ, don't bother
  26                 # types or sizes differ -- report different
  27                 return 0
  28         # same type and size -- look in the cache
  29         key = (f1, f2)
  30         try:
  31                 cs1, cs2, outcome = cache[key]
  32                 # cache hit
  33                 if s1 == cs1 and s2 == cs2:
  34                         # cached signatures match
  35                         return outcome
  36                 # stale cached signature(s)
  37         except KeyError:
  38                 # cache miss
  39                 pass
  40         # really compare
  41         outcome = do_cmp(f1, f2)
  42         cache[key] = s1, s2, outcome
  43         return outcome
  44
  45 def sig(st): # Return signature (i.e., type, size, mtime) from raw stat data
  46         # 0-5: st_mode, st_ino, st_dev, st_nlink, st_uid, st_gid
  47         # 6-9: st_size, st_atime, st_mtime, st_ctime
  48         type = st[0] / 4096
  49         size = st[6]
  50         mtime = st[8]
  51         return type, size, mtime
  52
  53 def do_cmp(f1, f2): # Compare two files, really
  54         bufsize = 8*1024 # Could be tuned
  55         fp1 = open(f1, 'r')
  56         fp2 = open(f2, 'r')
  57         while 1:
  58                 b1 = fp1.read(bufsize)
  59                 b2 = fp2.read(bufsize)
  60                 if b1 <> b2: return 0
  61                 if not b1: return 1