Lib/lib-old/cmp.py

   1 """Efficiently compare files, boolean outcome only (equal / not equal).
   2
   3 Tricks (used in this order):
   4     - Files with identical type, size & mtime are assumed to be clones
   5     - Files with different type or size cannot be identical
   6     - We keep a cache of outcomes of earlier comparisons
   7     - We don't fork a process to run 'cmp' but read the files ourselves
   8 """
   9
  10 import os
  11
  12 cache = {}
  13
  14 def cmp(f1, f2, shallow=1):
  15     """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 shallow and 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):
  46     """Return signature (i.e., type, size, mtime) from raw stat data
  47     0-5: st_mode, st_ino, st_dev, st_nlink, st_uid, st_gid
  48     6-9: st_size, st_atime, st_mtime, st_ctime"""
  49     type = st[0] / 4096
  50     size = st[6]
  51     mtime = st[8]
  52     return type, size, mtime
  53
  54 def do_cmp(f1, f2):
  55     """Compare two files, really."""
  56     bufsize = 8*1024 # Could be tuned
  57     fp1 = open(f1, 'rb')
  58     fp2 = open(f2, 'rb')
  59     while 1:
  60         b1 = fp1.read(bufsize)
  61         b2 = fp2.read(bufsize)
  62         if b1 != b2: return 0
  63         if not b1: return 1