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Guido van Rossumc6360141990-10-13 19:23:40 +00001# 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
11import posix
12
13cache = {}
14
15def 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(posix.stat(f1)), sig(posix.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 RuntimeError:
38 # cache miss
39 pass
40 # really compare
41 outcome = do_cmp(f1, f2)
42 cache[key] = s1, s2, outcome
43 return outcome
44
45def 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
53def do_cmp(f1, f2): # Compare two files, really
54 bufsize = 8096 # 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