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- # Module 'cmp'
-
- # Efficiently compare files, boolean outcome only (equal / not equal).
-
- # Tricks (used in this order):
- # - Files with identical type, size & mtime are assumed to be clones
- # - Files with different type or size cannot be identical
- # - We keep a cache of outcomes of earlier comparisons
- # - We don't fork a process to run 'cmp' but read the files ourselves
-
- import os
-
- cache = {}
-
- def cmp(f1, f2): # Compare two files, use the cache if possible.
- # Return 1 for identical files, 0 for different.
- # Raise exceptions if either file could not be statted, read, etc.
- s1, s2 = sig(os.stat(f1)), sig(os.stat(f2))
- if s1[0] <> 8 or s2[0] <> 8:
- # Either is a not a plain file -- always report as different
- return 0
- if s1 == s2:
- # type, size & mtime match -- report same
- return 1
- if s1[:2] <> s2[:2]: # Types or sizes differ, don't bother
- # types or sizes differ -- report different
- return 0
- # same type and size -- look in the cache
- key = (f1, f2)
- try:
- cs1, cs2, outcome = cache[key]
- # cache hit
- if s1 == cs1 and s2 == cs2:
- # cached signatures match
- return outcome
- # stale cached signature(s)
- except KeyError:
- # cache miss
- pass
- # really compare
- outcome = do_cmp(f1, f2)
- cache[key] = s1, s2, outcome
- return outcome
-
- def sig(st): # Return signature (i.e., type, size, mtime) from raw stat data
- # 0-5: st_mode, st_ino, st_dev, st_nlink, st_uid, st_gid
- # 6-9: st_size, st_atime, st_mtime, st_ctime
- type = st[0] / 4096
- size = st[6]
- mtime = st[8]
- return type, size, mtime
-
- def do_cmp(f1, f2): # Compare two files, really
- bufsize = 8*1024 # Could be tuned
- fp1 = open(f1, 'r')
- fp2 = open(f2, 'r')
- while 1:
- b1 = fp1.read(bufsize)
- b2 = fp2.read(bufsize)
- if b1 <> b2: return 0
- if not b1: return 1
-