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62 lines
1.7 KiB
Python
62 lines
1.7 KiB
Python
# Module 'cmp'
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# Efficiently compare files, boolean outcome only (equal / not equal).
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# Tricks (used in this order):
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# - Files with identical type, size & mtime are assumed to be clones
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# - Files with different type or size cannot be identical
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# - We keep a cache of outcomes of earlier comparisons
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# - We don't fork a process to run 'cmp' but read the files ourselves
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import posix
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cache = {}
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def cmp(f1, f2): # Compare two files, use the cache if possible.
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# Return 1 for identical files, 0 for different.
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# Raise exceptions if either file could not be statted, read, etc.
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s1, s2 = sig(posix.stat(f1)), sig(posix.stat(f2))
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if s1[0] <> 8 or s2[0] <> 8:
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# Either is a not a plain file -- always report as different
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return 0
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if s1 = s2:
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# type, size & mtime match -- report same
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return 1
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if s1[:2] <> s2[:2]: # Types or sizes differ, don't bother
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# types or sizes differ -- report different
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return 0
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# same type and size -- look in the cache
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key = f1 + ' ' + f2
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try:
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cs1, cs2, outcome = cache[key]
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# cache hit
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if s1 = cs1 and s2 = cs2:
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# cached signatures match
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return outcome
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# stale cached signature(s)
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except RuntimeError:
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# cache miss
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pass
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# really compare
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outcome = do_cmp(f1, f2)
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cache[key] = s1, s2, outcome
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return outcome
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def sig(st): # Return signature (i.e., type, size, mtime) from raw stat data
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# 0-5: st_mode, st_ino, st_dev, st_nlink, st_uid, st_gid
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# 6-9: st_size, st_atime, st_mtime, st_ctime
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type = st[0] / 4096
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size = st[6]
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mtime = st[8]
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return type, size, mtime
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def do_cmp(f1, f2): # Compare two files, really
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bufsize = 8096 # Could be tuned
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fp1 = open(f1, 'r')
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fp2 = open(f2, 'r')
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while 1:
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b1 = fp1.read(bufsize)
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b2 = fp2.read(bufsize)
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if b1 <> b2: return 0
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if not b1: return 1
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