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5
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1 # Module 'cmp'
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2
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3 # Efficiently compare files, boolean outcome only (equal / not equal).
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4
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5 # Tricks (used in this order):
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6 # - Files with identical type, size & mtime are assumed to be clones
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7 # - Files with different type or size cannot be identical
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8 # - We keep a cache of outcomes of earlier comparisons
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9 # - We don't fork a process to run 'cmp' but read the files ourselves
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10
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776
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11 import os
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5
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12
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13 cache = {}
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14
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15 def cmp(f1, f2): # Compare two files, use the cache if possible.
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16 # Return 1 for identical files, 0 for different.
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17 # Raise exceptions if either file could not be statted, read, etc.
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18 s1, s2 = sig(os.stat(f1)), sig(os.stat(f2))
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5
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19 if s1[0] <> 8 or s2[0] <> 8:
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20 # Either is a not a plain file -- always report as different
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21 return 0
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559
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22 if s1 == s2:
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23 # type, size & mtime match -- report same
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24 return 1
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25 if s1[:2] <> s2[:2]: # Types or sizes differ, don't bother
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26 # types or sizes differ -- report different
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27 return 0
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28 # same type and size -- look in the cache
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29 key = f1 + ' ' + f2
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30 try:
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31 cs1, cs2, outcome = cache[key]
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32 # cache hit
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33 if s1 == cs1 and s2 == cs2:
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34 # cached signatures match
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35 return outcome
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36 # stale cached signature(s)
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521
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37 except KeyError:
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38 # cache miss
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39 pass
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40 # really compare
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41 outcome = do_cmp(f1, f2)
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42 cache[key] = s1, s2, outcome
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43 return outcome
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44
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45 def sig(st): # Return signature (i.e., type, size, mtime) from raw stat data
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46 # 0-5: st_mode, st_ino, st_dev, st_nlink, st_uid, st_gid
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47 # 6-9: st_size, st_atime, st_mtime, st_ctime
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48 type = st[0] / 4096
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49 size = st[6]
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50 mtime = st[8]
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51 return type, size, mtime
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52
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53 def do_cmp(f1, f2): # Compare two files, really
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1965
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54 bufsize = 8*1024 # Could be tuned
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5
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55 fp1 = open(f1, 'r')
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56 fp2 = open(f2, 'r')
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57 while 1:
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58 b1 = fp1.read(bufsize)
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59 b2 = fp2.read(bufsize)
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60 if b1 <> b2: return 0
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61 if not b1: return 1
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