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diff --git a/Lib/cmp.py b/Lib/cmp.py
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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 posix
+
+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(posix.stat(f1)), sig(posix.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 RuntimeError:
+ # 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 = 8096 # 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