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+# Module 'cmpcache'
+#
+# Efficiently compare files, boolean outcome only (equal / not equal).
+#
+# Tricks (used in this order):
+#	- Use the statcache module to avoid statting files more than once
+#	- 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
+#
+# XXX There is a dependency on constants in <sys/stat.h> here.
+
+import posix
+import statcache
+
+
+# The cache.
+#
+cache = {}
+
+
+# Compare two files, use the cache if possible.
+# May raise posix.error if a stat or open of either fails.
+#
+def cmp(f1, f2):
+	# Return 1 for identical files, 0 for different.
+	# Raise exceptions if either file could not be statted, read, etc.
+	s1, s2 = sig(statcache.stat(f1)), sig(statcache.stat(f2))
+	if s1[0] <> 8 or s2[0] <> 8: # XXX 8 is S_IFREG in <sys/stat.h>
+		# 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
+	if cache.has_key(key):
+		cs1, cs2, outcome = cache[key]
+		# cache hit
+		if s1 = cs1 and s2 = cs2:
+			# cached signatures match
+			return outcome
+		# stale cached signature(s)
+	# really compare
+	outcome = do_cmp(f1, f2)
+	cache[key] = s1, s2, outcome
+	return outcome
+
+# Return signature (i.e., type, size, mtime) from raw stat data.
+#
+def sig(st):
+	# 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 # XXX dependent on S_IFMT in <sys/stat.h>
+	size = st[6]
+	mtime = st[8]
+	return type, size, mtime
+
+# Compare two files, really.
+#
+def do_cmp(f1, f2):
+	#print '    cmp', f1, f2 # XXX remove when debugged
+	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