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//usr/lib/python2.4/lib-old/cmp.py
"""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, shallow=1): """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 shallow and 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, 'rb') fp2 = open(f2, 'rb') while 1: b1 = fp1.read(bufsize) b2 = fp2.read(bufsize) if b1 != b2: return 0 if not b1: return 1