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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
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