cmpcache.py

"""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
"""

import os
from stat import *
import statcache


# The cache.
#
cache = {}


def cmp(f1, f2, shallow=1):
    """Compare two files, use the cache if possible.
    May raise os.error if a stat or open of either fails.
    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 not S_ISREG(s1[0]) or not S_ISREG(s2[0]):
        # 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
    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

def sig(st):
    """Return signature (i.e., type, size, mtime) from raw stat data."""
    return S_IFMT(st[ST_MODE]), st[ST_SIZE], st[ST_MTIME]

def do_cmp(f1, f2):
    """Compare two files, really."""
    #print '    cmp', f1, f2 # XXX remove when debugged
    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