cmp.py

minimal python variant for small footprint apps like embedded apps

"""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