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# Copyright (c) 1999 Tyler C. Sarna
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"""Compressed Layered Storage

This is a  storage that layers over another storage, compressing
objects larger than a certain threshold (default 2K).

The format of compressed pickles is an optional prefix byte followed by
data.  If the prefix byte is 1, the data is compressed.  If the
prefix byte is 0, the data is uncompressed.  If the first byte is not 0
or 1, the data is assumed uncompressed.  Since python pickles never
start with 0 or 1, they are stored directly without a prefix byte.  This
makes compatability with existing storages easier -- you can simply
stick a CompressedStorage atop an existing storage and it will work.

However, if some other layered storage sits atop this one, say an
encrypting storage, the "pickle" this layer receives may start with a 0
or 1.  In that event, if this layer won't be compressing it, a 0 byte
will be prefixed to assure that CompressedStorage doesn't try to
decompress data that isn't compressed.

"""
__version__='$Revision: 1.1 $'[11:-2]

import zlib


class ZWrap:
    """Wrap a function to produce a new one that can take a compressed
       string as the first argument"""

    def __init__(self, f):
        self.func = f

    def __call__(self, *args, **kw):
        if type(args) != type(()):
            args = tuple(args)
        if args[0][0] == '\1':
            args = (zlib.decompress(args[0][1:]),) + args[1:]
        elif args[0][1] == '\0':
            args = (args[0][1:],) + args[1:]
        return apply(self.func, args, kw)


class CompressedStorage:
    def __init__(self, base, thresh=2048):
        self._base = base
        self._zthresh = thresh

    def __getattr__(self, attr):
        """hokey pseudo-acquisition for base's stuff that we don't
        otherwise override"""

        return getattr(self._base, attr)

    def load(self, oid, version):
        p, serial = self._base.load(oid, version)
        if p[0] == '\1':
            p = zlib.decompress(p[1:])
        elif p[0] == '\0':
            p = p[1:]
        return p, serial

    def store(self, oid, serial, data, version, transaction):
        datalen = len(data)
        compressed = 0
        if datalen >= self._zthresh:
            ndata = '\1' + zlib.compress(data)
            if len(ndata) < datalen:
                data = ndata
                compressed = 1

        if not compressed and data[0] in '\0\1':
            data = '\0' + data

        return self._base.store(oid, serial, data, version, transaction)

    def pack(self, t, referencesf):
        return self._base.pack(t, ZWrap(referencesf))