deflater/inflater and dictionnary for huffman

Discussion in 'Java' started by NOBODY, Oct 17, 2003.


    NOBODY Guest

    (deflater/inflater and dictionnary for huffman)

    I want to compress/decompress small data like strings of 1k or less.
    I know huffman compression with static dictionnary is the best solution.

    Seams I could use the with strategy Huffman_only and
    maybe prepare a dictionnary.

    Does anyone can explain the way to use this? I can't find any docs after 2
    hours on google and the groups! Javadoc is useless.

    NOBODY, Oct 17, 2003
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  2. Roedy Green

    Roedy Green Guest

    On Fri, 17 Oct 2003 03:05:06 GMT, NOBODY <> wrote or
    quoted :

    >Seams I could use the with strategy Huffman_only and
    >maybe prepare a dictionnary.

    For sample code see

    I don't know if you can force Java's GZIP to use only Huffman though.

    Canadian Mind Products, Roedy Green.
    Coaching, problem solving, economical contract programming.
    See for The Java Glossary.
    Roedy Green, Oct 17, 2003
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  3. NOBODY <> writes:

    > Seams I could use the with strategy Huffman_only and
    > maybe prepare a dictionnary.
    > Does anyone can explain the way to use this? I can't find any docs after 2
    > hours on google and the groups! Javadoc is useless.

    Yep, it is knowledge that is passed from father to son with a secret
    handshake. I once tried to interest some Java journals in publishing an
    article about "all" the magic of the Zip an Jar classes, but no one was

    First of all, consider using a DeflaterOutputStream instead of a raw
    Deflater. It handles all the ugly details of feeding data to the
    compression algorithm and writing the results. If you want to have the
    result in memory, provide a ByteArrayOutputStream to the
    DeflaterOutputStream's constructor.

    If you need to do it by your own, I suggest to study the
    DeflaterOutputStream's source code (source comes with the J2SDK in a
    file called or src.jar).

    Please note that DeflaterOutputStream uses a Deflater differently than
    the Deflater documentation suggests. The documentation suggests to
    check needsInput() if deflate() returns with a 0. DeflaterOutputStream
    ignores the return value and just loops until needsInput() returns
    true. They can do this, because they ensure their output buffer has at
    least a size of 1, and they immediately write out any result and reuse
    the buffer.

    For using Deflator, you need two sets of "pointers" (this is where the
    underlying C library shines through). One "pointer" points to the part
    of the data that still needs to be processed, the other to some
    storage location to which compressed data should be written to.

    Of course, you don't have pointers in Java, so what the API expects is
    a pair of a byte[] and an integer. The byte[] holds the data, the
    integer serves as a pointer into the byte[]. In addition, you need
    something to know the remaining data in the byte[], that's another
    integer. So for the input data, and the output data you have a triplet

    byte[] b; // memory holding the data
    int off; // position "pointer" into b
    int len; // For input: remaining input data in this buffer
    // For output: remaining free memory in this buffer
    // In both cases: off + len <= b.length

    You provide such a triplet to setInput() to tell the Defalter where to
    get the data from. And you provide another such triplet to deflate() to
    tell the Deflater where to place the output.

    Now the really tricky part starts. You call deflate() and then you have
    to react according to the return value of deflate():

    deflate() == 0 && needsInput():

    All data provided via setInput() has been read (but maybe not
    completely processed). Either provide more data by calling
    setInput() again, or finish the compression.

    Finishing the compression is tricky, too:
    (a) Call finish()
    (b) Flush all data still in internal Deflater buffers. You do this
    by calling deflate() in a loop while finished() returns false.
    Check the return value of deflate(), because you still
    might need to provide more output memory.

    deflate() == 0 && !needsInput():

    This is undocumented, but important. The Deflater needs more
    output storage. Save the already compressed data in the
    output byte[], and call deflate() again with more memory.

    deflate() > 0

    Some output data is available. The data is in the byte[] provided
    to deflate(), starts at off as provided to deflate() and has a
    length as returned by deflate().

    Do whatever you want with the data, then adjust off and len if
    necessary, and call deflate() again.


    Thomas Weidenfeller, Oct 17, 2003
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