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Thread: Grouped LZW, Vol. 2

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    Grouped LZW, Vol. 2

    Hi all,
    ​in this episode I am glad to tell about an attempt of my own to dispense
    the problem previously encountered in Grouped (ROLZ) LZW:
    the fixed size of the groups (dictionaries attached to context).
    A way to proceed is illustrated by Nakano, Yahagi, Okada but I started
    from a different consideration.
    Every time a symbol occurs in a text, it gains an increasingly number of
    children and the chance for it to reappear is more and more smaller,
    while an entropy stage which takes its output assigns shorter codes. To
    conciliate this two opposite I settled down a schema where symbols
    belong to a set of lists entitled for the number of children, and each
    list is organized as LRU.
    A symbol then will be emitted by its list and rank inside it,
    respectively via the Witten Cleary arithmetic coder and the Elias delta.
    I choosed an AC because it is the solely that can mimic closely the
    fatherhood distribution among symbols, but this premise put me in front
    of the necessity to interleave its sequence.
    After a complicated period I realized that the solution must be based on
    two facts: (i) the encoder has to be ahead of two symbols because the
    decoder needs to start with 16 bit; (ii) the variables high and low (which
    define the focus interval) are in lockstep between the just mentioned two
    sides. To cope with this constraints I decoupled the LZW part from the
    entropy one by a queue data interface.
    At the moment the compressor is terrible, both in terms of
    speed and ratio, but I made it public as the interleaver could be of
    some interest.
    I have in mind to improve the performance of the compressor imposing on
    it a more canonical context apparatus, that should curtail the recency lists
    at the expense of memory consumption.
    I hope to be back soon,
    greetings, Marco Borsari
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    Last edited by Marco_B; 11th June 2020 at 12:58.

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    ​I finished to elaborate Lens with a standard context machinery,
    now every node in the trie has a presence in all the recency
    lists associated to the characters of an order 1 set: in case of an
    actually encountered context the node is placed at the head of the list,
    for other ones at the tail.
    This is necessary to keep the consistency of the specific system of the
    Lens series (see above) to transmit a symbol.
    I conserved a single AC statistic for the fatherhood instead to reply it
    for every context because in the mean the differences should be zero.
    I have been forced to decouple the LRU for the full dictionary from the
    list for zero children because it would be difficult to make a choice
    regarding the various rank of the symbols in respect to their contexts.
    Unfortunately, though the compression ratio is better than that of
    Lens3, it remains far worse than simply emitting the appropriate bit
    index as in classical LZW.
    So I must admit this is a sterile path and I stop it here.
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    Last edited by Marco_B; 4th April 2020 at 16:34.

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