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POLZW (Limpel-Ziv-Welch) Dictionary coding delimiter issue
 

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  1. POLZW (Limpel-Ziv-Welch) Dictionary coding delimiter issue
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    <p>This question may not be strictly limited to the LZW algorithm, and may cover other implementations of LZ77 and LZ78:</p> <p>I've been trying to write a compression/decompression utility involving the LZW dictionary coding scheme. The problem is that I've found it necessary to include a delimiting character (a space) after each codeword (or "code") is written out during the encoding phase. I've been doing this because I can't assume that the output is being streamed directly to the decoder, and could be stored in a compressed file to be decoded later (in which case, the decoder will need some way to detect what separates codewords - the delimiter).</p> <p>I've recently been told that this is unnecessary, and that the decoder should be able to "figure out" dynamically how much of the compressed file to read in each time, presumably based on the previously read code(s). This would supposedly eliminate the (costly) need to insert an extra byte after each code.</p> <p>I'm just not sure how the decoder can figure this out. Maybe someone that understands how it works could explain it to me? Thanks.</p> <p>EDIT:</p> <p>The dictionary is a hash table mapping "input strings" to integers (codes), and is built up in the usual way as more of the input data is read in. The codes are written out to the compressed file. The decoder reads in each code (integer) from the compressed file and either looks up its dictionary for the associated string to output, or if there's no entry for that code, then it figures out what the string should be in the usual way and updates its dictionary. </p> <p>"Why does it make a difference if the file is streamed or stored?" If the output of the encoder is streamed one code at a time to the decoder, then the decoder can work with each code as it receives them. But if the encoder writes all the codes to a file (the compressed file), and then later that file is fed into the decoder, then how does the decoder know where one code starts and another one begins. The file would just be a mashed up sequence of digits.</p> <p>For example: Delimited Compressed file: 127 32 45 22 228 122 209.... Non-Delimited Compressed file: 127324522228122209...</p> <p>-Rob</p>
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      1. USDmitri Zaitsev
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    1. COWhy does it make a difference if the file is streamed or stored? And where are you putting delimiters? I don't understand what you are doing.
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      1. USJeffrey L Whitledge
 

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