In this case, for clarity in the output tables, we use a missing byte marker (by default, this is the '~' character). In any case, the initial dictionary will have 256 items with codes from 0 to 255 for whatever encoding you choose.įor multibyte encodings, a dynamically generated dictionary will look quite exotic - its phrases will inevitably be composed of different parts of adjacent characters. Some encodings can take more than 1 byte per character, for example, UTF-8 encoding contains characters of variable length from 1 to 4 bytes. In the latter case, the phrase codes' size is the minimum number of bits required to express the number of characters in the dictionary. The size of phrase codes starts at 8 bits for the initial dictionary specified by standard encoding, or less for a dictionary entered by hand. Our calculators implement an endlessly growing vocabulary, which can be too expensive for huge data. Our implementation features Initial dictionary LZT - on overflow, removes from the dictionary a phrase that has not been used for the longest time.The algorithm monitors the compression ratio and, if it degrades significantly, resets the dictionary and forms it anew. When the maximum size is reached, the dictionary stops changing. LZC - the implementation of the algorithm in the compress utility limits the maximum dictionary size to 16 bits.There are known modifications to the algorithm trying to address this problem: In practice, this can lead to resource constraints when packing large amounts of data. In the compression algorithm described above, the size of the dictionary is not limited. Therefore, it is quite common to use dynamic code length, which changes every time the dictionary limit is reached. Still, this approach may even increase the length of the encoded message for small messages compared to the original text. Welch's original article intended to encode a phrase in a dictionary with a fixed-size 12-bit code. If the phrase with the WK code is not in the dictionary, return the phrase with the W code, and add the phrase with the WK code to the dictionary.Įlse, assign the WK code to the input phrase and go to 3. Put the first code to the input phrase W.Ĥ.If EOF, return the character having the code W, else:. It is recreated by itself in the process of decompression: To decode the data generated in this way, you do not need to store the dictionary. If WK phrase is already in the dictionary, W ⟵ WK, go to 3,Įlse return the code of W, add WK to the dictionary, W ⟵ K.
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