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For such a reduced alphabet, the full 12-bit codes yielded poor compression unless the image was large, so the idea of a variable-width code was introduced: codes typically start one bit wider than the symbols being encoded, and as each code size is used up, the code width increases by 1 bit, up to some prescribed maximum (typically 12 bits).

After the first time “I am an engineer" appeared, it was assigned the value *.Further refinements include reserving a code to indicate that the code table should be cleared and restored to its initial state (a "clear code", typically the first value immediately after the values for the individual alphabet characters), and a code to indicate the end of data (a "stop code", typically one greater than the clear code).The clear code allows the table to be reinitialized after it fills up, which lets the encoding adapt to changing patterns in the input data.Afterwards, that phrase is indicated only by the *. Actually, the real algorithm would compress the space-letter combinations as well, but that would confuse this example.So you can see how much shorter the second message is than the first.It was published by Welch in 1984 as an improved implementation of the LZ78 algorithm published by Lempel and Ziv in 1978.

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