LZWmenetelmällä

LZWmenetelmällä, often referred to as the LZW algorithm, is a lossless data compression technique developed by Abraham Lempel, Jacob Ziv, and Terry Welch. It was patented in 1985. The core idea behind LZW is to build a dictionary of phrases encountered in the input data and then represent these phrases with shorter codes.

The algorithm starts with an empty dictionary, often pre-populated with single characters. As it scans the input data, it reads characters sequentially. It looks for the longest string of characters that is already present in its dictionary. Once found, it outputs the code associated with that string. Then, it adds a new entry to the dictionary: the string it just found, followed by the next character from the input. This new entry is assigned a new, unique code. If the current string plus the next character is not in the dictionary, it outputs the code for the current string and then adds the new string (current string + next character) to the dictionary with a new code. This process continues until the end of the input data is reached.

During decompression, the algorithm uses the same dictionary-building process. It reads the codes from the compressed data and looks them up in its dictionary to reconstruct the original strings. When it encounters a code that is not yet in its dictionary (which happens when the compressor added a new entry that the decompressor has not yet created), it can infer the missing entry based on the preceding data and the structure of the LZW algorithm. LZW is known for its simplicity and effectiveness, particularly on repetitive data. It was widely used in early GIF image formats and in Unix compress utilities.