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What is LZW (Lempel-Ziv-Welch)? An Easy-to-Understand Explanation of the Basic Concepts of Data Compression
Data compression plays a crucial role in our digital world, enabling us to store and transmit information efficiently. One popular compression algorithm is LZW, also known as Lempel-Ziv-Welch, which has been widely used in various applications, including file compression formats like GIF and TIFF. In this blog post, we will delve into the basic concepts of LZW and explain how it achieves compression, all in a way that is easy to understand.
The Basics of LZW
LZW is a dictionary-based algorithm, meaning it utilizes a dictionary to replace repeated patterns of data with shorter codes. By doing so, LZW reduces the overall size of the data, resulting in compression. The algorithm works by initially building a dictionary of all possible characters or patterns in the input data.
Let’s illustrate this with a simple example. Consider the following sequence of characters: “ABABABAABABBABBA”. LZW would begin by adding all the individual characters (‘A’, ‘B’) to the dictionary. Then it would start scanning the input sequence, looking for repeated patterns.
Compression Process
1. The algorithm starts by reading the first character (‘A’).
2. The next character is ‘B’. Since the sequence “AB” is not yet in the dictionary, the algorithm outputs the code for ‘A’, which is 0.
3. The algorithm then adds the sequence “AB” to the dictionary with a new code, say 256.
4. The algorithm continues scanning and encounters ‘A’. The sequence “A” is already in the dictionary, so it appends ‘A’ to the current sequence.
5. The algorithm now encounters ‘B’, resulting in the sequence “AA”. This sequence is also already in the dictionary, so it appends ‘B’.
6. The algorithm repeats this process until it reaches the end of the input sequence.
In the end, the compressed output would be a sequence of codes: [0, 1, 0, 256, 0, 1, 258]. The original sequence of 14 characters has been compressed into a sequence of 7 codes.
Decompression Process
To decompress the data, the algorithm uses the same dictionary and follows a similar process. It initializes the dictionary with the same characters and codes as the compression phase. Then, it reads the compressed codes and reconstructs the original sequence.
Considering our previous example, the decompression process would work as follows:
1. Initialize the dictionary with ‘A’ and ‘B’ and their corresponding codes.
2. Read the first code, which is 0, and output the corresponding character from the dictionary (‘A’).
3. Continue reading the codes and outputting the corresponding characters until all codes have been processed.
In the end, the decompressed output of the codes [0, 1, 0, 256, 0, 1, 258] would be the original sequence “ABABABAABABBABBA”.
Conclusion
LZW is a powerful and widely used data compression algorithm, thanks to its simplicity and effectiveness. By replacing repeated patterns with shorter codes, LZW achieves compression while preserving the integrity of the original data. Understanding the basic concepts behind LZW can provide insights into the fascinating world of data compression and its applications in various fields like file storage, transmission, and beyond.
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