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An Overview of Different Compression Algorithms. Their application on compressing inverted files. Alternative Compression Algorithms. Arithmetic coding Huffman coding Character-based Word-based Dictionary-based coding – Ziv-Lempel family of coding. Pros and Cons of Different Algorithms.

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an overview of different compression algorithms

An Overview of Different Compression Algorithms

Their application on compressing inverted files

alternative compression algorithms
Alternative Compression Algorithms
  • Arithmetic coding
  • Huffman coding
    • Character-based
    • Word-based
  • Dictionary-based coding – Ziv-Lempel family of coding
choosing an compression algorithm for inverted files
Choosing an Compression Algorithm for inverted files
  • Factors need to be considered
    • Compression ratio
    • Speed
    • Random access
  • In modern IR system, Word-based Huffman coding is commonly used
  • There are a lot of research on Ziv-Lempel family coding to see if they can be applied to indices compression
an improved sliding window ziv lempel algorithm
An Improved Sliding-window Ziv-Lempel Algorithm
  • Conventional LZ family compression algorithms use a sliding window approach.
    • Based on longest matching length (m-length)
  • An improved sliding window LZ algorithm is proposed by Bender and Wolf.
    • Instead of m-length, the improved algorithm is based on the offset of the length (o-length) and the differential of the length (-length)
benefits of the improved algorithm
Benefits of the Improved Algorithm
  • Better compression ratio in the experiment
  • Still linear compression and searching: O(n).
  • It didn’t really provide an LZ algorithm that support random access.
another modified lz algorithm
Another Modified LZ algorithm
  • Proposed by Williams
    • Use literal/copy item;
    • Each step, transmit original if it is a literal item, a pointer if it is a copy item;
  • Aimed at faster compression speed and smaller memory footprint.
  • Better used in the embedded system where real-time compression is required.
  • Inappropriate for index compression.
conclusion
Conclusion
  • Up to date, the best practical compression algorithm for index is still word-based Huffman coding.
  • There are theoretical studies about Ziv-Lempel family coding. Non of them are practically applicable to our problem. But they can be used in other areas.
reference
Reference
  • An Improved Data Compression Algorithm Based on Ziv-Lempel Data Compression Algorithm, Paul Edward Bender and Jack Keil Wolf;
  • An Extremely Fast Ziv-Lempel Data Compression Algorithm, Ross N. Williams;
  • Modern Information Retrieval, Ricardo Baeza-Yates and Berthier Ribeiro-Neto;
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