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lec21 - Outline Introduction the data compression Human...

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Outline Introduction the data compression Huffman compression Lecture 21, Huffman Compression p. 1
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How data are stored fixed-length code: ASCII code — the to- tal length: number of characters × code length. Can we do better? For a random sequence of characters: no For English: yes variable-length code Frequently used characters: shorter code Rarely used characters: longer code extra code for frequently used combination For example, Lempel-Ziv compression (next week) Lecture 21, Huffman Compression p. 2
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Example:abbcccaaad Fixed-length code a: 00; b:01; c:10; d:11 total length: 10 × 2 = 20 bits Variable-length code a: 0; b: 110; c: 10; d:111 total length: 4 × 1 + 2 × 3 + 3 × 2 + 1 × 3 = 17 bits How to get the optimal? Huffman tree Lecture 21, Huffman Compression p. 3
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Build Huffman Trees Algorithm HuffmanTree(Σ, f)
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