1. File Compression
Even though disks have gotten bigger, we are still running short on disk space
A common technique is to compress files so that they take up less space on the disk
We can save space by taking advantage of the fact that most files have a relatively low “information content”
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Compression

2. Run Length Encoding
The simplest type of redundancy in a file is long runs of repeated characters
AAAABBBAABBBBBCCCCCCCC
This string can be represented more compactly by replacing each repeated string with a single occurrence of the character and a count
4A3B2A5B8C
For binary files a refined version of this method can yield dramatic savings
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Compression

3. Variable Length Encoding
Suppose we wish to encode
ABRACADABRA
Instead of using the standard 8 (or 16) bits to represent these letters, why not use 3?
A =
B = 001
C = 010
D = 011
R = 100
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Compression

4. We can do better!! Why use the same number of bits for each letter
This is not really a code because it depends on the blanks
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Compression

5. Consider this Tree B D A C R
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Compression

6. More Formally Start with a frequency table A 5 B 2 R C 1 D 4/7/2019
Compression

7. More Formally
Create a binary tree out of the two elements with the lowest frequencies
New frequency is the sum of the frequencies
Add new node to the frequency table 2
C, 1
D, 1
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Compression

8. More Formally Repeat until only one element is left in the table 11 62 4
C, 1
D, 1
B,2
R, 2
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Compression

9. Huffman Coding
The general method for finding this code was developed by D. Huffman in 1952
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Compression