Simple Data Compression
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The document discusses different methods for compressing data to reduce file sizes for storage or transmission, including: - Replacing common words with numbers (e.g. replacing "the" with "1") - Using run-length encoding (RLE) to represent repeated characters in images as numerical codes rather than writing out each character - Adding a rule that using the number "0" means to repeat the previous line when using RLE, which can further reduce the size of compressed image files.
Simple Data Compression
- 2. Rather than have to store every character in a file, it would be great if we could find a way of reducing the length of the file to allow it to be stored in a smaller space. This is the File Manager’s job
- 3. Also Rather than have to send every character in a message, it would be great if we could find a way of reducing the length of the message to allow it to be transmitted quicker. This is the Network Manager’s job
- 4. Whether the File Manager or the Network Manager both take a similar approach to compression.
- 5. Let’s look at an example. Let’s imagine we had to send the following message: The rain in Spain lies mainly in the plain
- 6. If we had to send this as it is down a wire: The rain in Spain lies mainly in the plain
- 7. The a total of 42 characters (including 8 spaces) The rain in Spain lies mainly in the plain
- 8. The a total of 42 characters (including 8 spaces) The rain in Spain lies mainly in the plain
- 9. Lets replace the word “the” with the number 1. The rain in Spain lies mainly in the plain
- 10. Lets replace the word “the” with the number 1. 1 rain in Spain lies mainly in 1 plain the =1
- 11. Lets replace the word “the” with the number 1. We’ve reduced the of characters to 38. 1 rain in Spain lies mainly in 1 plain the =1
- 12. Lets replace the letters “ain” with the number 2. 1 rain in Spain lies mainly in 1 plain the =1
- 13. Lets replace the letters “ain” with the number 2. We’ve reduced the of characters to 30. 1 r2 in Sp2 lies m2ly in 1 pl2 the =1 ain =2
- 14. Lets replace the letters “in” with the number 3. 1 r2 in Sp2 lies m2ly in 1 pl2 the =1 ain =2
- 15. Lets replace the letters “in” with the number 3. We’ve reduced the of characters to 28. 1 r2 3 Sp2 lies m2ly 3 1 pl2 the =1 ain =2 in = 3
- 16. Now lets say 1 means “the ”, so it’s “the” and a space 1 r2 3 Sp2 lies m2ly 3 1 pl2 the =1 ain =2 in = 3
- 17. Now lets say 1 means “the ”, so it’s “the” and a space We’ve reduced the of characters to 26. 1r2 3 Sp2 lies m2ly 3 1pl2 the =1 ain =2 in = 3
- 18. Now lets say 3 means “in ”, so it’s “in” and a space 1r2 3 Sp2 lies m2ly 3 1pl2 the =1 ain =2 in = 3
- 19. Now lets say 3 means “in ”, so it’s “in” and a space We’ve reduced the of characters to 24. 1r2 3Sp2 lies m2ly 31pl2 the =1 ain =2 in = 3
- 20. So that’s 24 characters for a 42 character message, not bad. The rain in Spain lies mainly in the plain 1r2 3Sp2 lies m2ly 31pl2 the =1 ain =2 in = 3
- 21. Let’s try a different example.
- 22. Let’s try a different example. Let’s say we are sending a list of jobs, with each item on the list is 10 characters long. Bookkeeper Teacher--- Porter---- Nurse----- Doctor----
- 23. Rather than sending the spaces we could just say how long they are: Bookkeeper Teacher--- Porter---- Nurse----- Doctor----
- 24. Rather than sending the spaces we could just say how long they are: Bookkeeper Teacher--- Porter---- Nurse----- Doctor---- • Bookkeeper • Teacher3- • Porter4- • Nurse5- • Doctor4-
- 25. We’ve gone from 50 to 42 characters: Bookkeeper Teacher--- Porter---- Nurse----- Doctor---- • Bookkeeper • Teacher3- • Porter4- • Nurse5- • Doctor4-
- 26. PROGRAM CompressExample: BEGIN Get Current Character; WHILE (NOT End_of_Line) DO Get Next Character; IF (Current Character != Next Character) THEN Get next char, and set current to next; Write out Current Character; ELSE Keep looping while the characters match; Keep counting; Get next char, and set current to next; When finished write out Counter; Write out Current Character; Reset Counter; ENDIF; ENDWHILE; END.
- 27. PROGRAM CompressExample: BEGIN char Current_Char, Next_char; int Counter; Current_Char := Get_char(); WHILE (NOT End_of_Line) DO Next_Char := Get_char(); IF (Current_Char != Next_char) THEN Current_Char := Next_Char; Next_Char := Get_char(); Write out Current_Char; ELSE WHILE (Current_Char == Next_char) DO Counter = Counter + 1; Current_Char := Next_Char; Next_Char := Get_char(); ENDWHILE; Write out Counter, Current_Char; Counter := 0; ENDIF; ENDWHILE; END.
- 28. Or let’s imagine we are sending a list of house prices. 350000 600000 550000 2100000 3000000
- 29. Now let’s use the # to indicate number of zeros: 350000 600000 550000 2100000 3000000
- 30. Now let’s use the # to indicate number of zeros: 350000 600000 550000 2100000 3000000 • 35#4 • 6#5 • 55#4 • 21#5 • 3#6
- 31. We’ve gone from 32 characters to 18 characters: 350000 600000 550000 2100000 3000000 • 35#4 • 6#5 • 55#4 • 21#5 • 3#6
- 33. Let’s think about images. Let’s say we are trying to display the letter ‘A’
- 34. Let’s think about images. Let’s say we are trying to display the letter ‘A’
- 35. We could encode this as: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW
- 36. We could compress this to: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW
- 37. We could compress this to: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW • 3W2B3W • 2WB2WB2W • WB4WBW • WB4WBW • W6BW • WB4WBW • WB4WBW • 8W
- 38. From 64 characters to 44 characters: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW • 3W2B3W • 2WB2WB2W • WB4WBW • WB4WBW • W6BW • WB4WBW • WB4WBW • 8W
- 39. We call this “run-length encoding” or RLE.
- 40. Now let’s add one more rule.
- 41. Now let’s add one more rule. Let’s imagine if we send the number ‘0’ it means repeat the previous line.
- 42. So now we had: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW • 3W2B3W • 2WB2WB2W • WB4WBW • WB4WBW • W6BW • WB4WBW • WB4WBW • 8W
- 43. And we get: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW • 3W2B3W • 2WB2WB2W • WB4WBW • WB4WBW • W6BW • WB4WBW • WB4WBW • 8W • 3W2B3W • 2WB2WB2W • WB4WBW • 0 • W6BW • WB4WBW • 0 • 8W
- 44. Going from 64 to 44 to 34 characters: WWWBBWWW WWBWWBWW WBWWWWBW WBWWWWBW WBBBBBBW WBWWWWBW WBWWWWBW WWWWWWWW • 3W2B3W • 2WB2WB2W • WB4WBW • WB4WBW • W6BW • WB4WBW • WB4WBW • 8W • 3W2B3W • 2WB2WB2W • WB4WBW • 0 • W6BW • WB4WBW • 0 • 8W
- 45. For most images, the lines are repeated frequently, so you can get massive savings from RLE.