74 lines
3.4 KiB
Org Mode
74 lines
3.4 KiB
Org Mode
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#+title: nalgh
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#&summary
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Showoff of a poor (but simple) data compression method
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#&
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#+license: bysa, page
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#+license: wtf, program
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* nalgh, the {n alg}&strong{orit}{h}&strong{m} (nalgh)
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nalgh is an algorithm for lossless compression. It generally compresses a text
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into 77%-87% of its original size. The focus of nalgh lies on simplicity and
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not speed and effeciency. See an example of nalgh in action [[http://projects.metanohi.name/nalgh/example.php][here]] (using part of
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a [[http://www.gutenberg.org/etext/22600][random text]] I found at the [[http://www.gutenberg.org/][Gutenberg Project's website]].
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** Pros
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+ It's simple
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** Cons
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+ It's ineffecient
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+ It's slow
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+ It delivers a very poor compression
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** How
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1. nalgh loops through input text and makes a list of the characters that are
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present.
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2. The list is organised so that the character with most appearances comes
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first.
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3. nalgh loops through input text and creates a new text with each character
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converted to their binary counterpart based on their appearance on the
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list. If a text contains most "a"s, second-most "e"s and third-most "r"s,
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then a becomes 0, e becomes 1, and r becomes 10. In the newly created text,
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the binary versions are kept as a string, each part split by a "2". This
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means that "are", based on the facts just stated, will turn into "021021".
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4. nalgh checks if the length of the new text string is directly dividable
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with 5. If that is not the case, a bunch of "2"s are appended onto the
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text. The length of "021021" is 6, which divided with 5 leaves a rest of 1,
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so it's not directly dividable. The final string would in this case be
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"0210212222".
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5. nalgh loops through the new text string and converts every 5 characters from
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trinary to their byte equivalent. Looking at the above string, "02102" is
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converted into the ASCII character of 65 ("A") and "12222" is converted into
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161 ("¡"). 5 characters of trinary has 243 possibilites, where a byte has
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256, so problems will never occur.
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6. nalgh is almost done now. To be able to easily decompress the text later on,
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it is neccesary to specify the numbered equivalents of the characters in the
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input text. To do so, nalgh creates a text string where the character with
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most appearances appear first, and so on. Continuing using the letters from
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the previous examples, this string would be "aer", because there are more
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"a"s than "e"s but more "e"s than "r"s.
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7. A character with the ASCII value of the amount of different input characters
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is joined with the text describing the appearances of the characters, which
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is then joined with the text string consisting of converted trinary
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numbers. Using the "aer" example, the *final* string would be "[ASCII
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character with the value 3]aerA¡".
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Decompressing is simply doing it backwards. At the same time, decompressing is
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a lot less stressful to the computer.
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For now nalgh only operates on text, but it shouldn't be too difficult to make
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it work with all byte stuff.
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The nalgh source code is available [[http://projects.metanohi.name/nalgh/nalgh.php][here]]. For now there's only a PHP version
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(and an uncommented one, at that), and I doubt that there will ever be a
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version in another language.</p>
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I sincerely hope that nalgh will never be used. I have not yet seen a data
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compression method cabaple of performing poorer than nalgh. At the same time, I
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do like the way nalgh works, keeping everything simple.
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