2005/06/17 Compression

A few years ago I had a call from someone who wanted me to look at his new compression tool. I'm not sure what he really wanted, money, exposure, advice - doesn't matter, because in a very few seconds I knew he was a genuine nut-case. He claimed that he could take already compressed data and compress it again, and then again, and so on with no data loss. I laughed, and then he went into the typical explanation abount how "they" don't want his discoveries published and somewhere in there I hung up. Oh well.

There are two basic ways to compress data. One depends upon only having to deal with a subset of characters. For example, if you have a file consisting only of the character set "0123455678", you can pack each character into three bits, which compresses the file more than 50%. That's not particularly good compression, and is obviously very limited in the data it can handle so that sort of scheme isn't often used (though that's exactly what we are doing when we use bit maps).

The other approach looks for patterns. If we had "aaaabbbcddd", whe could store that as "a4b3c1d3". If we have 30 "d" characters, we can store "d9d9d9d3". That can handle any character set, and if there is a lot of repetition, the compression could be pretty good. It's not a good general purpose compressor though.

Suppose we knew that certain characters occur more often than others. In English text, "ETAOIN SHRDLU" supposedly represents the most frequently used characters (though the space would actually come first and there is disagreement about the other characters). That's 13 characters, so if we add two more, we can encode any of those in four bits, and have one left over as an "escape" character so that we can handle all the not so popular characters: the 15 popular characters get packed into 4 bits, and anything else takes 12 (the escape plus the real character). That sort of scheme is simple to implement, can handle any 8 bit character set, and can produce decent compression, though obviously not more than 50%.

We could take it farther, though. Suppose that instead of common characters, we looked for common words. Pick the top 256 or top 512 or top any power of two words, subtract one (to leave room for an escape character) and you can replace a lot of text with a short run of bits. This is "dictionary" encoding. Anything not in the dictionary is output as the escape character, a length byte, and the original characters. Again, fairly simple to code, and the amount of compression can be pretty good.

But we're not always compressing English text, are we? Compression algorithms have to work with any kind of junk we toss at them. Therefor, they build their dictionaries using an adaptive approach - determining what the stored data means as it goes. Not as simple to code, of course, but this is how they do what they do. If you want to know more, http://www.fadden.com/techmisc/hdc/ has a much more detailed look at the details.

Buy Take Control of Your Domain Names

Jump to Comments

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.

We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.