FILE COMPRESSION  By Paul Mclachlan Ed: This is a great tutorial for all those who have been befuddled by "arc-ing" and "de-arcing" files, or what is the difference between "crunching" and "archiving" and "compressing".  ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| The whole concept of being able to take a file, and make it smaller has amused programmers on the computer for ages. Recently, there has been wonderful progress, with text file compression now 60%+. On the Amiga, we can categorise the types of compression we have to two major kinds: Archiving, and 'Crunching'.  ARCHIVING Not too long ago, when someone mentioned archives to me, I had sort of realised it was compression, but, I didn't know what, why or how. I have come a long way since then, but it would have been a lot easier if someone 'in the know' had sat down and written an article just like this one. Oh, the time that could have saved me. I used to hear how wonderful these directory utilities were. I agreed with that. Then I heard that the best part about them was how easy it was to archive and de-archive with them. Now, this got me confused. If it was so easy, how come I couldn't do it? Well, anyway, here's an explanation of archiving, and how to perform this feat from the CLI. Since the method of setting it up with the various directory utilities around varies so much, I'll leave that to you.  WHAT? Archiving, in simple terms, means taking a bunch of files, compressing them, and combining them into one big file that can be moved around easier. This is exactly what an archive program does. Simple really.  HOW? The next step in archiving is the ability to perform this feat using the archiving programs around. This can be reasonably complicated. A good start is to simply run the program from the CLI without any parameters. (Just type in the name of the program and hit return). For the purpose of this tutorial, I will take the example of LHarc, a popular arciving program on the Amiga. 1> lharc -- Lharc -- v 1.10 Jan 29 1990 by Paolo Zibetti (FidoNet 2:331/101.6) Usage:lharc[] [] [] summary of commands: summary of switches: e,x extract files from archive -p pause after loading l,v show archives contents -m no messages for queries p print extracted files to screen -x consider extended file names t test archive integrity -n no progress indicator a add files to archives -w set working directory m move files into archives -P set priority d delete files from archives -a consider file attributes u update files in archives -u convert file names to uppercase f freshen files in archives -r recursively collect files -S set sort criteria must end with `:' or `/' 1> Making sense of it Well, that's an interesting display of information. After this tutorial, a display such as that one should be enough to enable you to use that particular archiving program with no problems at all. The whole thing centres on the line that begins 'Usage:lharc['... This line tells you what order everything must go in. So, immediately after the colon, you should type 'lharc' (which incidentally is the name of the program), and then you should type the "switches" that you would like to use. You should then enter the command for the program, and then the archive name. You should end by typing a destination path and file patterns, where applicable. Let's begin with the terminology of this line. Anything enclosed in square brackets [] means that it is OPTIONAL. Anything enclosed by <> brackets means that you should not type the word literally, but replace there for the individual case. Now we notice that the switches are optional, and not always required. Let's take an example, to help show how this process is used. For example, extracting digitally... Okay. Someone has just handed you a disk with a 'great PD utility' on it. On the disk is a single file: 'util.lzh' or something of that nature. The '.lzh' at the end denotes that you should use this program to 'de-arc' it. So obviously, when something is followed by the three letter code lzh, it means it is a group of files archived by the program LHarc. You now want to do a process which is known as EXTRACT FILES from the archive. Which simply means take files out. So the command you want is e or x. It doesn't matter which you use. They both do exactly the same thing. Scanning down the list of switches, there's nothing there that really seems vital, so, since it's optional, leave it out. Next on the list is the ''. This actually refers to the archive name. We already know that the archive is called 'util.lzh', and so these are the letters that we put here. The next part of the command line is the '[]'. This is optional, but it's important for this operation. The destination path is where you want the files that come out of the archive to go to. And so, you can put a blank, unformatted disk in DF1:, and put DF1: here, which will tell the program to write the utility onto the disk in drive 1. You could put any drive here, DF0:, RAM:, DH0:. Anything that can store files. You can also put a directory on a disk, say 'DF1:Util' Which would inspire the program to put the files away into a drawer for you. Just like a secretary. If the drawer 'Util' is not there, our secretary will make a new one. The last option in the command line is relatively advanced. It deals with selecting only some files from the group. If you are familiar with WILDCARDS (ie, pattern-matching of names - see Megadisc's MegaDos manual for info) , this is where they are used. If not, don't worry. You probably won't need it anyway. So, here's our completed command line, ready to be typed in, to copy those files from the archive, to DF1: 1> lharc x util.lzh DF1: There, not so hard, was it? We could use our secretary analogy to easily describe the rest of the commands in the archive. Let us first describe the analogy in a more detail. Think of files as pieces of paper. These pieces of paper, when left to float around, tend to take up a lot of space, but, if they were to be put in a box, and stored in a corner somewhere, then they would take less space. Also the delivery boy, who wants to take the files to the office upstairs could simply pick up the box and carry it up. Which is much easier than picking up every single file, or sheet of paper. Now, we'll look at the most important of the commands which are used for LHarc. e,x Extract files from archive. With this command, you are effectively asking your secretary to go to the archive, take out the files that you specify in file patterns, or if not specified, all the files, make a photocopy of them. Put the originals back in the archive, and take the photocopied files over to you in single sheet format. By using destination path, you can tell the secretary where to put the files. l,v List archive contents. When this command is given, your secretary goes to the archive, and sorts through it, making a list of the files in the archive. a Add files to archive. When this command is given, you need to give your secretary some files, and then he/she will go and place photocopies of those files in the archive you specify. If the archive that you specify does not exist, then the secretary will create a new one by that name. m Move files into archive. This command causes your secretary to do the same thing as when adding files, but you are placing the originals into the archive, not leaving a copy from where it came from. d Delete files from archive. This command causes your secretary to take out the files that you specify from the archive, and put them through the paper shredder. You may notice that we've dealt with WHAT, and HOW. But not WHY. Here it comes....  WHY? Why should we bother? I mean, after all, how long must it take to continually archive and de-arcive the files that you want. As I've said before in the secretary analogy, an archive of files takes less space. This in itself is a good reason. If very little used files are archives, such as Hard Drive backups, then much less space is taken up. You don't expect to de-arc very often either. Another good reason is that once archived, the delivery boy will find it much easier and quicker to deliver the files. For this reason, archiving is almost always used when MODEM transfer is involved. Why? Because time online costs money, and it takes less time to deliver one file, than enter every file. It's also quicker because it's been compressed. Well. That's why!  CRUNCHING This type of process involves the compression of single files, and leaving them single. To follow the great tradition of analogies, you could look at the file, as still being a piece of paper. Once compressed, however, it is a piece of paper written in shorthand. It takes up less room, but, before it can be read, must be translated quickly back. This is possible for all types of files on the Amiga. It makes the files smaller, but doesn't make them unusable. The price? They're not as small as in the archive. But, nevertheless, they are smaller. Crunching uses a much larger program to do its work, and these programs have long docs, and a user-friendly interface. Programs such as PowerPacker, and Imploder do this very well, and are the most often used crunchers there are. PowerPacker is widely considered as the standard, as well as the best. MED is to Music, what PowerPacker and Imploder are to Crunching. To crunch a file, it is much easier to read the documentation to the program that you want to use. Both PowerPacker and Imploder use mouse interfaces. It should be no problem.  ARCHIVING vs. CRUNCHING ? No. This isn't a logical comparison. They are both used for different things. Archiving can compress more than crunching, but crunching leaves the files accessible. So, it comes down to this. Are you willing to sacrifice accessibility for less size? If so, then archive. If not the crunch. The wise user uses both of these techniques to the best advantage.  ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 ||| 28 |||