Simtel MSDOS 1992 June

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Diff - A Change Bar Tool Peter van Es CompuServe 72677,1417 As a person who has to maintain large amounts of documentation and needs to be able to track changes through successive revisions of the same document, I quickly felt the need for a tool which would insert change bars into documents. Remembering an article in Dr. Dobb's Journal, August 1987, by Don Krantz, I found a utility written in C which would insert Change Bars into existing document files in normal ASCII, exactly what I needed. I entered the program on my system, converting it to Turbo C (mainly using the Turbo C library routines for string comparisons) and tested it. Whereas the program worked fine on small text files, I had files for a document of 900 K text to process. The program's limitations became painfully obvious very rapidly. As it was written, the program could not: - deal with differences of over 200 lines, since the resync module only looked 200 lines ahead; - handle large text files since both uppercase and normal versions of the line were stored in memory. Furthermore the program was very slow on dealing with small changes (1 line inserted or deleted) due to the way the resynchronization algorithm was implemented. Rather than re-designing the program I went through a series of revisions on the program, tackling problems in small chunks. Perhaps it would have been better to re-design the program, but the resulting program has solved all of these limitations, and works just fine for my needs. Some of the existing limitations are documented at the end of this document. Why have I documented the change process here? Well, I dislike two things: - reinventing the wheel; - optimizing programs by delving into optimizers, or transforming parts into assembler routines. I believe in optimizing programs through the use of different, more efficient algorithms. Changing a high level language into assembler in selected places may give you a 10% speed increase, or, if you are lucky, a 50% increase. By altering the algorithm you can achieve improvements of 100% to 1000% much more easily (depending on the problem, of course). I hope that my thought processes might be of use to someone else, which is why I document them in this document. Page 1 Diff - A Change Bar Tool The Improvements The first problem was easy to solve. The original program stored a line read into memory both in normal format and in uppercase format to allow case insensitive change bars to be inserted. Since Turbo C provides a subroutine to compare two strings either with case sensitivity (strcmp()) or without (stricmp()) the memory use of the program was halved at the cot of some processing speed (deciding which one of the two comparison routines to use). Since these routines have been optimized by Borland, they actually performed faster than the routine that was originally in the program. The program included a good memory management scheme, which ensured that every line of text was only read once, and allocated memory would be freed at the earliest possible opportunity, when a match has been found. Since this scheme is quite efficient, I decided to keep it as an essential part of the program. Upon inspection the program was still CPU bound, spending most of its time in string comparisons, with the disk drive being idle. In order to eliminate the time on most of these comparisons, where the same line of text is compared over and over again with other lines, a simple hash value of the characters in the line is added to the structure containing the line. This hash value, or checksum, is an exclusive or of all the characters in the line. Instead of comparing lines, the program now compares the pre-calculated checksums, and only if they are equal, will it compare the line character for character. This reduced the amount of processing on similar lines considerably. However, the program was still very slow, which was very noticeable on files with lots of small changes. Furthermore, the program couldn't cope with changes of more than 200 lines of text. Checking the program revealed why this was so. The program would always take a line from file 1, and look up to 200 lines ahead in file 2, to find the matching line. Then it would take line 2 of file 1, and look 200 lines ahead from the current line of file 2 again. And so on, until it had found a minimum of re-sync (which is a command line modifiable value, normally set at 5) of un-changed lines. This meant that in order to detect a 1 line change, the program could perform up to a 1000 line comparisons. Since most changes two documents are small ones, I modified the program so that it now starts by checking only 10 lines ahead. If that fails, it looks 20 lines ahead, then 40, 80 and so on until it runs out of memory. There is no upper limit on the number of lines, other than constrained by memory. As a result, a small change requires far fewer comparisons and is detected far more rapidly. Page 2 Diff - A Change Bar Tool Big changes can also be detected, since the program does not have the artificial constraint of 200 lines look ahead anymore. If it doesn't match on 80 lines it tries 160, then 320 and so on until it runs out of memory. Since the memory management function is quite efficient (discarding lines as soon as a match has been found) quite a few lines can be stored in memory. That is also why the compact compilation model is used, maximizing the amount of allocatable memory for a small program. There still turned out a problem with this. The program could now almost always synchronize, but on extremely large changes this was still very slow. The reason for this is, that every time the lookahead number of lines is doubled, the program has to start at line 1 of file 1 again, but now checking half the number of lines in file 2 for the second time, and half for the first time. Ie, the same line would be compared against the same line lots of times anyway. In order to minimize the amount of repeat lookups which occur every time that the lookahead lines are increased, the program now assumes that after 40 lines couldn't match, the change is probably very big, and the lookahead is set to 400 lines. If it still doesn't match, it sets them at 800, 1600 etc until it runs out of memory. Regrettably, although this helped a little bit, it was only a little bit, since for each of the 400 lines in file 1 each and everyone of the 400 lines in file 2 are checked, leaving the program still CPU bound. Then a brainwave hit. If we can use the checksum (a value between 0 and 255) to compare lines in the files, why can't we use it to quickly locate a line already read in for file 2? So an array was created, with an entry for each possible checksum value. This entry is the start of an ordered linked list, pointing to lines with the same checksum. Rather than having to check all lookahead lines in order to resync, the program uses the checksum value of the line in file 1 to index into the array of linked lists. It then traverses the list of lines with the same checksum, comparing the lines until one has been found which is equal. If none are found, the program takes the next line from file 1, and repeats the process. If no match can be found at all, the program restarts the lookahead procedure with twice the number of lines to look ahead. In order to ensure that sufficient lines are present, lookahead lines are read every time, which means that the memory management still works and memory gets cleared every time a synchronization has been detected. At one point a resync will either be found, or end of file reached (or memory is full -- this will not happen soon since matched lines are discarded from memory, so the change would have to exceed the size of memory). Page 3 Diff - A Change Bar Tool However, since file 2 is not scanned sequentially but randomly, the last line before a match (which is used to be able to produce the "deleted from file 2" difference list) is not available any more. In order to allow for this, the line structure now includes a pointer to the previous line, so that the difference summary can still be printed. The nextline() and discard() functions maintain the line structure list, and have been modified to maintain the checksum array and the previous line list as well. Resync has been altered substantially in order to take advantage of the array. Minor modifications have been made to other routines in order to provide extra error checking. As a result of these modifications the program now is much faster especially with lots of small changes (due to the lookahead line policy), and all large changes (due to the checksum linked list). So much so, in fact, that it is also I/O bound (ie the disk drive is almost operating continuously). And it can handle bigger changes. The program now can put change bars into my 900 K of document faster than my word processor can print it. Limitations of the program. The main limitation of the program is due to the resynchro- nization algorithm. The program considers a file resynchronized if it discovers a number of matched lines. This number is set at 3 as a default, but can be modified using the -r option. However, the results of this algorithm can be surprising. When text has been duplicated (with more than resync lines) and sandwiched in between new text, and the old text has been left unchanged after the inserted text (as in the following example), a match will be detected. In the example assume that resync is 3. file 1 file 2 A P B _______ Q C \ R D ______ \__________________ B E ____ \ C F \ \___________________ D G \ S H _ \ T I \ \ B \ \ C \ \ D \ \________________ E A false match... F \ G \___________________ H Page 4 Diff - A Change Bar Tool As can be seen above, BCD in file 1 will match with the first occurrence of BCD in file 2. And as a result, the second BCD in file 2 will not match with the BCD in file 1. (The remainder, EFGH will match again). A more logical match would have been to leave the PQBCDRST sequence as a new insert, and just match on the sequence BCDEFGH. (Note that the first match could have been avoided by selecting the number of lines to be resynced to be larger than 3). A different algorithm, called the "longest common subsequence" algorithm, will do just that automatically. It will select the longest matching sequence in the file and use that as a basis for selecting the second longest matching sequence both above and below the longest matching sequence just found, and so on until all sequences have been found. This method is also particularly suitable for implementation using hashing. However, a disadvantage of this algorithm is that no lines can be discarded until both files have been processed in their entirety. As a result either lines have to be kept on disk (for instance storing the lseek() value in the line buffer with the checksum, so that lines can be retrieved from disk using direct access), or the size of the files that can be processed is limited. Furthermore, since the program will recursively check for matching subsequences (compare it for instance with CAR Hoare's QuickSort algorithm) this algorithm will produce better difference reports only at the cost of much more computer time. Thus this algorithm is used most often on mini-computer systems. Note that a mix between the "longest common subsequence" algorithm as described above, and the "scan until next match" as implemented in the program presented here can be implemented to good effect. Rather than having a minimum number of lines before resync is achieved, select a maximum number of lines after which a matching subsequence is considered the longest, causing the file to be split. By setting this number as one larger than the largest sequence which occurs more than once in a file (eg 100 lines) the program will produce the same results as the unconstrained version of the program, in less processing time. A final limitation of the program is due to the fact that it compares files on a line by line basis. This is fine for for instance program source code, but gives problems with most automatic paragraph reformatting features available in word processors. The result of inserting a word in one sentence can often be a re-format of several lines of text in the paragraph. Although in reality those sentences have not changed, the program will think they are changed. The solution is to perform the comparison on a sentence by sentence (or word by word) basis rather than line by line. The difficulty is however not in the synchronization algorithm (since that can stay as it is), but in the Page 5 Diff - A Change Bar Tool algorithm which inserts change bars on the correct line in the correct place in the document. You would have to store a representation of the original line of text somewhere, and use a pointer to associate every word with the line from which it came. This modification is left as an exercise for the reader, however. If I ever get to it, I'll put the resulting code in Public Domain again. Using the diff program. diff - text file differencer and change barrer usage: diff [option{option}] newfile oldfile [barfile] Where the options are optional and can be inserted in any order except for the -n and -o option (which must appear in that order). Newfile and Oldfile are required (you don't want to compare a file against nothing, do you?). Barfile is optional, but required if you want a file with change bars in it as output. The options are: -t trace operation, default off -b n column of barfile for change bar, default 78 -h n lines at top of page to skip for headers, default 0 -f n lines at bottom of page to skip for footers, default = 0 -p n lines per page (embedded form feeds override), default = 66 -c uppercase/lowercase is significant (default is off) -r n lines that must match before files are considered synced after differences are found. default = 3 -w blank lines are considered significant (default is not) -s screen listing off (default is on) -n n pages in NEWFILE to skip before compare. Also sets - o. default = 0 -o n pages in OLDFILE to skip before compare. Must come after -n. default = 0 The Trace option (-t) only shows if the program has been compiled with the debug option on. It shows debug messages. The Bar-column option (-b) selects the column in which the vertical bar will be placed. The default is column 78. Column 0 will put it in the left most position on the page. If your program contains embedded tabs, these are not expanded automatically to ensure that when the bar should appear in column 78, it actually does appear there. So use the option -b 0 in this case. Page 6 Diff - A Change Bar Tool The Header option (-h) and Footer option (-f) allow you to specify the number of lines to skip at the top of the page and the bottom of the page for headers and footers. You don't usually want change bars just because the date or page numbering of a file has changed. The Page length option (-p) allows you to specify the number of lines per page. 66 is the Default. The Case Sensitive (-c) option will insert change bars if just the case of text has changed. The default is no case sensitivity, so that for instance changing "New york" into "New York" does not generate a change bar. For case sensitive language's program listings you'll want to use the -c option. The Resync option (-r) specifies the number of lines that must match before the files are considered re-synced. The limitations of this parameter have been discussed above. The default is 3. For program source code you'll want to experiment with slightly larger values. The White option (-w) makes blank lines significant. The default is that they are not significant. I never use this option, but perhaps you have a need for it. The Screen option (-s) turns off the screen listing of the differences. This speeds up the program when you are not interested in the differences list, or when you use the program in a batch file. Note that the program will give an error message if you specify the -s option and no bar file (since then all it would do is waste computer time). The differences list to screen has the following format: 002:12<This text has been added to the newfile on page 2, 002:13<lines 12 and 13. 002:12>This line has been deleted from oldfile, page 2 and 002:13>lines 12 and 13 and 14. Note that every time a match 002:14>has been found a blank line is printed first. (Note the direction of the arrows: < for insertion into the newfile, >for deletion (extraction if you like) from the oldfile to the newfile). The Newfile skip option (-n) and the Oldfile skip option (-o) allow you to skip pages at the beginning of the file for header pages and contents lists and the like, where you are not interested in change bars. Note that the -n option sets the -o option (unless you override the -o setting by explicitly including it in case contents lists are different lengths). Page 7