Delta algorithms: An empirical analysis

Delta Algorithms: An Empirical Analysis JAMES J. HUNT University of Karlsruhe KIEM-PHONG VO AT&T Laboratories and WALTER F. TICHY University of Karlsruhe Delta algorithms compress data by encoding one file in terms of another. This type of compression is useful in a number of situations: storing multiple versions of data, displaying differences, merging changes, distributing updates, storing backups, transmitting video se- quences, and others. This article studies the performance parameters of several delta algorithms, using a benchmark of over 1,300 pairs of files taken from two successive releases of GNU software. Results indicate that modern delta compression algorithms based on Ziv-Lempel techniques significantly outperform diff, a popular but older delta compressor, in terms of compression ratio. The modern compressors also correlate better with the actual difference between files without sacrificing performance. Categories and Subject Descriptors: D.2.7 [Software Engineering]: Distribution and Main- tenance—version control; D.2.8 [Software Engineering]: Metrics—performance measures; E.4 [Data]: Coding and Information Theory—data compaction and compression; E.5 [Data]: Files—backup/recovery General Terms: Algorithms, Experimentation, Measurement, Performance Additional Key Words and Phrases: Benchmark, delta encoding, differencing 1. INTRODUCTION Delta algorithms, i.e., algorithms that compute differences between two files or strings, have a number of uses when multiple versions of data objects must be stored, transmitted, or processed. The major early applica- An earlier version of this article appeared in the Proceedings of the ICSE’96 SCM-6 Workshop (March 25–26, 1996), pp. 49 – 66. Authors’ addresses: J. J. Hunt and W. F. Tichy, Institut fuer Programmstrukturen und Datenorganisation, University of Karlsruhe, Am Fasanengarten 5, Postfach 6980, D-76128 Karlsruhe Germany; K.-P. Vo, Research, AT&T Laboratories, 180 Park Avenue, Florham Park, NJ 07932 . Permission to make digital / hard copy of part or all of this work for personal or classroom use is granted without fee provided that the copies are not made or distributed for profit or commercial advantage, the copyright notice, the title of the publication, and its date appear, and notice is given that copying is by permission of the ACM, Inc. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and / or a fee. © 1998 ACM 1049-331X/98/0400 –0192 $5.00 ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998, Pages 192–214.  Delta Algorithms: An Empirical Analysis • 193 tion of delta algorithms occured in revision control systems such as SCCS and RCS [Rochkind 1975; Tichy 1985]. By storing deltas relative to a base revision, these systems save substantial amounts of disk space compared with storing every revision in its entirety. Much less information need be stored for each revision because changes from one revision to the next are typically small. Other well-known applications are the display of differ- ences between files and the merging together of the changes in two different files relative to a common base. The classic program for generating deltas is Unix diff [Hunt et al. 1976; 1977]. Both SCCS and RCS use it for storage and display of differences; RCS also uses it for merging. Since diff is limited to text files, so are SCCS and RCS. However, users wish to place binary code under revision control as well, not just source text. A simple technique is to map the binary code into text and then apply diff. While this works reliably and is widely used in practice, the deltas produced are typically larger than the originals! Newer algorithms such as bdiff [Tichy 1984] and suff [Obst 1987] do not exhibit this problem. Today, binary differencing capability has become mandatory. There are now many binary source formats that users need to manage, such as word processor files, spreadsheet data, electrical and mechanical CAD data, audio, and images. Thus, revision control system must handle binary files in a space efficient way. The display of differences and merging is also more difficult for these complex formats than for simple text. Compression not only saves space, but also reduces I/O and thus can speed up programs. Compressing I/O is interesting because the speed differential between processors and bulk storage devices is about three orders of magnitude and increasing. Reading a delta from disk can be faster than reading the corresponding full file, even with the decompression time added in. Deltas have found a number of applications outside revision control. For example, backup programs save space by storing deltas. Checkpoints of large data spaces can be compressed dramatically with deltas and can then be reloaded rapidly. Display updates can be performed efficiently using a delta that moves lines or rectangles around on the display [Gosling 1981]. Deltas are also needed for sequence comparison in molecular biology. The recent explosive expansion of the World Wide Web has accentuated another use of deltas: they can be used for distributing updates for software and other data. Besides saving time, space, and network bandwidth, distributing deltas has another interesting benefit: a delta is effectively an encrypted form of the new version. It can only be decoded if the original is available. Thus, software updates in the form of deltas could be made available while strongly reducing the threat of piracy. Of course, revision control is also needed for simplifying web maintenance and providing access to old web pages. Obviously, choosing the best delta algorithm for any given application requires reliable and comparable information about the performance of each algorithm; however, the state of empirical comparisons of delta ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 194 • J. J. Hunt et al. algorithms is poor. Miller and Myers [1985] compare the runtime of their delta program, fcomp, with that of Unix diff. Their first test involves two pairs of (highly atypical) files, and fcomp fails on one of them. Additional tests were run, but not enough particulars are given to repeat the tests independently. Obst [1987] compares several delta algorithms on programs of about 3MB. No details are given that would permit the repetition of their experiment. In both instances, the claims are quite dubious. The require- ments of a proper benchmark—a well-defined metric and a large, accessi- ble, domain-appropriate data set—are not met. Nor are statistical methods brought to bear on the results. The unreliability of the observations is underscored by outliers and irregularities. The authors’ goals in this article are threefold: to provide a metric for comparing delta algorithms objectively, to suggest an appropriate bench- mark for the comparison, and to present the results of applying the metric and benchmark to an interesting set of delta algorithms, i.e., Unix diff and two modern delta compressors. Results are presented for text, object code, and a mixture of binary code and text and report compression ratio, compression speed, and decompression speed. Enough information is given so that anyone can repeat the experiment. 2. METRIC Designing a metric for comparing delta algorithms objectively is not as easy as it sounds. While it is straightforward to measure time, measuring compression ratio is more difficult. It is not enough to simply use a set of file pairs and determine the cumulative size of the deltas. Such summary information is not helpful without knowledge about the characteristics of the differences and the content of files being compressed. For instance, how large is a delta if the difference is small? What happens if the difference is (relatively) large, but the file size is small? In other words, how well does the delta size track the actual difference for various file sizes? How do different formats such as text, binary code, image data, etc., affect the delta size? A new metric based on the Longest Common Subsequence (LCS) addresses these concerns. The performance of a delta algorithm depends on the size of the differ- ence between pairs of files. The authors propose using the LCS as the reference against which to measure compression. This metric applies mainly to one-dimensional data, but it may also apply to higher-dimen- sional data that can be linearized without fragmenting typical changes. Given two strings of characters, an LCS is a longest sequence of charac- ters that is contained within both strings. The characters in the LCS need not be contiguous in the two strings. Thus, the length of the LCS is a good measure of the commonality of two strings. The difference between two files can then be expressed as the average size of the two files1 minus the size of the LCS (all in bytes): 1 The average of the sizes of both files is used to eliminate any bias in the direction in which a delta is computed. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 195 size~ file1! 1 size~ file2! difference 5 2 size~ LCS! (1) 2 An LCS is computed by an algorithm using dynamic programming [Hunt et al. 1977; Nakatsu et al. 1982]. The algorithm’s runtime is O ~ nm ! where n and m are the files sizes, so it is not practical for general use. Practical delta algorithms find approximations of the LCS or use other techniques to find common segments of strings. 3. BENCHMARK Though a metric is useful for providing a basis of analysis, truly meaning- ful comparison requires a standard data set or benchmark as well. This benchmark combined with an objective metric makes the comparison repeatable and the results verifiable. Repeatability is one of the essential ingredients of scientific experiments. Results that cannot be reproduced in an independent trial are not trustworthy.2 The first problem encountered when defining a benchmark is finding an appropriate data set that is both large enough for the results to be statistically significant and representative of real-world applications. For delta algorithms, the most important quality of any benchmark is that it contain a wide spectrum of change examples. This means that both the size of the changes represented and the size of the files involved should vary considerably. Large changes on small files and small changes on large files should be included as well as small changes on small files and large changes on large files. Furthermore, the benchmark should contain a variety of formats, partic- ularly, pure text, pure object code, and pseudotext. Pseudotext is what word processors produce: stretches of text with interspersed binary data and infrequent line breaks. Pseudotext and binary formats have become more important as word processors, spread sheets, and multimedia data have become prevalent. Data sets that meet these requirements can be found in the GNU software. Thanks to the Free Software Foundation, quite a number of software projects are available in successive versions. The authors chose two versions of GNU emacs—19.28 and 19.29 —and two versions of GNU gcc—2.7.0 and 2.7.1—as their test suite. These versions provide a broad spectrum of variation between one revision of any given file and the next. Files range from 0 to over 200KB in size with differences from 0 to 90%. The benchmark contains 810 text files (C programs, Lisp programs, docu- mentation) and 300 files with Lisp byte code (a pseudotext format). The authors also compiled the 201 C program files present in both versions and included the resulting object code in the study. The total comes to 1344 file pairs. 2 The history of benchmarking teaches that benchmarks cannot be completely static; they must be extended from time to time to prevent overfitting of algorithms to their benchmark. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 196 • J. J. Hunt et al. Another important advantage of GNU software is that it is freely available. Thus, it is easy to confirm or reject our results independently. It also allows the developers of new delta algorithms to compare their algorithms with those presented below without duplicating this entire study. A potential problem with GNU software is that it was produced under unique circumstances. However, we are not claiming that the GNU soft- ware is representative for all development environments. Since we report results dependent on the variables format, change size, and file size, it is unimportant what mix of these variables is actually present in the GNU software. Thus, our results permit easy determination of an appropriate algorithm for given circumstances: simply select the ranges of the variables appropriate for a given environment and read-off the performance of the algorithms. 4. ALGORITHMS Three delta algorithms are examined in this study: Unix diff, bdiff, and vdelta .3 Unix diff is still the most widely used delta algorithm in revision control and configuration management systems. It finds an approximation of the LCS by considering whole lines instead of characters as indivisible units [Hunt et al. 1976]. It is much faster than computing a byte-level LCS because it does not examine all possible combinations of byte positions. However, only common lines can be found with diff. As stated above, revision control for binary data has become essential, but diff can only handle text files. A simple solution to this problem is to map binary files to text before handing them to diff. The most widely available program used for this conversion is uuencode. Since uuencode recodes data with fixed line lengths, it is not hard to convince oneself that it is a poor encoding choice. One could come up with a better solution, but that is not the purpose of this study. Instead the uuencode 1 diff combina- tion is examined because it is frequently used: it is used in at least one commercial product and the widely available RCS. It is important to include the uuencode 1 diff results to show how poor a choice uuencode actually is. Folding binary files into the text range is not necessary for any of the other algorithms, so uuencode is only used with diff. The other two algorithms—bdiff and vdelta—piece together the second file out of blocks from the first file. Unlike diff, these algorithms are applicable to any byte stream. They exploit reordering of blocks to produce short differences. Further details about bdiff and vdelta are given in the appendices. Bdiff and vdelta offer additional compression on the resultant delta. For this reason, diff coupled with gzip postprocessing is included in the study as well. All algorithms run enough faster than a byte-level LCS computa- tion to have practical applications. 3 Vdelta is described in Appendix B. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 197 Table I. Software System Examples: GNU Emacs Files Size 19.28 Size 19.29 LCS Changed Ratio (%) Text 640 16,640,810 17,538,744 14,966,051 2,572,693 15.5 ELC 305 4,287,642 4,674,901 3,519,338 1,155,563 27.0 Object 60 3,035,984 3,441,120 2,598,887 842,233 27.7 Totals 1,005 23,964,436 25,654,765 21,084,276 4,570,489 19.1 Table II. Software System Examples: GNU GCC Files Size 2.7.0 Size 2.7.1 LCS Changed Ratio (%) Text 196 12,319,992 12,422,167 12,198,290 223,877 1.8 Object 143 10,818,472 10,888,376 10,158,623 729,753 6.7 Totals 339 23,138,464 23,310,543 22,356,913 953,630 4.1 Both Bdiff and vdelta are comparable in utility to diff. Not only do they produce deltas suitable for compression and three-way file merging, but their output can also be used to display differences. Since bdiff and vdelta break lines apart, minor postprocessing of the deltas is needed to produce output identical to diff ’s. Other human-readable output that takes advan- tage of the finer granularity of the output of these algorithms can be produced using color-coding techniques. In addition, both bdiff and vdelta can compress a single file with itself. 5. SUMMARY RESULTS Before delving into the details of the experiment, one can get a reasonable sense of performance from a simple overview. A delta is often one to two orders of magnitude smaller than the original and significantly smaller than a direct compression of the original. For example, Tables I and II summarize the differences between two versions of the two different GNU projects mentioned above: GNU emacs, releases 19.28 and 19.29, and of GNU gcc, releases 2.7.0 and 2.7.1. The data are divided into text and objects code files. For emacs there is an additional row for byte-compiled Lisp (ELC) files. The column headed with Changed in Tables I and II is the size of what was changed from or added to the first release to obtain the second. One can see that quite large space savings can be obtained simply by storing a reference release and a sequence of changes instead of a full copy of each release. One would expect that the better the delta algorithm is, the closer the delta produced would be to the LCS size. Tables III and IV give summary figures for all the delta algorithms. The ratio is computed as a percentage of the resultant delta and the size of the second version. The output from diff is always longer than the LCS-based delta. Furthermore, binary files, which are preprocessed with uuencode before diff is applied, are signifi- cantly longer than the originals in two of the three cases in Tables III and IV. Even with gzip postcompression, the resultant deltas from diff are still ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 198 • J. J. Hunt et al. Table III. Results Summary: GNU Emacs Changed diff diff1gzip bdiff vdelta Text Size 2,572,693 4,509,529 1,530,684 1,528,763 1,277,287 Ratio 15.5% 27.1% 9.2% 9.2% 7.7% ELC Size 1,155,563 6,359,727 2,697,410 574,216 542,577 Ratio 27.0% 148.3% 62.9% 13.4% 12.7% Object Size 842,233 4,313,537 1,579,490 962,562 1,000,759 Ratio 27.7% 142.1% 52.0% 31.7% 33.0% Totals Size 4,570,489 15,182,793 5,807,584 3,065,541 2,820,623 Ratio 19.1% 63.4% 24.2% 12.8% 11.8% Table IV. Results Summary: GNU GCC Changed diff diff 1 gzip bdiff vdelta Text Size 223,877 647,079 175,300 140,549 146,204 Ratio 1.8% 5.3% 1.4% 1.1% 1.2% Object Size 729,753 9,746,789 3,346,493 1,359,273 1,536,779 Ratio 6.7% 90.1% 30.9% 12.6% 14.2% Totals Size 953,630 10,393,868 3,521,793 1,499,822 1,682,983 Ratio 4.1% 44.9% 15.2% 6.5% 7.3% significantly worse than bdiff and vdelta. The deltas from bdiff and vdelta are very close in compression ratio overall. Vdelta seems to compress text data better, and bdiff appears to compress object code better. One can see that in some cases, bdiff and vdelta outperform LCS. That is because the algorithms postcompress the resultant delta. Tables III and IV just give a broad overview of the Emacs and GCC results. How the compression rate depends on the actual difference between the files being compared is not apparent. For this reason, it is important to investigate how the delta of each algorithm compares with the LCS size as well as how fast deltas can be computed and what effects additional compression might have. 6. METHOD In order to obtain a more detailed view of the performance of the various delta algorithms, a comparison was done based on the difference metric defined in Section 2. As above, the Longest Common Subsequence was computed for each pair. Then each algorithm was run on each pair of files. This procedure was repeated for the compiled C code of both versions of GNU emacs and GNU gcc. Files that existed in one version and not the other and files that did not differ at all were eliminated. All computation was carried out on a DEC Alpha system. The precise combination of algorithms chosen were Unix diff -n (as used by RCS),4 Unix diff followed by compressing the results with gzip, bdiff, and vdelta. The files were broken up into three types: text files (mostly C 4 Unlike the default and -c output, the -n output of diff is quite dense, as it is not designed for human consumption, but rather as a script for automatic processing. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 199 Fig. 1. Plot of delta size for text files. and Elisp code), byte-compiled Elisp code (ELC files), and object files. As stated above, since diff was designed to work only with text files, uuencode was used to convert ELC and binary files to text. Each algorithm ran with each file pair both forward and reverse, e.g., revision 19.28 then 19.29 and revision 19.29 then 19.28. Each forward and reverse pair was then averaged together to give a single value for compar- ison. This procedure averages out the effect on differences where one file is much smaller than the other. Removing large sections from one file results in a small delta, and adding large sections results in a large delta. In practice, this phenomena is also averaged out in revisioning systems, since one revision must be stored in its entirety. Two types of data were collected: the size of the delta file and the time needed to encode and decode each pair. Since no dedicated decoder is available for Unix diff -n files, RCS was used to time diff -n encoding and decoding. (The authors tried using diff -e and ed, but that proved to be ridiculously slow.) Unix wc measured file sizes and Unix time measure duration. The results report byte count and user plus system times. 7. DETAILED RESULTS There are two important measures for compression algorithms: the result- ant compression ratio and the execution speed. The authors present the results of their study in graphic form below. Results are given separately ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 200 • J. J. Hunt et al. Fig. 2. Plot of delta size for ELC files. Fig. 3. Plot of delta size for object files. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 201 Fig. 4. Log plot of delta sie for text files. for text files, ELC files, and object files, since the behavior of some of the algorithms differ for these classes. ELC files were not combined with object files because their content is mostly text and thus similar to many common word processor formats. 7.1 Compression The metric in Section 2 is the basis for measuring differences here. An alternative would have been to just examine file sizes and compare them to the sum of the delta sizes. Although this number is also interesting, the LCS comparison used here sheds more light on the correlation between the actual changes and the size of the delta. Each graph contains a point for each file pair and each algorithm. The overall trend of the data is illus- trated with a linear regression line for each algorithm. The correlation coefficients provide the reader with an indication of how linear each of the depicted compression relationships are.5 In the first set of three graphs—Figures 1, 2, and 3—the average size of the forward delta and reverse delta is plotted against the difference. The x-axis is simply the average size of the two files in each pair minus the LCS size. The y-axis is the average size of the forward and reverse delta for each 5 The correlation coefficient does not indicate how good the algorithm is, rather how consistent the compression is with the actual difference between versions. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 202 • J. J. Hunt et al. Fig. 5. Log plot of delta size for ELC files. Fig. 6. Log plot of delta size for object files. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 203 Fig. 7. Plot of compression ratio for text files. algorithm. Here, one can see how much better bdiff and vdelta correlate6 to the difference than diff and diff with gzip. Though diff with gzip performs as well as bdiff and vdelta for text files (Figure 1), it performs much more poorly for ELC and object files. The outlying points correspond to files where the difference is much smaller than the file size. All algorithms performed more poorly on the object file set (Figure 3) than on the other sets. The next set of three graphs—Figures 4, 5, and 6 —presents the same data as a log-log plot. The logarithmic scales permit the presentation of a much greater range of differences and delta sizes. As expected, the linear regression lines are not straight due to their nonzero y-intercepts. This is caused by a constant overhead factor in the delta format. Here one can see the low end of the graph in more detail. The separation between the data points for the different algorithms is clearly visible, and the band for diff with gzip is much closer for those of bdiff and vdelta for text files than for the other categories. The final set of three graphs—Figures 7, 8, and 9 —presents the average compression ratio against the ratio of difference to average file size. Here, the x-axis is given as one minus the LCS size divided by the average file size in each pair, thus expressing how much the two files differ as a ratio. The y-axis is the size of the delta produced by a given algorithm divided by 6 Correlation coefficients are provided for each graph. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 204 • J. J. Hunt et al. Fig. 8. Plot of compression ratio for ELC files. Fig. 9. Plot of compression ration for object files. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 205 Fig. 10. Plot of encoding time for text files. the average file size for the pair, i.e., the size of the delta as a ratio to the file size. Here it becomes clear how badly uuencode disrupts the Unix diff algorithm. Good correlation is obtained for bdiff and vdelta, whereas diff and diff with gzip appear to be independent of the difference ratio. All these graphs show clear trends in the performance of diff, diff 1 gzip, bdiff, and vdelta. 7.2 Efficiency Time performance for both compression and decompression is also impor- tant in evaluating delta algorithms. Time, given as a sum of system and user time as given by the Unix time utility, is plotted against the average file size. The first three plots—Figures 10, 11, and 12—present encoding times in seconds and the remaining three plots—Figures 13, 14, and 15—show decoding times in seconds. The scale is not the same for all plots, but the aspect ratio is held constant in each group. The reader should note the change in aspect ratio between the encode plots and the decode plots. Decoding is much faster for all algorithms. Though, the relative perfor- mance of the algorithms vary between encoding and decoding, vdelta is almost as fast as diff (the fastest) for encoding and much faster than all algorithms for decoding. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 206 • J. J. Hunt et al. Fig. 11. Plot of encoding time for ELC files. Fig. 12. Plot of encoding time for object files. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 207 Fig. 13. Plot of decoding time for text files. 8. CONCLUSION Vdelta is the best algorithm overall. Its coding and decoding performance is high enough to be used for interactive applications. For example, it could be used to improve performance of raster display updates over relatively slow network links. Though bdiff generates output that is comparable in size to vdelta, vdelta is much faster. Both vdelta and bdiff result in delta sizes that correlate well with the difference. This is not true for diff. In the best case—text files—diff only reaches the effectiveness of vdelta and bdiff when it is combined with gzip. As expected, using uuencode is not a good idea for binary files, since it breaks diff ’s algorithm for detecting un- changed sequences. This property was anticipated, because uuencode es- sentially removes all natural newlines and adds new ones at constant intervals. This means that only changes that do not modify the positions of unmodified characters or change the file length by an exact multiple of the constant interval can be effectively processed by diff. 9. FUTURE WORK The result of this article apply only to one-dimensional data. Other studies should consider two- and higher-dimensional data such as images and video data. In addition, this test suite could be used to fine tune both bdiff and vdelta and to determine what effect postcompression has on run time and delta size. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 208 • J. J. Hunt et al. Fig. 14. Plot of decoding time for ELC files. Fig. 15. Plot of decoding time for object files. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 209 APPENDIX A. BDIFF Bdiff is a modification of W. F. Tichy’s block-move algorithm [Tichy 1984]. It uses a two-stage approach. First it computes the difference between the two files. Then it uses a second step to compress the resulting difference description. These two parts run concurrently in that the first stage calls the second each time it generates output. In the first phase, bdiff builds an index, called a suffix tree, for the first file. This tree is used to look up blocks, i.e., substrings, of the second file to find matches in the first file. A greedy strategy is used, i.e., every possible match is examined to ensure that the longest possible match is found. The output from this phase is a sequence of copy blocks and character inser- tions that encode the second file in terms of the first. It can be shown that the algorithm produces the smallest number of blocks and runs in linear time. It also discovers crossing blocks, i.e., blocks whose order was per- muted in the second file. The second phase efficiently encodes the output of the first. A block is represented as a length and an offset into the first file. Characters and block lengths are encoded in the same space by adding 253 (256 minus the three unused lengths) to lengths before encoding. Blocks of lengths less than four bytes are converted to character insertions. Characters and lengths are then encoded using a common splay tree [Jones 1988]. The splay tree is used to generate a character encoding that ensures that frequently encoded characters are shorter than uncommon characters. Splay trees dynamically adapt to the statistics of the source without requiring an extra pass. A separate splay tree encodes the offsets. Bdiff actually uses a sliding window of 64KB on the first file, moving it in 16KB increments. This means that the first phase actually builds four suffix trees that index 16KB each of the first file. The window is shifted forward whenever the encoding of the second file crosses a 16KB boundary, but in such a fashion that the top window position in the first file is always at least 16KB ahead of the current encoding position in the second file. Whenever the window is shifted, the oldest of the four suffix trees is discarded and a new one built in its space. The decoder has to track the window shifts, but does not need to build the suffix trees. Position informa- tion is given as an offset from the beginning of the window. B. VDELTA Vdelta is a new technique that combines both data compression and data differencing. It is a refinement of W. F. Tichy’s block-move algorithm [Tichy 1984], in that, instead of a suffix tree, vdelta uses a hash table approach inspired by the data parsing scheme in the 1978 Ziv-Lempel compression technique [Ziv and Lempel 1978]. Like block-move, the Ziv-Lempel tech- nique is also based on a greedy approach in which the input string is parsed by longest matches to previously seen data. Both Ziv-Lempel and ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 210 • J. J. Hunt et al. block-move techniques have linear-time implementations [McCreight 1976]. However, implementations of both of these algorithms can be mem- ory intensive, and without careful consideration, they can also be slow because the work required at each iteration is large. Vdelta generalizes Ziv-Lempel and block-move by allowing for string matching to be done both within the target data and between a source data and a target data. For efficiency, vdelta relaxes the greedy parsing rule so that matching prefixes are not always maximally long. This modification allows the construction of a simple string-matching technique that runs efficiently and requires minimal main memory. B.1 Building Difference For encoding, data differencing can be thought of as compression, where the compression algorithm is run over both sequences but output is only generated for the second sequence. The idea is to construct a hash table with enough indexes into the sequence for fast string matching. Each index is a position which is keyed by the four bytes starting at that position. In order to break a sequence into fragments and construct the necessary hash table, the sequence is processed from start to end; at each step the hash table is searched to find a match. Processing continues at each step as follows: (1) if there is no match, (a) insert an index for the current position into the hash table, (b) move the current position forward by 1, and (c) generate an insert when in output mode; or (2) if there is a match, (a) insert into the hash table indexes for the last 3 positions of the matched portion, (b) move the current position forward by the length of the match, and (c) generate a copy block when in output mode. Each comparison is done by looking at the last three bytes of the current match plus one unmatched byte and checking to see if there is an index in the hash table that corresponds to a match. The new match candidate is checked backward to make sure that it is a real match before matching forward to extend the matched sequence. If there is no current match, i.e., just starting a new match, use the four bytes starting at the current position. As an example, assume the sequence below with the beginning state as indicated (the s indicates the current position): s 0 1 2 3 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 b c d e a b c d a b c d a b c d e f g h ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 211 The algorithm starts at position 0. At this point the rest of the sequence is the entire sequence, so there is no possible match to the left. Case 1 requires position 0 to be entered into the hash table (indicated with an asterisk under it) then to advance the current position by 1. s 0 1 2 3 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 b c d e a b c d a b c d a b c d e f g h * This process continues until position 8 is reached. At that time, we have this configuration: s 0 1 2 3 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 b c d e a b c d a b c d a b c d e f g h * * * * * * * * Now the rest of the sequence is “abcdabcdedfg.” The longest possible match to some part previously processed is “abcdabcd,” which starts at location 4. Case 1 dictates entering the last 3 positions of the match (i.e., 13, 14, 15) into the hash table, then moving the current position forward by the length of the match. Thus the current position becomes 16 in this example. s 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 b c d e a b c d a b c d a b c d e f g h * * * * * * * * * * * The final step is to match “efgh” (which fails), so the last mark is on position 16. The current position moves to position 17 which now does not have enough data left for the next hash code, so the algorithm stops after outputting the last three characters. s 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 b c d e a b c d a b c d a b c d e f g h * * * * * * * * * * * * Note that the matching algorithm above will actually find the longest match if indexes are kept for every location in the string. The skip in step ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998. 212 • J. J. Hunt et al. (2b) prevents the algorithm from being able to always find the longest prefix; however, this rule saves considerable processing time and memory space. In fact, it is easy to see from the above construction rules that the space requirement is directly proportional to the output. The more com- pressible a target data set is, the faster it is to compress it. B.2 Difference Encoding In order to minimize the output generated, the block-move list generated above must be encoded. The output of vdelta consists of two types of instructions: add and copy. The add instruction has the length of the data followed by the data itself. The copy instruction has the size of the data followed by its address. Two caches are maintained as references to minimize the space required to store this address information. Each instruction is coded starting with a control byte. Eight bits of the control byte are divided into two parts. The first 4 bits represent numbers from 0 to 15, each of which defines a type of instruction and a coding of some auxiliary information. Below is an enumeration of the first 10 values of the first 4 bits: 0: an add instruction, 1,2,3: a copy instruction with position in the QUICK cache, 4: a copy instruction with position coded as an absolute offset from the beginning of the file, 5: a copy instruction with position coded as an offset from current location, and 6,7,8,9: a copy instruction with position in the RECENT cache. For the add instruction and the copy instructions above, the second 4 bits of the control byte, if not zero, code the size of the data involved. If these bits are 0, the respective size is coded as a subsequent sequence of bytes. The above-mentioned caches—QUICK and RECENT— enable more com- pact coding of file positions. The QUICK cache is an array of size 768 ~ 3*256 ! . Each index of this array contains the value p of the position of a recent copy instruction such that p modulo 768 is the array index. This cache is updated after each copy instruction is output (during coding) or processed (during decoding). A copy instruction of type 1, 2, or 3 will be immediately followed by a byte whose value is from 0 to 255 that must be added to 0, 256, or 512 respectively to compute the array index where the actual position is stored. The RECENT cache is an array with 4 indices storing the most recent 4 copying positions. Whenever a copy instruction is output (during coding) or processed (during decoding), its copying position replaces the oldest position in the cache. A copy instruction of type 6, 7, 8, or 9 corresponds to cache index 1, 2, 3, or 4, respectively. Its copying position is guaranteed to be larger than the position stored in the corre- sponding cache index, and only the difference is coded. ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.  Delta Algorithms: An Empirical Analysis • 213 It is a result of this encoding method that an add instruction is never followed by another add instruction. Frequently, an add instruction has data size less than or equal to 4, and the following copy instruction is also small. In such cases, it is advantageous to merge the two instructions into a single control byte. The values from 10 to 15 of the first 4 bits code such merged pairs of instructions. In such a case, the first 2 bits of the second 4 bits in the control byte code the size of the add instruction, and the remaining 2 bits code the size of the copy instruction. Below is an enumeration of the values from 10 to 15 of the first 4 bits: 10: a merged add/copy instruction with copy position coded as itself, 11: a merged add/copy instruction with copy position coded as difference from the current position, 12,13,14,15: a merge add/copy instruction with copy position coded from a RECENT cache. In order to elucidate the overall encoding scheme, consider the following files: Version1: abcdabcdabcdefgh Version2: abcdxyxyxyxybcdef The block-move output would be 1. copy 4 0 01000100 0 2. add 2 “xy” which encodes to 00000010 “xy” 3. copy 6 20 (instruction 01000110 20 4. copy 5 9 in binary) 01000101 9 Note that the third instruction copies from Version2. The address 20 for this instruction is 16 1 4 where 16 is the length of Version1. Note also that the data to be copied are also being reconstructed, i.e., vdelta knows about periodic sequences. 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Received: September 1996; revised: January 1997 and October 1997; accepted: November 1997 ACM Transactions on Software Engineering and Methodology, Vol. 7, No. 2, April 1998.