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In the course of another recent project I had occasion to read up on Tarjan's bridge-finding algorithm. This analyses an arbitrary graph and finds 'bridges', i.e. edges whose removal would increase the number of connected components. This is precisely the dual problem to error-highlighting loops in games like Slant that don't permit them, because an edge is part of some loop if and only if it is not a bridge. Having understood Tarjan's algorithm, it seemed like a good idea to actually implement it for use in these puzzles, because we've got a long and dishonourable history of messing up the loop detection in assorted ways and I thought it would be nice to have an actually reliable approach without any lurking time bombs. (That history is chronicled in a long comment at the bottom of the new source file, if anyone is interested.) So, findloop.c is a new piece of reusable library code. You run it over a graph, which you provide in the form of a vertex count and a callback function to iterate over the neighbours of each vertex, and it fills in a data structure which you can then query to find out whether any given edge is part of a loop in the graph or not.
This is the README accompanying the source code to Simon Tatham's puzzle collection. The collection's web site is at <http://www.chiark.greenend.org.uk/~sgtatham/puzzles/>. If you've obtained the source code by downloading a .tar.gz archive from the Puzzles web site, you should find several Makefiles in the source code. However, if you've checked the source code out from the Puzzles Subversion repository, you won't find the Makefiles: they're automatically generated by `mkfiles.pl', so run that to create them. The Makefiles include: - `Makefile.am', together with the static `configure.ac', is intended as input to automake. Run `mkauto.sh' to turn these into a configure script and Makefile.in, after which you can then run `./configure' to create an actual Unix Makefile. - `Makefile.vc' should work under MS Visual C++ on Windows. Run 'nmake /f Makefile.vc' in a Visual Studio command prompt. - `Makefile.cyg' should work under Cygwin / MinGW. With appropriate tweaks and setting of TOOLPATH, it should work for both compiling on Windows and cross-compiling on Unix. - `Makefile.osx' should work under Mac OS X, provided the Xcode tools are installed. It builds a single monolithic OS X application capable of running any of the puzzles, or even more than one of them at a time. - `Makefile.wce' should work under MS eMbedded Visual C++ on Windows and the Pocket PC SDK; it builds Pocket PC binaries. Many of these Makefiles build a program called `nullgame' in addition to the actual game binaries. This program doesn't do anything; it's just a template for people to start from when adding a new game to the collection, and it's compiled every time to ensure that it _does_ compile and link successfully (because otherwise it wouldn't be much use as a template). Once it's built, you can run it if you really want to (but it's very boring), and then you should ignore it. DO NOT EDIT THE MAKEFILES DIRECTLY, if you plan to send any changes back to the maintainer. The makefiles are generated automatically by the Perl script `mkfiles.pl' from the file `Recipe' and the various .R files. If you need to change the makefiles as part of a patch, you should change Recipe, *.R, and/or mkfiles.pl. The manual is provided in Windows Help format for the Windows build; in text format for anyone who needs it; and in HTML for the Mac OS X application and for the web site. It is generated from a Halibut source file (puzzles.but), which is the preferred form for modification. To generate the manual in other formats, rebuild it, or learn about Halibut, visit the Halibut website at <http://www.chiark.greenend.org.uk/~sgtatham/halibut/>.