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Thanks to Glen Sawyer for reporting it. This is surely a consequence of separating interpret_move from execute_move - if I'd done things in the more obvious way, then this bug would never have happened, because once the autofollow code had gone once round the loop it would find that the starting edge was no longer in the same state it was looking for. But since we don't start changing the states of edges until execute_move(), it's possible for interpret_move() to go round and round a cycle for ever. Fortunately, it can _only_ happen if the edge you clicked on was part of a loop which is the whole of its connected component - i.e. every vertex in the cycle has degree 2 - and therefore we don't need O(N) space to detect it (e.g. by recording whether each edge has been visited already), but instead we can simply check if we've come back to the starting edge.
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 git 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/>.