Friday 17 August 2012

Friday Puzzles #170

So what with having a thesis to finish off ASAP, perhaps, dearest reader, you'll forgive me for being a little obsessed with joining up fragments of loop in (sometimes punctured!) topological discs.

I've also been pondering the similarities and the flaws in two seemingly different puzzles: Slalom (or Suraromu to give it its imperfectly Anglicised title), and Bahnhof.  The latter is not German in origin - I've used Bahnhof purely because I had only ever seen the puzzle on the German based croco puzzle website - and roughly translates to "Railway Stations."  Rather simpler would have been to use the title "Railroad Tracks" - thanks motris for setting me straight on this one.

Anyhow, the idea to Bahnhof (I'm going to stick with that rather than make more drastic edits to the post, that's just the way it's going to be for now) is simple: draw a single loop in the grid using all the cells, and so that the loop travels through each numbered "station" without turning and in order.  The loop is only allowed to cross itself at marked intersection points.

I'd like to take elements from both these puzzles to try and get past some of the flaws to both.  I don't much like the intersection points in Bahnhof; pretty much every Bahnhof puzzle I've solved seems to use these as  the main break-ins to the puzzles.  I also don't like how most puzzles only very loosely use the ordering constraint.  Slalom, as I'm sure I've blogged before, is a beautiful idea for a puzzle making strong use of the ordering constraint, but it is somewhat crippled by its susceptibility to uniqueness strategies, in much the same way that Numberlink thrives upon them.  I'm not sure the constraint of using every square a la Bahnhof is the way to save Slalom but watch out in the future for some attempts to reconcile everything nicely.  For now, here are some plain old Bahnhof puzzles, without crossings.  Enjoy!
    #203 Bahnhof – rated medium
    #204 Bahnhof – rated medium
All puzzles © Tom Collyer 2009-12.


  1. Just as Sudoku is not Japanese, Bahnhof is not German. I'm fairly certain it is Canadian, created by Craig Kasper for the 2000 WPC in the USA under the name Railroad Tracks. The crossing junctions were a design advance for that type yours lack - which means they are probably closer to an earlier loop style that Railroad Tracks if we are being very rigorous about it.

  2. Thanks for setting me straight - like I said before the sneaky edit I have no time to do any proper digging at the moment. If you - or anyone else - has any more information about any earlier version I'd be interested to know!

  3. 203 is a good puzzle, and so is 203.

  4. I've no idea what you mean, I'm sure ;) - thanks!

  5. I think the crossings are fun. It takes away the directional element away from all-cell loop puzzles. Without them it can turn a bit into Ariadne's thread puzzles, that were on the last WPC. With crossings, the way the loop will run to the next number, is usually less obvious as without them.


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