A few months ago I got a friendly email from a user of the 'Scramble Squares' solver who mentioned that he'd adapted it to solve 'Raging Rapids', a similarly constrained puzzle, after he had inadvertantly knocked over a friend's solved set.

Raging Rapids is made by the ThinkFun company, who make a wide range of excellent kid's games, among them the classic 'Sliding 15' puzzle and a personal favorite, 'Rush Hour'.

Like Scramble Squares, Raging Rapids is a grid puzzle and the goal is to get all 12 pieces in the 4x3 grid into the right position. The Raging Rapids board looks like a whitewater raft, and the pieces are 12 passengers holding oars. 6 have long blond hair and may be women, or miniature Fabios for all I know ... the plastic figures give few anatomical hints to distinguish further.

Unlike Scramble Squares, the board has a definite orientation (there is a front to the boat distinguishable from the back, which has a rope).

The sides of each piece have bumps or sockets like a jigsaw puzzle piece, although these bumps and sockets do not vary in position or shape. The goal is to get all boaters sitting in four even rows facing front.

To complete the challenge, the interior edge of the raft also sports a pattern of bumps and sockets, so the rafters need to interlock among themselves, as well as along all of the edges.

The depth-first search algorithm from Scramble Squares adapts easily ... the biggest challenge is simplifying it correctly. Since the pieces are longer than they are wide (one bump or socket at the north and south, with a set of two on the east and west edges), and everyone in the raft is expected to face front, only one orientation of each piece has to be checked with its neighbors. On the other hand, a little special code has to be added to keep tabs on the raft edges (the outer edge of the Scramble Squares puzzle was never a concern ... it was always straight and didn't have to match anything).

Each of the 12 rafters have a distinctive pattern of bumps/sockets along their edges. As with the Scramble Squares Solver, you have to set out your puzzle pieces and note these patterns before you start. The ThinkFun people have simplified this step ... on the underside of each rafter you will find a letter from A through L embossed in the plastic (some sort of bizarre ritualistic branding we can use to our advantage.)

So, step 1, have a look at these letters and set out your rafters in a row in front of you, A through L. Ready? Here's the answer:

+----+----+----+
| L  | D  | A  |
+----+----+----+
| K  | G  | F  |
+----+----+----+
| C  | I  | J  |
+----+----+----+
| E  | H  | B  |
+----+----+----+

Of course, this assumes that ThinkFun sells only one configuration of Raging Rapids (that the interior edge of the raft, and perimeters of pieces A through L are always the same). I suspect this is the case (everything about the design screams "mass production!"). But no worries ... this perl script can be easily adapted to solve a new configuration (replace the ".txt" extension with ".pl" after downloading ... it is named that way so that the script won't try to run when you click the link.)

Sort of anti-climactic, but as I mentioned, Raging Rapids is in many ways a constrained puzzle. Most players find that with a little persistence they can find a way to get all of the rafters to 'fit' if they spin one or more of them 180 degrees (so that a rafter is turned to face the back of the raft). There are 16,768 such solutions in all, only one of which results in all of the rafters facing front.

Of course, having figured this out, you start wondering if there are any novel solutions to the puzzle. For example, is it possible to find a solution where all rafters face the back? It turns out that yes, there is one (the suffix 'f' indicates a flipped piece):

+-----+-----+-----+
| Ff  | Df  | Gf  |
+-----+-----+-----+
| Hf  | Af  | Lf  |
+-----+-----+-----+
| Bf  | Jf  | If  |
+-----+-----+-----+
| Ef  | Kf  | Cf  |
+-----+-----+-----+

And, it is possible to, for example, specify a solution where everyone in a middle seat faces the rear and everyone else faces front:

+----+-----+----+
| L  | Gf  | D  |
+----+-----+----+
| C  | Ff  | J  |
+----+-----+----+
| K  | Af  | H  |
+----+-----+----+
| E  | If  | B  |
+----+-----+----+

Here are two solutions where the inside seat faces front, and everyone else faces the back of the raft:

+-----+----+-----+
| Ff  | D  | Gf  |
+-----+----+-----+
| Hf  | E  | Lf  |
+-----+----+-----+
| Jf  | A  | If  |
+-----+----+-----+
| Kf  | B  | Cf  |
+-----+----+-----+

and ...

+-----+----+-----+
| Ff  | A  | Gf  |
+-----+----+-----+
| Hf  | D  | Lf  |
+-----+----+-----+
| Jf  | E  | If  |
+-----+----+-----+
| Kf  | B  | Cf  |
+-----+----+-----+