Date: Fri, 03 May 96 12:13:14 -0400 (EDT)
From: Dan Hoey <hoey@aic.nrl.navy.mil >
~~~ Subject: Re: TripleCross by Binary Arts

Dale Newfield writes about the Binary Arts puzzle TripleCross. In
case anyone in cube-lovers hasn't seen it, it's a nice mechanical
puzzle involving the permutation of 18 pieces. Stripped of the
mechanical parts, it has 18 sliding pieces in an array shaped like:

```             1  2
3  4  5  6  7  8  9
10 11 12 13 14 15 16
17 18
```

Pieces 8, 9, 15, 16, 17, and 18 are distinguishably marked. Pieces
3, 4, and 5 are marked identically. The remaining nine pieces are
unmarked.

Pieces are permuted by a plunger-like action. One kind of plunge is
to move pieces 3-16 left or right one unit. The other is to move the
second and third columns (4,5,11,12,17,18) up one unit while
simultaneously moving the fifth and sixth columns (1,2,7,8,14,15) down
one unit. At the end of any process, we it is traditional to restore
both plungers to their original position. Calling the horizontal
plunger positions Left, Center, and Right and the vertical plunger
positions Up and Down, we may reduce any process to a sequence of ULDC
and URDC and their inverses LUCD and RUCD. Cycle forms for the first
two are

```ULDC = (1,7,14,15,16,9,2)(4,5,6,13,18,17,11) and
URDC = (1,2,8,15,14,13,6)(3,10,17,18,12,5,4).
```

Since these are both even permutations, it's clear the group must be a
subgroup of A18. GAP confirms that all 18!/2 positions are reachable.

Dale writes:

... I've been doing most of my investigations
with respect to a fairly arbitrary subgroup--that of distinguishable
positions (On the actual puzzle there are 9 indestinguishable blank
tiles, and 3 indestinguishable tiles that each contain one orange dot).
In this group there are just under 3 billion possible positions:
18!/(9!*3!)....

Right, though this is not actually a group, but a collection of
cosets. For instance, you could have two indistinguishable
manipulations that would provide distinguishable outcomes when
preceded by some other manipulation.

Dale writes of his work on a breadth-first search:

... I can encode a position (theoretically ~31.5 bits of
information) into a 32 bit unsigned long, along with a 2 bit number
representing which direction to go to get to start.

There is a more compressed approach that has proven valuable with some
of the smaller cubes. The idea is to use the 32-bit number as the
index into a large vector of distances. Initialize all the distances
to -1, set the distance of SOLVED to zero, and repeatedly scan the
whole array checking neighbors of positions of distance D; if their
distance is -1 set the distance to D+1.

This procedure admits some useful variations. For one thing, you can
store D mod 3 in two bits, with a fourth value in place of -1. This
reduces the storage to 735 megabytes. Also, instead of relying on
your virtual memory system, you can store the vector in a big
random-access file (or several files). In both cases, it's probably
easiest to use bit-fields of the index to specify which file (if
multiple files), which block of the file (choose a useful power of
two), which byte in the block, and which part of the byte (if packed).
Third, it may be better to generate lists of a few thousand neighbor
indices, sort them, and then scan for neighbors, to reduce thrashing.

If you implement this, I'd be interested in knowing:

Number of positions at each distance,
Maximal-distance positions (up to ten or twenty),
Number of local maxima at each distance (these show up as
positions none of whose neighbors get set),
Number of nonstrict local maxima at each distance (i.e. which have
neighbors at the same distance).

Dan Hoey
Hoey@AIC.NRL.Navy.Mil