Conversation with Richard and Chris yesterday gave rise to a more

Conversation with Richard and Chris yesterday gave rise to a more
sensible means of generating an initial gridful of rectangles. This
was previously a stupidly non-scalable bit of the Rectangles puzzle
generator: it filled a ludicrously large array with every possible
rectangle that could go anywhere in the grid, picked one at random
and winnowed the list by removing anything that overlapped that one,
then repeated until the list was empty (and therefore the grid was
full except for remaining singleton squares). Total cost was O(N^4)
in both time and space; not pretty.

Richard and Chris's sensible alternative was to place each rectangle
by randomly choosing a so-far-uncovered _square_, and then picking a
random rectangle from the possible ones covering that square. This
means we only have to deal with a small fragment of the rectangle
list at any one time, and we don't have to store the whole lot in
memory; so it's _much_ faster and more scalable, and has virtually
no memory cost.

A side effect of this algorithmic change is that the probability
distribution has altered. When you line up all the possible
_rectangles_ and pick one at random, then obviously the small ones
are going to be in the majority since lots of small ones can fit
into the space taken up by any given big one. So the original
algorithm tends to favour fiddly grids full of lots of tiny
rectangles, which don't tend to be very interesting. But if you
first pick a square and then think about the rectangles that can
surround that square, the small ones are suddenly going to be in the
_minority_ because there are only two ways you can place (say) a 2x1
containing a given square compared to 36 ways you can place a 6x6.
So this algorithm favours more large rectangles, which I generally
consider to be an improvement.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5982 cda61777-01e9-0310-a592-d414129be87e