#### 2012-05-07: Professor Layton and the Last Specter, Puzzle #146

Categories: Puzzles Racket

This is for Puzzle 146. Here is the puzzle:

> A bookcase has seven shelves each filled with 10 books of the same size.

> When you choose a book, you must also take the two books above, below, to the left, and to the right of the one you actually want. If there are fewer than two books in any of these directions, you cannot choose that book.

> Choosing as many books as possible, how many books will you end up taking?

-

I encoded the bookcase as a hash table in Racket, where the key is the coordinate of the book and the value is whether the book has been taken.

 (define bookcase (for*/hash ([shelf (in-range 7)] [book (in-range 10)]) (values (cons shelf book) #t)))

I didn’t know when I wrote it, but I assumed that it would not matter what order you picked the books in, so I just considered taking them from left to right, and top to bottom. After consider each book, I would return the new bookcase, after having removed some books. The number of books in this bookcase, subtracted from 70, would give me the number selected:

 (define final-bookcase (for*/fold ([bookcase bookcase]) ([shelf (in-range 7)] [book (in-range 10)]) )) (- 70 (hash-count final-bookcase))

Inside the loop, it will be convenient to bind an escape continuation to return early. The actual loop body is divided into three parts. First, we’ll make sure that the book we’re consider is actually there. Then, we’ll make sure it has two books in every directions. Finally, we’ll return the updated bookcase. The code will look like this:

 (let/ec return )

The first part is really simple: just call our predicate and if it isn’t there, call the escape continuation to jump past the rest of the code and leave the bookcase unchanged.

 (unless (hash-has-key? bookcase (cons shelf book)) (return bookcase))

The middle part is the most complicated. Here’s the idea: we’ll loop over the eight other different books (the two above, below, to the left, and to the right) and remove them from the bookcase, if they are there. If any book isn’t there, we’ll return from the outer loop with the original bookcase, because the conditions aren’t met, using the escape continuation.

 (define other-book-offsets (list (cons 1 0) (cons 2 0) (cons -1 0) (cons -2 0) (cons 0 1) (cons 0 2) (cons 0 -1) (cons 0 -2))) (define new-bookcase (for*/fold ([new-bookcase bookcase]) ([diff (in-list other-book-offsets)]) (define new-book (cons (+ shelf (car diff)) (+ book (cdr diff)))) (if (hash-has-key? new-bookcase new-book) (hash-remove new-bookcase new-book) (return bookcase))))

If the earlier two tests haven’t returned, then when we get to the third, we’ll know all the right conditions are met, so we can remove the current book from the bookcase (the one after the other books were removed):

(hash-remove new-bookcase (cons shelf book))

This technique makes inherent use of functional data-structures, because the bookcase is not destructively modified during the trial deletions of the eight books. If it were, then we couldn’t just return the original bookcase. Instead, we’d have to undo the changes, or do the inner loop twice: once to check if we should and once to actually remove them. Either way, we’d be doing about twice as much work.

This program also demonstrates how useful early return, escape continuations can be. If we didn’t have them, then in the first case, we’d just have to change the syntactic structure of the program, by putting the rest of the loop body on the false side of if. In the second case, however, it would be more complicated, because we’d have to continue the loop past the point of usefulness and check if it was successful at the end—maybe seeing how many books were removed or by keeping a boolean on the side. Either way, escape continuations saved the day.

In retrospect, I think a good way to solve this would be to think of it has a packing problem where you have a plus sign where the legs are each two units and you are trying to fit as many as possible on a 7x10 grid.

Was this faster or slower than doing it the old fashion way...? Who knows.

Can you work out what the answer is...?

By the way, if you use this code at home, make sure you put the code in this order:

<*> ::=