#### 2013-06-03: Towers of Hanoi in Racket

The source for this post is online at 2013-06-03-hanoi.rkt.

I was explaining Towers of Hanoi to a young person at church the other day and decided to make a short version in Racket with a GUI to display the movements.

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##### 1Towers of Hanoi

The Towers of Hanoi is a classic physical puzzle. The objective is to move a stack of discs from one pole to another using a third pole as an intermediate stack. You can only move one disc at a time, you can only take the top disc, and you can’t put a large disc on a small disc.

It is a classic recursive problem, as well, because to move three discs from A to C, you can first move two discs from A to B then from B to C after moving the biggest disc.

Here’s a solution in Racket that returns a lazy stream of movements:

 (struct move (from to)) (define (towers n from to extra) (if (= n 0) empty-stream (stream-append (towers (sub1 n) from extra to) (stream-cons (move from to) (towers (sub1 n) extra to from)))))

It’s important, using racket/stream, to use the stream-cons because if you wrote a stream-append with three arguments, then the second recursive call wouldn’t be lazily evaluated and you’d save nothing by using streams over lists. That’s because stream-append is a function, while stream-cons is a macro that thunks its arguments.

##### 2Visualization

The program is interesting enough, but the more fun thing to do is display it graphically. I decided to use 2htdp/universe, because it’s so simple.

When using 2htdp/universe, you need to define a structure to represent the state of your simulation, called the world. I used a list of lists of disc diameters, a list of moves to go forward in the puzzle, and a list of moves to go backwards.

I defined a function called show that shows the solution for any given n. It uses towers to lazily generate all the forward moves and a few helper functions to display and browse the state.

<show> ::=
 (struct world (stacks forward backward)) (define (show n) (define ms (towers n 0 2 1)) (define s (list (build-list n add1) empty empty)) (define (draw-world w) (draw-stacks n (world-stacks w))) (define (move-disc w k) (cond [(equal? k "left") (move-disc/backward w)] [(equal? k "right") (move-disc/forward w)] [else w])) (big-bang (world s ms empty) (on-key move-disc) (to-draw draw-world)))

Since we do things lazily, you can very easily view huge problems, like (show 40).

##### 3Displaying

The display of a bunch of stacks is very simple. A disc is just a rectangle. A stack is a bunch of discs above one another. A set of stacks is a bunch of stacks next to one another. The only weird thing is creating the blank space for the portion of the stack that is empty. This is necessary, because otherwise the alignment on the picture doesn’t look nice.

 (define disc-width 10) (define disc-height 10) (define (draw-disc d) (rectangle (* d disc-width) disc-height 'solid "black")) (define (draw-stack s) (apply above empty-image empty-image (map draw-disc s))) (define (draw-stacks n ss) (apply beside/align 'top (map (λ (s) (define m (- n (length s))) (above (rectangle (* n disc-width) (* m disc-height) 'solid "white") (draw-stack s))) ss)))

##### 4Browsing

When the user press the left and right key, we must act on the next move and move it to the other list so we can go back. There’s nothing spectacular about the code, but it’s nice in its simplicity.

 (define (move-disc/forward w) (match-define (world ss fs bs) w) (cond [(stream-empty? fs) w] [else (match-define (and m (move from to)) (stream-first fs)) (world (list-move ss from to) (stream-rest fs) (cons m bs))])) (define (move-disc/backward w) (match-define (world ss fs bs) w) (cond [(empty? bs) w] [else (match-define (and m (move to from)) (first bs)) (world (list-move ss from to) (stream-cons m fs) (rest bs))]))

I think it is a bit ugly that the two programs are so close but not clearly abstractable. I tried to write a macro to avoid this:

 (define-syntax-rule (define-move-disc move-disc/forward fs bs cond:e m from to mfrom mto first:e nfs nbs) (define (move-disc/forward w) (match-define (world ss fs bs) w) (cond [cond:e w] [else (match-define (and m (move from to)) first:e) (world (list-move ss mfrom mto) nfs nbs)]))) (define-move-disc move-disc/forward fs bs (stream-empty? fs) m from to from to (stream-first fs) (stream-rest fs) (cons m bs)) (define-move-disc move-disc/backward fs bs (empty? bs) m to from from to (first bs) (stream-cons m fs) (rest bs))

But this is an example of a really bad macro, it doesn’t really represent an abstraction that is meaningful, and instead inherently relies on the user understanding exactly what the expansion will be. So, I don’t think you should use it.

Finally, to actually move the disc from one stack to another, I came up with a cute little thing to make sure the list of stacks stayed in the same order:

<move> ::=
 (define (list-move ss from to) (define from-disc (first (list-ref ss from))) (for/list ([s (in-list ss)] [i (in-naturals)]) (cond [(= i from) (rest s)] [(= i to) (list* from-disc s)] [else s])))

##### 5Yo! It’s almost time to go!

But first let’s remember what we did today!

We learned that simple recursive algorithms are short and elegant in Racket and you can easily use laziness where appropriate.

We saw that using 2htdp/image to render things algebraically is simple and great for little visualizations like this.

Finally, we saw that 2htdp/universe lets us create an elegant little GUI in just 18 lines.

There wasn’t anything especially deep in this week’s post, but I think it is useful to remember that Racket doesn’t just blow your mind with crazy stuff you’ve never thought of. It also blows your mind with cute little elegant versions of classic ideas.

If you’d like to run this exact code at home, you should put it in this order:

<*> ::=
 (require racket/stream racket/list racket/match 2htdp/image 2htdp/universe) (let ()  (void)) (show 40)