Re: Graph coloring with Scheme

[Andy Wingo <wingo@pobox.com>]

What a delight!  Thank you for this elegant snippet :)

Andy

On Fri 14 Nov 2014 19:16, Jan Wedekind <jan@wedesoft.de> writes:

> Hi,
> Here is an implementation [1] of Chaitin's graph coloring algorithm
> using GNU Guile and Graphviz. Any feedback and suggestions are
> welcome. Let me know if you can make the implementation more concise
> ;)
>
> Regards Jan
>
> (use-modules (srfi srfi-1)
>              (srfi srfi-26))
> (define (dot graph colors)
>   (apply string-append
>          (append (list "graph g {")
>                  (map (lambda (color) (format #f " ~a [style=filled, fillcolor=~a];" (car color) (cdr color))) colors)
>                  (map (lambda (edge) (format #f " ~a -- ~a;" (car edge) (cdr edge))) graph)
>                  (list " }"))))
> (define (graphviz graph colors) (system (format #f "echo '~a' | dot -Tpng | display -" (dot graph colors))))
> (define (nodes graph) (delete-duplicates (append (map car graph) (map cdr graph))))
> (define (has-node? edge node) (or (eq? (car edge) node) (eq? (cdr edge) node)))
> (define (adjacent graph node) (nodes (filter (cut has-node? <> node) graph)))
> (define (remove-node graph node) (filter (lambda (edge) (not (has-node? edge node))) graph))
> (define (argmin fun lst)
>   (let* [(vals   (map fun lst))
>          (minval (apply min vals))]
>     (list-ref lst (- (length lst) (length (member minval vals))))))
> (define (order graph nodes)
>   (if (null? nodes) '()
>     (let [(target (argmin (lambda (node) (length (adjacent graph node))) nodes))]
>       (cons target (order (remove-node graph target) (delete target nodes))))))
> (define (assign-colors graph nodes colors)
>   (if (null? nodes) '()
>     (let* [(target    (car nodes))
>            (coloring  (assign-colors (remove-node graph target) (delete target nodes) colors))
>            (blocked   (map (cut assq-ref coloring <>) (adjacent graph target)))
>            (available (lset-difference eq? colors blocked))]
>       (cons (cons target (car available)) coloring))))
> (define (coloring graph colors) (assign-colors graph (nodes graph) colors))
> (let [(graph '((b . a) (a . c) (d . c)))] (graphviz graph (coloring graph '(red green blue))))
>
> [1] http://wedesoft.de/graph-coloring.html