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typesetting/csp/solver.rkt

121 lines
5.1 KiB
Racket

#lang racket/base
(require racket/class sugar/unstable/container sugar/debug racket/list
racket/bool racket/generator racket/match "helper.rkt")
(provide (all-defined-out))
(define solver%
;; Abstract base class for solvers
(class object%
(super-new)
(abstract get-solution)
(abstract get-solutions)
(abstract get-solution-iter)))
(define solver%? (is-a?/c solver%))
(struct vvp (variable values pushdomains))
(define-syntax-rule (pop-vvp-values! vvps)
(if (empty? vvps)
(error 'pop-vvp-values! (format "~a is null" vvps))
(let ([vvp (car vvps)])
(set! vvps (cdr vvps))
(values (vvp-variable vvp) (vvp-values vvp) (vvp-pushdomains vvp)))))
#|
(define (recursive-backtracking assignment csp)
(if (complete? assignment)
assignment
(let ([var (select-unassigned-variable csp-variables, assignment, csp)])
(for/or ([value (in-list (order-domain-values var assignment csp))])
if ((value . consistent-with? . assignment csp-constraints))
(add-to assignment var value)
(define result (recursive-backtracking assignment csp))
(when result
(and result (remove-from assignment var value)))
#f))))
|#
(define backtracking-solver%
;; Problem solver with backtracking capabilities
(class solver%
(super-new)
(init-field [forwardcheck #t])
(field [_forwardcheck forwardcheck])
(define/override (get-solution-iter domains constraints vconstraints)
(define sorted-variables (sort (hash-keys domains) list-comparator
#:key (λ(var)
(list (- (length (hash-ref vconstraints var)))
(length ((hash-ref domains var)))
var))))
;; state-retention variables
(define possible-solution (make-hash))
(define variable-queue null)
(define variable #f)
(define values null)
(define pushdomains null)
(define (get-next-unassigned-variable)
(for/first ([sorted-variable (in-list sorted-variables)]
#:unless (hash-has-key? possible-solution sorted-variable))
(set! variable sorted-variable)
(set! values ((hash-ref domains variable)))
(set! pushdomains
(if _forwardcheck
(for/list ([(var domain) (in-hash domains)]
#:unless (and (equal? variable var)
(hash-has-key? possible-solution var)))
domain)
null))
variable))
(define (set!-previous-variable)
(set!-values (variable values pushdomains) (pop-vvp-values! variable-queue))
(for-each-send pop-state pushdomains))
(let/ec exit-k
;; mix the degree and minimum-remaining-values (MRV) heuristics
(forever
(unless (get-next-unassigned-variable)
(yield (hash-copy possible-solution)) ; if there are no unassigned variables, solution is complete.
(if (empty? variable-queue)
(exit-k) ; all done, no other solutions possible.
(set!-previous-variable))) ; otherwise return to previous variable
(let value-checking-loop () ; we have a variable. Do we have any values left?
(when (empty? values) ; no, so try going back to last variable and getting some values
(forever/until
(when (empty? variable-queue) (exit-k)) ; no variables left, so solver is done
(hash-remove! possible-solution variable)
(set!-previous-variable)
(not (empty? values))))
;; Got a value. Check it.
(hash-set! possible-solution variable (car-pop! values))
(for-each-send push-state pushdomains)
(unless (for/and ([constraint+variables (in-list (hash-ref vconstraints variable))])
(let ([constraint (car constraint+variables)]
[variables (cadr constraint+variables)])
(send constraint is-true? variables domains possible-solution pushdomains)))
;; constraint failed, so try again
(for-each-send pop-state pushdomains)
(value-checking-loop)))
;; Push state before looking for next variable.
(set! variable-queue (cons (vvp variable values pushdomains) variable-queue)))
(error 'get-solution-iter "impossible to reach this"))
(void))
(define (call-solution-generator domains constraints vconstraints #:first-only [first-only #f])
(for/list ([solution (in-generator (get-solution-iter domains constraints vconstraints))] #:final first-only)
solution))
(define/override (get-solution . args)
(car (apply call-solution-generator #:first-only #t args)))
(define/override (get-solutions . args)
(apply call-solution-generator args))))
(define backtracking-solver%? (is-a?/c backtracking-solver%))