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typesetting/csp/solver.rkt

118 lines
4.8 KiB
Racket

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#lang racket/base
(require racket/class sugar/container racket/list racket/generator racket/match "helper.rkt")
(provide (all-defined-out))
(define solver%
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;; Abstract base class for solvers
(class object%
(super-new)
(abstract get-solution)
(abstract get-solutions)
(abstract get-solution-iter)))
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(define solver%? (is-a?/c solver%))
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(struct vvp (variable values pushdomains))
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(define-syntax-rule (pop-vvp-values! vvps)
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(if (null? 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)))))
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(define backtracking-solver%
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;; Problem solver with backtracking capabilities
(class solver%
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(super-new)
(init-field [forwardcheck #t])
(field [_forwardcheck forwardcheck])
(define/override (get-solution-iter domains constraints vconstraints)
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(define forwardcheck _forwardcheck)
(define assignments (make-hash))
(define queue null)
(define values null)
(define pushdomains null)
(define variable #f)
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(define work-list null)
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(define want-to-return #f)
(define return-k #f)
(let/ec break-loop1
(set! return-k break-loop1)
(let loop1 ()
;; Mix the Degree and Minimum Remaing Values (MRV) heuristics
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(set! work-list (sort (for/list ([variable (in-hash-keys domains)])
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(list (* -1 (length (hash-ref vconstraints variable)))
(length ((hash-ref domains variable)))
variable)) list-comparator))
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;(report lst)
(let/ec break-for-loop
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(for ([last-item (in-list (map last work-list))]
#:when (not (hash-has-key? assignments last-item)))
; Found unassigned variable
(set! variable last-item)
(set! values ((hash-ref domains variable)))
(set! pushdomains
(if forwardcheck
(for/list ([x (in-hash-keys domains)]
#:when (and (not (hash-has-key? assignments x))
(not (equal? variable x))))
(hash-ref domains x))
null))
(break-for-loop))
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;; if it makes it through the loop without breaking, then there are
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;; no unassigned variables. We've got a solution.
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(yield (hash-copy assignments))
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;; Return to previous variable in queue if possible, otherwise all done
(cond
[(not (null? queue))
(set!-values (variable values pushdomains) (pop-vvp-values! queue))
(for-each-send pop-state pushdomains)]
[else
(set! want-to-return #t) (return-k)]))
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(let constraint-checking-loop ()
;; We have a variable. Do we have any values left?
(when (null? values)
;; No. Go back to last variable, if there is one, otherwise solver is done.
(for/or ([i (in-naturals)])
(hash-remove! assignments variable)
(when (null? queue) (set! want-to-return #t) (return-k))
(set!-values (variable values pushdomains) (pop-vvp-values! queue))
(for-each-send pop-state pushdomains)
(not (null? values))))
;; Got a value. Check it.
(hash-set! assignments variable (car-pop! values))
(for-each-send push-state pushdomains)
(when (for/or ([cvpair (in-list (hash-ref vconstraints variable))])
(match-define (list constraint variables) cvpair)
(not (send constraint is-true? variables domains assignments pushdomains)))
;; constraint failed, so try again
(for-each-send pop-state pushdomains)
(constraint-checking-loop)))
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;; Push state before looking for next variable.
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(set! queue (cons (vvp variable values pushdomains) queue))
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(loop1)))
(if want-to-return
(void)
(error 'get-solution-iter "Whoops, broken solver")))
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(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)
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solution))
(define/override (get-solution . args)
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(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%))