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