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139 lines
6.3 KiB
Racket
139 lines
6.3 KiB
Racket
#lang racket/base
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(require racket/class sugar/container sugar/list sugar/debug racket/list "helper.rkt" "variable.rkt")
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(provide (all-defined-out))
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(define constraint%
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(class object%
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(super-new)
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(define/public (broken? variables domains assignments [forward-check? #f])
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;; Perform the constraint checking
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;; If the forwardcheck parameter is not false, besides telling if
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;; the constraint is currently broken or not, the constraint
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;; implementation may choose to hide values from the domains of
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;; unassigned variables to prevent them from being used, and thus
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;; prune the search space.
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#t)
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(define/public (preprocess variables domains constraints vconstraints)
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;; todo: functionalize this
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;; Preprocess variable domains
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;; This method is called before starting to look for solutions,
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;; and is used to prune domains with specific constraint logic
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;; when possible. For instance, any constraints with a single
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;; variable may be applied on all possible values and removed,
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;; since they may act on individual values even without further
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;; knowledge about other assignments.
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(when (= (length variables) 1)
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(define variable (car variables))
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(define domain (hash-ref domains variable))
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(set-field! _list domain
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(for/fold ([domain-values (domain)])
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([value (in-list (domain))]
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#:when (not (broken? variables domains (make-hash (list (cons variable value))))))
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(remove value domain-values)))
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(set! constraints (remove (list this variables) constraints))
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(hash-update! vconstraints variable (λ(val) (remove (list this variables) val)))))
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;; Helper method for generic forward checking
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;; Currently, this method acts only when there's a single
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;; unassigned variable.
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(define/public (forward-check variables domains assignments [_unassigned Unassigned])
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(define unassigned-variables
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(filter-not (λ(v) (hash-has-key? assignments v)) variables))
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(cond
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;; Remove from the unassigned variable's domain
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;; all values that break our variable's constraints.
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[(= (length unassigned-variables) 1)
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(define unassigned-variable (car unassigned-variables))
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(define unassigned-variable-domain (hash-ref domains unassigned-variable))
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(for ([value (in-list (unassigned-variable-domain))])
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(hash-set! assignments unassigned-variable value)
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(when (not (broken? variables domains assignments))
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(send unassigned-variable-domain hide-value value)))
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(hash-remove! assignments unassigned-variable)
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(not (null? unassigned-variable-domain))] ; if domain had no remaining values, the constraint will be impossible to meet, so return #f
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[else #t]))
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))
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(define constraint%? (is-a?/c constraint%))
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(define function-constraint%
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(class constraint%
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(super-new)
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(init-field func [assigned #t])
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(field [_func func][_assigned assigned])
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(inherit forward-check)
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(define/override (broken? variables domains assignments [forward-check? #f] [_unassigned Unassigned])
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(define parms (map (λ(v) (hash-ref assignments v _unassigned)) variables))
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(define missing (length (filter (λ(p) (equal? p _unassigned)) parms)))
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(if (> missing 0)
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(and (or _assigned (apply _func parms))
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(or (not forward-check?) (not (= missing 1))
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(forward-check variables domains assignments)))
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(apply _func parms)))))
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(define function-constraint%? (is-a?/c function-constraint%))
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;; Constraint enforcing that values of all given variables are different
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(define all-different-constraint%
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(class constraint%
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(super-new)
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(define/override (broken? variables domains assignments [forward-check? #f] [_unassigned Unassigned])
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(define-values (assigned-vars unassigned-vars)
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(partition (λ(var) (hash-has-key? assignments var)) variables))
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(define assigned-values (map (λ(var) (hash-ref assignments var)) assigned-vars))
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(cond
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[(not (members-unique? assigned-values)) #f] ; constraint failed because they're not all different
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[(and forward-check?
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(for*/or ([unassigned-var-domain (in-list (map (λ(uv) (hash-ref domains uv)) unassigned-vars))]
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[assigned-value (in-list assigned-values)]
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#:when (member assigned-value (unassigned-var-domain)))
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(send unassigned-var-domain hide-value assigned-value)
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(null? unassigned-var-domain))) #f] ; if domain had no remaining values, the constraint will be impossible to meet, so return #f
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[else #t]))))
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(define all-different-constraint%? (is-a?/c all-different-constraint%))
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;; Constraint enforcing that values of all given variables are different
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(define all-equal-constraint%
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(class constraint%
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(super-new)
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(define/override (broken? variables domains assignments [forward-check? #f] [_unassigned Unassigned])
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(define singlevalue _unassigned)
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(define value #f)
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(define domain #f)
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(define return-value (void))
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(let/ec return-k
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(for ([variable (in-list variables)])
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(set! value (if (hash-has-key? assignments variable)
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(hash-ref assignments variable)
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_unassigned))
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(cond
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[(equal? singlevalue _unassigned) (set! singlevalue value)]
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[(and (not (equal? value _unassigned)) (not (equal? value singlevalue)))
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(set! return-value #f)
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(return-k)]))
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(when (and forward-check? (not (equal? singlevalue _unassigned)))
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(for ([variable (in-list variables)])
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(when (not (variable . in? . assignments))
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(set! domain (hash-ref domains variable))
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(when (not (singlevalue . in? . (domain)))
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(set! return-value #f)
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(return-k))
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(for ([value (in-list (domain))])
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(when (not (equal? value singlevalue))
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(send domain hide-value value))))))
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(set! return-value #t)
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(return-k))
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return-value)))
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(define all-equal-constraint%? (is-a?/c all-equal-constraint%))
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