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@ -348,11 +348,12 @@
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;; csp is arc-consistent if every pair of variables (x y)
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;; has values in their domain that satisfy every binary constraint
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(define checkable-names (cons ref-name (filter-not (λ (vn) (assigned-name? prob vn)) (map var-name (vars prob)))))
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(define starting-arcs (two-arity-constraints->arcs (for/list ([const (in-constraints prob)]
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#:when (and (two-arity? const)
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(for/and ([cname (in-list (constraint-names const))])
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(memq cname checkable-names))))
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const)))
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(define starting-arcs
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(two-arity-constraints->arcs (for/list ([const (in-constraints prob)]
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#:when (and (two-arity? const)
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(for/and ([cname (in-list (constraint-names const))])
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(memq cname checkable-names))))
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const)))
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(for/fold ([prob prob]
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[arcs (sort starting-arcs < #:key (λ (a) (length (find-domain prob (arc-name a)))) #:cache-keys? #true)]
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#:result (prune-singleton-constraints prob))
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@ -360,22 +361,21 @@
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#:break (empty? arcs))
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(match-define (cons (arc name proc) other-arcs) arcs)
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(define reduced-csp (reduce-domain prob (arc name proc)))
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(values reduced-csp (if (= (length (find-domain prob name)) (length (find-domain reduced-csp name)))
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;; revision did not reduce the domain, so keep going
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other-arcs
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;; revision reduced the domain, so supplement the list of arcs
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(remove-duplicates (append (starting-arcs . terminating-at? . name) other-arcs))))))
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(define (domain-reduced? name)
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(= (length (find-domain prob name)) (length (find-domain reduced-csp name))))
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(values reduced-csp (if (domain-reduced? name)
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(remove-duplicates (append (starting-arcs . terminating-at? . name) other-arcs))
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other-arcs))))
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(define/contract (forward-check-var prob ref-name vr)
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(csp? name? var? . -> . var?)
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(cond
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(match vr
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;; don't check against assigned vars, or the reference var
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;; (which is probably assigned but maybe not)
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[(assigned-var? vr) vr]
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[(eq? (var-name vr) ref-name) vr]
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[else
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(match-define (var name vals) vr)
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[(? assigned-var? vr) vr]
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[(var (== ref-name eq?) _) vr]
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[(var name vals)
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(match ((constraints prob) . relating-only . (list ref-name name))
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[(? empty?) vr]
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[constraints
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@ -383,14 +383,13 @@
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(define new-vals
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(for/list ([val (in-list vals)]
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#:when (for/and ([const (in-list constraints)])
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(let ([proc (constraint-proc const)])
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(if (eq? name (first (constraint-names const)))
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(proc val ref-val)
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(proc ref-val val)))))
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(match const
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[(constraint (list (== name eq?) _) proc) (proc val ref-val)]
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[(constraint _ proc) (proc ref-val val)])))
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val))
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(checked-variable name new-vals (cons (cons ref-name ref-val) (match vr
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[(checked-variable _ _ history) history]
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[else null])))])]))
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[_ null])))])]))
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(define/contract (prune-singleton-constraints prob [ref-name #false])
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((csp?) ((or/c #false name?)) . ->* . csp?)
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@ -419,7 +418,7 @@
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;; so we can discover the *most recent past var* that could be the culprit.
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;; If we just bail out at the first conflict, we may backjump too far based on its history
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;; (and thereby miss parts of the search tree)
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(when (pair? conflict-set)
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(unless (empty? conflict-set)
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(backtrack! conflict-set))
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;; Discard constraints that have produced singleton domains
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;; (they have no further use)
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@ -514,8 +513,8 @@
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(λ (bt)
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(define bths (backtrack-histories bt))
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(append conflicts (remq name (remove-duplicates
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(for*/list ([bth bths]
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[rec bth])
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(for*/list ([bth (in-list bths)]
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[rec (in-list bth)])
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(car rec)) eq?))))])
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(let* ([prob (assign-val prob name val)]
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;; reduce constraints before inference,
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