148 lines
4.7 KiB
Scheme
148 lines
4.7 KiB
Scheme
; Copyright (c) 1993-1999 by Richard Kelsey. See file COPYING.
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; Code to find the strongly connected components of a graph.
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; (TO <vertex>) are the vertices that have an edge to <vertex>.
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; (SLOT <vertex>) and (SET-SLOT! <vertex> <value>) is a settable slot
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; used by the algorithm.
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;
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; The components are returned in a backwards topologically sorted list.
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(define (strongly-connected-components vertices to slot set-slot!)
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(make-vertices vertices to slot set-slot!)
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(let loop ((to-do vertices) (index 0) (stack #t) (comps '()))
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(let ((to-do (find-next-vertex to-do slot)))
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(cond ((null? to-do)
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(for-each (lambda (n) (set-slot! n #f)) vertices)
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comps)
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(else
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(call-with-values
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(lambda ()
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(do-vertex (slot (car to-do)) index stack comps))
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(lambda (index stack comps)
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(loop to-do index stack comps))))))))
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(define (find-next-vertex vertices slot)
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(do ((vertices vertices (cdr vertices)))
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((or (null? vertices)
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(= 0 (vertex-index (slot (car vertices)))))
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vertices)))
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(define-record-type vertex
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(data ; user's data
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)
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((edges '()) ; list of vertices
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(stack #f) ; next vertex on the stack
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(index 0) ; time at which this vertex was reached in the traversal
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(parent #f) ; a vertex pointing to this one
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(lowpoint #f) ; lowest index in this vertices strongly connected component
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))
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(define (make-vertices vertices to slot set-slot!)
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(let ((maybe-slot (lambda (n)
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(let ((s (slot n)))
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(if (vertex? s)
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s
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(error "graph edge points to non-vertex" n))))))
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(for-each (lambda (n)
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(set-slot! n (vertex-maker n)))
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vertices)
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(for-each (lambda (n)
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(set-vertex-edges! (slot n) (map maybe-slot (to n))))
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vertices)
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(values)))
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; The numbers are the algorithm step numbers from page 65 of Graph Algorithms,
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; Shimon Even, Computer Science Press, 1979.
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; 2
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(define (do-vertex vertex index stack comps)
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(let ((index (+ index '1)))
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(set-vertex-index! vertex index)
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(set-vertex-lowpoint! vertex index)
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(set-vertex-stack! vertex stack)
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(get-strong vertex index vertex comps)))
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; 3
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(define (get-strong vertex index stack comps)
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(if (null? (vertex-edges vertex))
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(end-vertex vertex index stack comps)
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(follow-edge vertex index stack comps)))
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; 7
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(define (end-vertex vertex index stack comps)
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(call-with-values
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(lambda ()
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(if (= (vertex-index vertex) (vertex-lowpoint vertex))
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(unwind-stack vertex stack comps)
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(values stack comps)))
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(lambda (stack comps)
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(cond ((vertex-parent vertex)
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=> (lambda (parent)
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(if (> (vertex-lowpoint parent) (vertex-lowpoint vertex))
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(set-vertex-lowpoint! parent (vertex-lowpoint vertex)))
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(get-strong parent index stack comps)))
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(else
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(values index stack comps))))))
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(define (unwind-stack vertex stack comps)
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(let loop ((n stack) (c '()))
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(let ((next (vertex-stack n))
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(c (cons (vertex-data n) c)))
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(set-vertex-stack! n #f)
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(if (eq? n vertex)
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(values next (cons c comps))
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(loop next c)))))
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; 4
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(define (follow-edge vertex index stack comps)
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(let* ((next (pop-vertex-edge! vertex))
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(next-index (vertex-index next)))
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(cond ((= next-index 0)
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(set-vertex-parent! next vertex)
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(do-vertex next index stack comps))
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(else
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(if (and (< next-index (vertex-index vertex))
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(vertex-stack next)
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(< next-index (vertex-lowpoint vertex)))
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(set-vertex-lowpoint! vertex next-index))
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(get-strong vertex index stack comps)))))
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(define (pop-vertex-edge! vertex)
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(let ((edges (vertex-edges vertex)))
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(set-vertex-edges! vertex (cdr edges))
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(car edges)))
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; GRAPH is ((<symbol> . <symbol>*)*)
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;(define (test-strong graph)
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; (let ((vertices (map (lambda (n)
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; (vector (car n) #f #f))
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; graph)))
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; (for-each (lambda (data vertex)
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; (vector-set! vertex 1 (map (lambda (s)
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; (first (lambda (v)
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; (eq? s (vector-ref v 0)))
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; vertices))
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; (cdr data))))
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; graph
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; vertices)
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; (map (lambda (l)
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; (map (lambda (n) (vector-ref n 0)) l))
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; (strongly-connected-components vertices
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; (lambda (v) (vector-ref v 1))
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; (lambda (v) (vector-ref v 2))
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; (lambda (v val)
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; (vector-set! v 2 val))))))
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