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br-parser-tools/collects/parser-tools/private-yacc/graph.ss

61 lines
1.6 KiB
Scheme

#cs
(module graph mzscheme
(provide digraph)
;; digraph:
;; ('a list) * ('a -> 'a list) * ('a -> 'b) * ('b * 'b -> 'b) * 'b
;; -> ('a -> 'b)
;; DeRemer and Pennello 1982
;; Computes (f x) = (f- x) union Union{(f y) | y in (edges x)}
;; We use a hash-table to represent the result function 'a -> 'b set, so
;; the values of type 'a must be comparable with equal?.
(define (digraph nodes edges f- union fail)
(letrec (
;; Will map elements of 'a to 'b sets
(results (make-hash-table 'equal))
(f (lambda (x) (hash-table-get results x (lambda () fail))))
;; Maps elements of 'a to integers.
(N (make-hash-table 'equal))
(get-N (lambda (x) (hash-table-get N x (lambda () 0))))
(set-N (lambda (x d) (hash-table-put! N x d)))
(stack null)
(push (lambda (x) (set! stack (cons x stack))))
(pop (lambda () (begin0 (car stack) (set! stack (cdr stack)))))
(depth (lambda () (length stack)))
;; traverse: 'a ->
(traverse
(lambda (x)
(push x)
(let ((d (depth)))
(set-N x d)
(hash-table-put! results x (f- x))
(for-each (lambda (y)
(if (= 0 (get-N y))
(traverse y))
(hash-table-put! results
x
(union (f x) (f y)))
(set-N x (min (get-N x) (get-N y))))
(edges x))
(if (= d (get-N x))
(let loop ((p (pop)))
(set-N p +inf.0)
(hash-table-put! results p (f x))
(if (not (equal? x p))
(loop (pop)))))))))
(for-each (lambda (x)
(if (= 0 (get-N x))
(traverse x)))
nodes)
f))
)