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