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54 lines
1.5 KiB
Racket
54 lines
1.5 KiB
Racket
#lang racket/base
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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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(define results (make-hasheq))
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(define (f x) (hash-ref results x fail))
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;; Maps elements of 'a to integers.
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(define N (make-hasheq))
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(define (get-N x) (hash-ref N x zero-thunk))
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(define (set-N x d) (hash-set! N x d))
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(define stack null)
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(define (push x) (set! stack (cons x stack)))
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(define (pop) (begin0
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(car stack)
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(set! stack (cdr stack))))
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(define (depth) (length stack))
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;; traverse: 'a ->
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(define (traverse x)
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(push x)
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(define d (depth))
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(set-N x d)
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(hash-set! results x (f- x))
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(for-each (λ (y)
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(when (= 0 (get-N y))
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(traverse y))
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(hash-set! 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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(when (= 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-set! results p (f x))
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(when (not (eq? x p))
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(loop (pop))))))
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;; Will map elements of 'a to 'b sets
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(for ([x (in-list nodes)]
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#:when (zero? (get-N x)))
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(traverse x))
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f)
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