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brag/parser-tools/private-yacc/graph.rkt

53 lines
1.5 KiB
Racket

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