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;;; "tsort.scm" Topological sort
;;; Copyright (C) 1995 Mikael Djurfeldt
;
; This code is in the public domain.
;;; The algorithm is inspired by Cormen, Leiserson and Rivest (1990)
;;; "Introduction to Algorithms", chapter 23
(require 'hash-table)
(require 'primes)
(define (topological-sort dag pred)
(if (null? dag)
'()
(let* ((adj-table (make-hash-table
(car (primes> (length dag) 1))))
(insert (hash-associator pred))
(lookup (hash-inquirer pred))
(sorted '()))
(letrec ((visit
(lambda (u adj-list)
;; Color vertex u
(insert adj-table u 'colored)
;; Visit uncolored vertices which u connects to
(for-each (lambda (v)
(let ((val (lookup adj-table v)))
(if (not (eq? val 'colored))
(visit v (or val '())))))
adj-list)
;; Since all vertices downstream u are visited
;; by now, we can safely put u on the output list
(set! sorted (cons u sorted)))))
;; Hash adjacency lists
(for-each (lambda (def)
(insert adj-table (car def) (cdr def)))
(cdr dag))
;; Visit vertices
(visit (caar dag) (cdar dag))
(for-each (lambda (def)
(let ((val (lookup adj-table (car def))))
(if (not (eq? val 'colored))
(visit (car def) (cdr def)))))
(cdr dag)))
sorted)))
(define tsort topological-sort)
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