Import guile-1.8 as multiple upstream tarball component

[lilypond.git] / guile18 / ice-9 / q.scm

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+;;;; q.scm --- Queues
+;;;;
+;;;;   Copyright (C) 1995, 2001, 2004, 2006 Free Software Foundation, Inc.
+;;;;
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 2.1 of the License, or (at your option) any later version.
+;;;; 
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;; 
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+;;;;
+
+;;; Commentary:
+
+;;; Q: Based on the interface to
+;;;
+;;; "queue.scm"  Queues/Stacks for Scheme
+;;;  Written by Andrew Wilcox (awilcox@astro.psu.edu) on April 1, 1992.
+
+;;; {Q}
+;;;
+;;; A list is just a bunch of cons pairs that follows some constrains,
+;;; right?  Association lists are the same.  Hash tables are just
+;;; vectors and association lists.  You can print them, read them,
+;;; write them as constants, pun them off as other data structures
+;;; etc. This is good.  This is lisp.  These structures are fast and
+;;; compact and easy to manipulate arbitrarily because of their
+;;; simple, regular structure and non-disjointedness (associations
+;;; being lists and so forth).
+;;;
+;;; So I figured, queues should be the same -- just a "subtype" of cons-pair
+;;; structures in general.
+;;;
+;;; A queue is a cons pair:
+;;;            ( <the-q> . <last-pair> )
+;;;
+;;; <the-q> is a list of things in the q.  New elements go at the end
+;;; of that list.
+;;;
+;;; <last-pair> is #f if the q is empty, and otherwise is the last
+;;; pair of <the-q>.
+;;;
+;;; q's print nicely, but alas, they do not read well because the
+;;; eq?-ness of <last-pair> and (last-pair <the-q>) is lost by read.
+;;;
+;;; All the functions that aren't explicitly defined to return
+;;; something else (a queue element; a boolean value) return the queue
+;;; object itself.
+
+;;; Code:
+
+(define-module (ice-9 q)
+  :export (sync-q! make-q q? q-empty? q-empty-check q-front q-rear
+          q-remove! q-push! enq! q-pop! deq! q-length))
+
+;;; sync-q!
+;;;   The procedure
+;;;
+;;;            (sync-q! q)
+;;;
+;;;   recomputes and resets the <last-pair> component of a queue.
+;;;
+(define (sync-q! q)
+  (set-cdr! q (if (pair? (car q)) (last-pair (car q))
+                 #f))
+  q)
+
+;;; make-q
+;;;  return a new q.
+;;;
+(define (make-q) (cons '() #f))
+
+;;; q? obj
+;;;   Return true if obj is a Q.
+;;;   An object is a queue if it is equal? to '(() . #f)
+;;;   or it is a pair P with (list? (car P))
+;;;                      and (eq? (cdr P) (last-pair (car P))).
+;;;
+(define (q? obj)
+  (and (pair? obj)
+       (if (pair? (car obj))
+          (eq? (cdr obj) (last-pair (car obj)))
+          (and (null? (car obj))
+               (not (cdr obj))))))
+
+;;; q-empty? obj
+;;;
+(define (q-empty? obj) (null? (car obj)))
+
+;;; q-empty-check q
+;;;  Throw a q-empty exception if Q is empty.
+(define (q-empty-check q) (if (q-empty? q) (throw 'q-empty q)))
+
+;;; q-front q
+;;;  Return the first element of Q.
+(define (q-front q) (q-empty-check q) (caar q))
+
+;;; q-rear q
+;;;  Return the last element of Q.
+(define (q-rear q) (q-empty-check q) (cadr q))
+
+;;; q-remove! q obj
+;;;  Remove all occurences of obj from Q.
+(define (q-remove! q obj)
+  (set-car! q (delq! obj (car q)))
+  (sync-q! q))
+
+;;; q-push! q obj
+;;;  Add obj to the front of Q
+(define (q-push! q obj)
+  (let ((h (cons obj (car q))))
+    (set-car! q h)
+    (or (cdr q) (set-cdr! q h)))
+  q)
+
+;;; enq! q obj
+;;;  Add obj to the rear of Q
+(define (enq! q obj)
+  (let ((h (cons obj '())))
+    (if (null? (car q))
+       (set-car! q h)
+       (set-cdr! (cdr q) h))
+    (set-cdr! q h))
+  q)
+
+;;; q-pop! q
+;;;  Take the front of Q and return it.
+(define (q-pop! q)
+  (q-empty-check q)
+  (let ((it (caar q))
+       (next (cdar q)))
+    (if (null? next)
+       (set-cdr! q #f))
+    (set-car! q next)
+    it))
+
+;;; deq! q
+;;;  Take the front of Q and return it.
+(define deq! q-pop!)
+
+;;; q-length q
+;;;  Return the number of enqueued elements.
+;;;
+(define (q-length q) (length (car q)))
+
+;;; q.scm ends here