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; NREV - A "safe" implementation of something like nreverse
; Copyright (C) 2014 Centaur Technology
;
; Contact:
; Centaur Technology Formal Verification Group
; 7600-C N. Capital of Texas Highway, Suite 300, Austin, TX 78731, USA.
; http://www.centtech.com/
;
; License: (An MIT/X11-style license)
;
; Permission is hereby granted, free of charge, to any person obtaining a
; copy of this software and associated documentation files (the "Software"),
; to deal in the Software without restriction, including without limitation
; the rights to use, copy, modify, merge, publish, distribute, sublicense,
; and/or sell copies of the Software, and to permit persons to whom the
; Software is furnished to do so, subject to the following conditions:
;
; The above copyright notice and this permission notice shall be included in
; all copies or substantial portions of the Software.
;
; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
; FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
; DEALINGS IN THE SOFTWARE.
;
; Original authors: Jared Davis <jared@centtech.com>
; Sol Swords <sswords@centtech.com>
(in-package "NREV")
; Basic implementation:
;
; ACC is NIL == We have the empty list.
;
; ACC is (CONS HEAD TAIL) otherwise, where:
;
; HEAD points at the list which has been constructed IN ORDER and hence
; does not need to be reversed
;
; TAIL points at its tail, so we can rplacd things in as needed.
(defun nrev$c-copy (nrev$c)
(let* ((acc (nrev$c-acc nrev$c))
(head (car acc)))
;; We must make a deep copy of the elements, because otherwise our next
;; PUSH would extend a list that someone else has gotten hold of.
(loop for e in head collect e)))
(defun nrev-replace-common-suffix (orig new)
;; Semantics: This may destructively modify NEW, but not ORIG, and it returns
;; a list equal to NEW, but perhaps using some suffix of ORIG.
;; We'll scan the two lists for the longest common suffix. First-same will
;; be the longest suffix of orig that is the same as a suffix of new, and
;; last-diff will be the cons previous to that suffix in new. We check the
;; lists starting from the cars. At each iteration our variables are set as
;; though the rest of the two lists are the same. So initially, last-diff is
;; a dummy cons whose cdr is all of new, and first-same is orig.
(let* ((head-handle (cons nil new))
(last-diff head-handle)
(first-same orig)
(orig-suffix orig)
(new-suffix new))
(declare (dynamic-extent head-handle))
(loop while (and (consp new-suffix)
(consp orig-suffix))
do
(unless (eql (car orig-suffix)
(car new-suffix))
(setq last-diff new-suffix)
(setq first-same (cdr orig-suffix)))
(setq orig-suffix (cdr orig-suffix))
(setq new-suffix (cdr new-suffix)))
(if (eql orig-suffix new-suffix)
(progn
;; Replace the common suffix with the tail from orig.
(setf (cdr last-diff) first-same)
(cdr head-handle))
;; If the final cdrs differ (or if the lists are different lengths),
;; there are no common conses to be used; just return new unchanged.
new)))
(defun nrev$c-finish (nrev$c)
(let* ((acc (nrev$c-acc nrev$c))
(hint (nrev$c-hint nrev$c))
(head (car acc))
(nrev$c (update-nrev$c-acc nil nrev$c)))
(if hint
(mv (nrev-replace-common-suffix hint head)
nrev$c)
;; No need to deep copy; nobody else has a handle to our conses.
(mv head nrev$c))))
(defun nrev$c-push (a nrev$c)
(let* ((acc (nrev$c-acc nrev$c))
(new-cons (cons a nil)))
(if (atom acc)
(update-nrev$c-acc (cons new-cons new-cons) nrev$c)
(let* ((tail (cdr acc)))
(acl2::memoize-flush nrev$c)
(rplacd tail new-cons)
(rplacd acc new-cons)
nrev$c))))
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