/usr/share/acl2-4.3/books/str/firstn-chars.lisp is in acl2-books-source 4.3-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 | ; ACL2 String Library
; Copyright (C) 2009-2010 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/
;
; This program is free software; you can redistribute it and/or modify it under
; the terms of the GNU General Public License as published by the Free Software
; Foundation; either version 2 of the License, or (at your option) any later
; version. This program 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 General Public License for
; more details. You should have received a copy of the GNU General Public
; License along with this program; if not, write to the Free Software
; Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335, USA.
;
; Original author: Jared Davis <jared@centtech.com>
(in-package "STR")
(include-book "doc")
(local (include-book "arithmetic"))
(local (include-book "unicode/take" :dir :system))
(defund firstn-chars-aux (x n acc)
(declare (xargs :guard (and (stringp x)
(natp n)
(< n (length x))))
(type string x)
(type integer n))
(if (mbe :logic (zp n)
:exec (= n 0))
(cons (char x 0) acc)
(firstn-chars-aux x
(- n 1)
(cons (char x n) acc))))
(defund firstn-chars (n x)
":Doc-Section Str
Efficient way to take leading characters from a string ~/
This function is logically equal to
~bv[]
(take (min n (length x)) (coerce x 'list)),
~ev[]
but is implemented more efficiently via ~c[char]. ~/ "
(declare (xargs :guard (and (stringp x)
(natp n))
:verify-guards nil)
(type string x))
(mbe :logic
(take (min n (length x)) (coerce x 'list))
:exec
(let ((n (min n (length x))))
(if (= n 0)
nil
(firstn-chars-aux x (- n 1) nil)))))
(local (defthm lemma
(implies (and (stringp x)
(natp n)
(< n (length x)))
(equal (firstn-chars-aux x n acc)
(append (take (+ n 1) (coerce x 'list))
acc)))
:hints(("Goal" :in-theory (enable firstn-chars-aux)))))
(verify-guards firstn-chars)
(defthm character-listp-of-firstn-chars
(implies (force (stringp x))
(character-listp (firstn-chars n x)))
:hints(("Goal" :in-theory (enable firstn-chars))))
(defund append-firstn-chars (n x y)
(declare (xargs :guard (and (natp n)
(stringp x))
:verify-guards nil))
(mbe :logic
(append (firstn-chars n x) y)
:exec
(let ((n (min n (length x))))
(if (= n 0)
y
(firstn-chars-aux x (- n 1) y)))))
(verify-guards append-firstn-chars
:hints(("Goal"
:in-theory (enable firstn-chars))))
(defthm character-listp-of-append-firstn-chars
(implies (and (force (stringp x))
(force (character-listp y)))
(character-listp (append-firstn-chars n x y)))
:hints(("Goal" :in-theory (enable append-firstn-chars))))
#||
:q
;; 6.82 seconds, 3.68 GB allocated
(time (loop for i fixnum from 1 to 10000000
do
(take 5 (coerce "Hello, World!" 'list))))
;; 2.00 seconds, 800 MB allocated
(time (loop for i fixnum from 1 to 10000000
do
(str::firstn-chars 5 "Hello, World!")))
||#
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