/usr/share/acl2-6.3/books/str/firstn-chars.lisp is in acl2-books-source 6.3-5.
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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 | ; ACL2 String Library
; Copyright (C) 2009-2013 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 "ieqv")
(local (include-book "arithmetic"))
(local (include-book "std/lists/take" :dir :system))
(local (include-book "std/lists/equiv" :dir :system))
(local (in-theory (disable acl2::take-redefinition)))
(defsection firstn-chars
:parents (str)
:short "Efficient way to take leading characters from a string."
:long "<p>@(call firstn-chars) is logically equal to:</p>
@({ (take (min n (length x)) (explode x)) })
<p>But it is implemented more efficiently, via @(see char).</p>"
(defund firstn-chars-aux (x n acc)
(declare (xargs :guard (and (stringp x)
(natp n)
(< n (length x))))
(type string x)
(type (integer 0 *) n))
(if (zp n)
(cons (char x 0) acc)
(firstn-chars-aux x
(the (integer 0 *) (- n 1))
(cons (char x n) acc))))
(defund firstn-chars (n x)
(declare (xargs :guard (and (stringp x)
(natp n))
:verify-guards nil)
(type string x)
(type (integer 0 *) n))
(mbe :logic
(take (min (nfix n) (len (explode x))) (explode x))
:exec
(let ((n (min n (length x))))
(if (zp n)
nil
(firstn-chars-aux x
(the (integer 0 *) (- n 1))
nil)))))
(local (in-theory (enable firstn-chars-aux
firstn-chars)))
(defthm firstn-chars-aux-removal
(implies (and (stringp x)
(natp n)
(< n (length x)))
(equal (firstn-chars-aux x n acc)
(append (take (+ n 1) (coerce x 'list))
acc))))
(verify-guards firstn-chars)
(defthm character-listp-of-firstn-chars
(character-listp (firstn-chars n x)))
(defcong streqv equal (firstn-chars n x) 2)
(defcong istreqv icharlisteqv (firstn-chars n x) 2))
(defsection append-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 (zp n)
y
(firstn-chars-aux x (- n 1) y)))))
(local (in-theory (enable append-firstn-chars)))
(verify-guards append-firstn-chars
:hints(("Goal" :in-theory (enable firstn-chars))))
(defthm character-listp-of-append-firstn-chars
(equal (character-listp (append-firstn-chars n x y))
(character-listp y)))
(defcong streqv equal (append-firstn-chars n x y) 2)
(defcong istreqv icharlisteqv (append-firstn-chars n x y) 2
:hints(("Goal" :in-theory (disable istreqv))))
(defcong list-equiv list-equiv (append-firstn-chars n x y) 3)
(defcong charlisteqv charlisteqv (append-firstn-chars n x y) 3)
(defcong icharlisteqv icharlisteqv (append-firstn-chars n x y) 3))
(defthm consp-of-firstn-chars
;; May be expensive, leaving enabled for now
(equal (consp (firstn-chars n x))
(and (posp n)
(consp (explode x))))
:hints (("Goal" :in-theory (enable firstn-chars length))))
(defthm consp-of-firstn-chars-of-1
;; Improved version of a lemma added by David Rager
(equal (consp (firstn-chars 1 x))
(consp (explode x)))
:hints (("Goal" :in-theory (enable firstn-chars length))))
#||
: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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