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bfe1ca4ef7
actually traverse the given alist. [this change copied from 2003 in the head, hence that copyright year]
146 lines
4.5 KiB
Scheme
146 lines
4.5 KiB
Scheme
;;; installed-scm-file
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;;;; Copyright (C) 1996, 2001, 2003 Free Software Foundation, Inc.
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;;;;
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;;;; This program is free software; you can redistribute it and/or modify
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;;;; it under the terms of the GNU General Public License as published by
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;;;; the Free Software Foundation; either version 2, or (at your option)
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;;;; any later version.
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;;;;
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;;;; This program is distributed in the hope that it will be useful,
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;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;;;; GNU General Public License for more details.
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;;;;
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;;;; You should have received a copy of the GNU General Public License
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;;;; along with this software; see the file COPYING. If not, write to
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;;;; the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
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;;;; Boston, MA 02110-1301 USA
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;;;;
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;;;; As a special exception, the Free Software Foundation gives permission
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;;;; for additional uses of the text contained in its release of GUILE.
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;;;;
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;;;; The exception is that, if you link the GUILE library with other files
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;;;; to produce an executable, this does not by itself cause the
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;;;; resulting executable to be covered by the GNU General Public License.
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;;;; Your use of that executable is in no way restricted on account of
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;;;; linking the GUILE library code into it.
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;;;;
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;;;; This exception does not however invalidate any other reasons why
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;;;; the executable file might be covered by the GNU General Public License.
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;;;;
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;;;; This exception applies only to the code released by the
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;;;; Free Software Foundation under the name GUILE. If you copy
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;;;; code from other Free Software Foundation releases into a copy of
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;;;; GUILE, as the General Public License permits, the exception does
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;;;; not apply to the code that you add in this way. To avoid misleading
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;;;; anyone as to the status of such modified files, you must delete
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;;;; this exception notice from them.
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;;;;
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;;;; If you write modifications of your own for GUILE, it is your choice
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;;;; whether to permit this exception to apply to your modifications.
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;;;; If you do not wish that, delete this exception notice.
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;;;;
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(define-module (ice-9 poe)
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:use-module (ice-9 hcons)
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:export (pure-funcq perfect-funcq))
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;;; {Pure Functions}
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;;;
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;;; A pure function (of some sort) is characterized by two equality
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;;; relations: one on argument lists and one on return values.
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;;; A pure function is one that when applied to equal arguments lists
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;;; yields equal results.
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;;;
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;;; If the equality relationship on return values can be eq?, it may make
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;;; sense to cache values returned by the function. Choosing the right
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;;; equality relation on arguments is tricky.
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;;;
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;;; {pure-funcq}
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;;;
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;;; The simplest case of pure functions are those in which results
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;;; are only certainly eq? if all of the arguments are. These functions
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;;; are called "pure-funcq", for obvious reasons.
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;;;
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(define funcq-memo (make-weak-key-hash-table 523)) ; !!! randomly selected values
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(define funcq-buffer (make-gc-buffer 256))
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(define (funcq-hash arg-list n)
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(let ((it (let loop ((x 0)
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(arg-list arg-list))
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(if (null? arg-list)
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(modulo x n)
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(loop (logior x (hashq (car arg-list) 4194303))
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(cdr arg-list))))))
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it))
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;; return true if lists X and Y are the same length and each element is `eq?'
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(define (eq?-list x y)
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(if (null? x)
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(null? y)
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(and (not (null? y))
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(eq? (car x) (car y))
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(eq?-list (cdr x) (cdr y)))))
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(define (funcq-assoc arg-list alist)
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(if (null? alist)
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#f
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(if (eq?-list arg-list (caar alist))
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(car alist)
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(funcq-assoc arg-list (cdr alist)))))
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(define (pure-funcq base-func)
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(lambda args
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(let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
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(if cached
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(begin
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(funcq-buffer (car cached))
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(cdr cached))
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(let ((val (apply base-func args))
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(key (cons base-func args)))
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(funcq-buffer key)
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(hashx-set! funcq-hash funcq-assoc funcq-memo key val)
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val)))))
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;;; {Perfect funq}
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;;;
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;;; A pure funq may sometimes forget its past but a perfect
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;;; funcq never does.
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;;;
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(define (perfect-funcq size base-func)
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(define funcq-memo (make-hash-table size))
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(lambda args
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(let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
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(if cached
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(begin
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(funcq-buffer (car cached))
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(cdr cached))
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(let ((val (apply base-func args))
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(key (cons base-func args)))
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(funcq-buffer key)
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(hashx-set! funcq-hash funcq-assoc funcq-memo key val)
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val)))))
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