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116 lines
3.3 KiB
Scheme
116 lines
3.3 KiB
Scheme
;;; installed-scm-file
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;;;; Copyright (C) 1996, 2001, 2006, 2011 Free Software Foundation, Inc.
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;;;;
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;;;; This library is free software; you can redistribute it and/or
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;;;; modify it under the terms of the GNU Lesser General Public
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;;;; License as published by the Free Software Foundation; either
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;;;; version 3 of the License, or (at your option) any later version.
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;;;;
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;;;; This library 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 GNU
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;;;; Lesser General Public License for more details.
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;;;;
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;;;; You should have received a copy of the GNU Lesser General Public
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;;;; License along with this library; if not, write to the Free Software
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;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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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 not-found (list 'not-found))
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(define (pure-funcq base-func)
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(lambda args
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(let* ((key (cons base-func args))
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(cached (hashx-ref funcq-hash funcq-assoc funcq-memo key not-found)))
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(if (not (eq? cached not-found))
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(begin
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(funcq-buffer key)
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cached)
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(let ((val (apply 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* ((key (cons base-func args))
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(cached (hashx-ref funcq-hash funcq-assoc funcq-memo key not-found)))
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(if (not (eq? cached not-found))
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(begin
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(funcq-buffer key)
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cached)
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(let ((val (apply 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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