ice-9/poe.scm

   1 ;;; installed-scm-file
   2
   3 ;;;;    Copyright (C) 1996, 2001 Free Software Foundation, Inc.
   4 ;;;;
   5 ;;;; This library is free software; you can redistribute it and/or
   6 ;;;; modify it under the terms of the GNU Lesser General Public
   7 ;;;; License as published by the Free Software Foundation; either
   8 ;;;; version 2.1 of the License, or (at your option) any later version.
   9 ;;;;
  10 ;;;; This library is distributed in the hope that it will be useful,
  11 ;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 ;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13 ;;;; Lesser General Public License for more details.
  14 ;;;;
  15 ;;;; You should have received a copy of the GNU Lesser General Public
  16 ;;;; License along with this library; if not, write to the Free Software
  17 ;;;; Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
  18 ;;;;
  19
  20 \f
  21 (define-module  (ice-9 poe)
  22   :use-module (ice-9 hcons)
  23   :export (pure-funcq perfect-funcq))
  24
  25 \f
  26
  27
  28 ;;; {Pure Functions}
  29 ;;;
  30 ;;; A pure function (of some sort) is characterized by two equality
  31 ;;; relations: one on argument lists and one on return values.
  32 ;;; A pure function is one that when applied to equal arguments lists
  33 ;;; yields equal results.
  34 ;;;
  35 ;;; If the equality relationship on return values can be eq?, it may make
  36 ;;; sense to cache values returned by the function.  Choosing the right
  37 ;;; equality relation on arguments is tricky.
  38 ;;;
  39
  40 \f
  41 ;;; {pure-funcq}
  42 ;;;
  43 ;;; The simplest case of pure functions are those in which results
  44 ;;; are only certainly eq? if all of the arguments are.  These functions
  45 ;;; are called "pure-funcq", for obvious reasons.
  46 ;;;
  47
  48
  49 (define funcq-memo (make-weak-key-hash-table 523)) ; !!! randomly selected values
  50 (define funcq-buffer (make-gc-buffer 256))
  51
  52 (define (funcq-hash arg-list n)
  53   (let ((it (let loop ((x 0)
  54                        (arg-list arg-list))
  55               (if (null? arg-list)
  56                   (modulo x n)
  57                   (loop (logior x (hashq (car arg-list) 4194303))
  58                         (cdr arg-list))))))
  59     it))
  60
  61 (define (funcq-assoc arg-list alist)
  62   (let ((it (and alist
  63                  (let and-map ((key arg-list)
  64                                (entry (caar alist)))
  65                    (or (and (and (not key) (not entry))
  66                             (car alist))
  67                        (and key entry
  68                             (eq? (car key) (car entry))
  69                             (and-map (cdr key) (cdr entry))))))))
  70     it))
  71
  72
  73
  74 (define (pure-funcq base-func)
  75   (lambda args
  76     (let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
  77       (if cached
  78           (begin
  79             (funcq-buffer (car cached))
  80             (cdr cached))
  81
  82           (let ((val (apply base-func args))
  83                 (key (cons base-func args)))
  84             (funcq-buffer key)
  85             (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
  86             val)))))
  87
  88 \f
  89
  90 ;;; {Perfect funq}
  91 ;;;
  92 ;;; A pure funq may sometimes forget its past but a perfect
  93 ;;; funcq never does.
  94 ;;;
  95
  96 (define (perfect-funcq size base-func)
  97   (define funcq-memo (make-hash-table size))
  98
  99   (lambda args
 100     (let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
 101       (if cached
 102           (begin
 103             (funcq-buffer (car cached))
 104             (cdr cached))
 105
 106           (let ((val (apply base-func args))
 107                 (key (cons base-func args)))
 108             (funcq-buffer key)
 109             (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
 110             val)))))
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