Import guile-1.8 as multiple upstream tarball component

[lilypond.git] / guile18 / ice-9 / poe.scm

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+;;; installed-scm-file
+
+;;;;   Copyright (C) 1996, 2001, 2006 Free Software Foundation, Inc.
+;;;; 
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 2.1 of the License, or (at your option) any later version.
+;;;; 
+;;;; This library 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
+;;;; Lesser General Public License for more details.
+;;;; 
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+;;;; 
+
+\f
+(define-module  (ice-9 poe)
+  :use-module (ice-9 hcons)
+  :export (pure-funcq perfect-funcq))
+
+\f
+
+
+;;; {Pure Functions}
+;;; 
+;;; A pure function (of some sort) is characterized by two equality
+;;; relations: one on argument lists and one on return values.
+;;; A pure function is one that when applied to equal arguments lists
+;;; yields equal results.
+;;;
+;;; If the equality relationship on return values can be eq?, it may make
+;;; sense to cache values returned by the function.  Choosing the right
+;;; equality relation on arguments is tricky.
+;;;
+
+\f
+;;; {pure-funcq}
+;;;
+;;; The simplest case of pure functions are those in which results
+;;; are only certainly eq? if all of the arguments are.  These functions
+;;; are called "pure-funcq", for obvious reasons.
+;;;
+
+
+(define funcq-memo (make-weak-key-hash-table 523)) ; !!! randomly selected values
+(define funcq-buffer (make-gc-buffer 256))
+
+(define (funcq-hash arg-list n)
+  (let ((it (let loop ((x 0)
+                      (arg-list arg-list))
+             (if (null? arg-list)
+                 (modulo x n)
+                 (loop (logior x (hashq (car arg-list) 4194303))
+                       (cdr arg-list))))))
+    it))
+
+;; return true if lists X and Y are the same length and each element is `eq?'
+(define (eq?-list x y)
+  (if (null? x)
+      (null? y)
+      (and (not (null? y))
+          (eq? (car x) (car y))
+          (eq?-list (cdr x) (cdr y)))))
+
+(define (funcq-assoc arg-list alist)
+  (if (null? alist)
+      #f
+      (if (eq?-list arg-list (caar alist))
+         (car alist)
+         (funcq-assoc arg-list (cdr alist)))))
+
+
+
+(define (pure-funcq base-func)
+  (lambda args
+    (let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
+      (if cached
+         (begin
+           (funcq-buffer (car cached))
+           (cdr cached))
+           
+         (let ((val (apply base-func args))
+               (key (cons base-func args)))
+           (funcq-buffer key)
+           (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
+           val)))))
+
+\f
+
+;;; {Perfect funq}
+;;;
+;;; A pure funq may sometimes forget its past but a perfect
+;;; funcq never does.
+;;;
+
+(define (perfect-funcq size base-func)
+  (define funcq-memo (make-hash-table size))
+
+  (lambda args
+    (let ((cached (hashx-get-handle funcq-hash funcq-assoc funcq-memo (cons base-func args))))
+      (if cached
+         (begin
+           (funcq-buffer (car cached))
+           (cdr cached))
+           
+         (let ((val (apply base-func args))
+               (key (cons base-func args)))
+           (funcq-buffer key)
+           (hashx-set! funcq-hash funcq-assoc funcq-memo key val)
+           val)))))
+
+
+
+
+
+
+
+