From a00edcbc54026ea956ca41029474ff0e6a838616 Mon Sep 17 00:00:00 2001
From: Pjotr Prins
Date: Sun, 6 Aug 2023 10:51:13 +0200
Subject: memoize: created separate module

---
 gn/cache/memoize.scm | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 gn/cache/memoize.scm

(limited to 'gn/cache')

diff --git a/gn/cache/memoize.scm b/gn/cache/memoize.scm
new file mode 100644
index 0000000..e7ccc33
--- /dev/null
+++ b/gn/cache/memoize.scm
@@ -0,0 +1,33 @@
+;; Defines simple memoization functions. Currently it uses an alist -
+;; we should change that to a hash. Also we want the cache to expire
+;; after some time. memoize2 is now a separate function to deal
+;; specifically with two return values. That is not ideal either.
+
+;; Basically lifted from
+;; https://lispdreams.wordpress.com/2016/04/08/lisp-memoization-techniques/
+
+(define-module (gn cache memoize)
+  #:export (memoize
+            memoize2))
+
+(define (memoize f)
+  "Simple memoize just uses alists at this point and does not expire"
+  (let ((result-table '()))
+    (lambda (. args)
+      (let ((cache-value (assoc args (pk result-table))))
+	(if (pk cache-value)
+	    (cdr cache-value)
+	    (let ((result (apply f args)))
+	      (set! result-table
+                    (alist-cons args result result-table)) result))))))
+
+(define (memoize2 f)
+  "Simple memoize functions that returns values pair and uses alists at this point and does not expire"
+  (let ((result-table '()))
+    (lambda (. args)
+      (let ((c (assoc args (pk result-table))))
+	(if (pk c)
+	    (values (car (cdr c)) (car (cdr (cdr c))))
+	    (let-values (((r1 r2) (apply f args)))
+	      (set! result-table
+                    (alist-cons args (list r1 r2) result-table)) (values r1 r2)))))))
-- 
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