Split out utils so that we don't need SRFI.

utils.scm

@@ -0,0 +1,55 @@
+;;
+;; utils.scm
+;;
+;; Various utilities so that no external libraries are needed.
+;;
+;; ISC License
+;;
+;; Copyright 2023 Brmlab, z.s.
+;; Dominik Pantůček <dominik.pantucek@trustica.cz>
+;;
+;; Permission to use, copy, modify, and/or distribute this software
+;; for any purpose with or without fee is hereby granted, provided
+;; that the above copyright notice and this permission notice appear
+;; in all copies.
+;; 
+;; THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+;; WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+;; WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+;; AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR
+;; CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
+;; OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
+;; NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+;; CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+;;
+
+(declare (unit utils))
+
+(module
+ utils
+ (
+  filter
+  utils-tests!
+  )
+
+ ;; Returns a list with elements matching pred? predicate.
+ (define (filter pred? lst)
+   (let loop ((lst lst)
+	      (res '()))
+     (if (null? lst)
+	 (reverse res)
+	 (if (pred? (car lst))
+	     (loop (cdr lst)
+		   (cons (car lst) res))
+	     (loop (cdr lst)
+		   res)))))
+
+ ;; Performs utils module self-tests.
+ (define (utils-tests!)
+   (run-tests
+    utils
+    (test-equal? filter (filter odd? '(1 2 3 4)) '(1 3))
+    (test-equal? filter (filter odd? '(2 4)) '())
+    ))
+
+ )