1 files changed, 49 insertions, 0 deletions
diff --git a/fibers/counter.scm b/fibers/counter.scm
new file mode 100644
index 0000000..569d74c
--- /dev/null
+++ b/fibers/counter.scm
@@ -0,0 +1,49 @@
+;; Counters
+
+;;;; Copyright (C) 2017 Christopher Allan Webber <cwebber@dustycloud.org>
+;;;;
+;;;; 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 3 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
+
+;;; General atomic counters; currently used for garbage collection.
+
+(define-module (fibers counter)
+  #:use-module (ice-9 atomic)
+  #:export (make-counter
+            counter-decrement!
+            counter-reset!))
+
+;;; Counter utilities
+;;;
+;;; Counters here are an atomic box containing an integer which are
+;;; either decremented or reset.
+
+;; How many times we run the block-fn until we gc
+(define %countdown-steps 42)  ; haven't tried testing for the most efficient number
+
+(define* (make-counter)
+  (make-atomic-box %countdown-steps))
+
+(define (counter-decrement! counter)
+  "Decrement integer in atomic box COUNTER."
+  (let spin ((x (atomic-box-ref counter)))
+    (let* ((x-new (1- x))
+           (x* (atomic-box-compare-and-swap! counter x x-new)))
+      (if (= x* x)  ; successful decrement
+          x-new
+          (spin x*)))))
+
+(define (counter-reset! counter)
+  "Reset a counter's contents."
+  (atomic-box-set! counter %countdown-steps))