From dc2fc9f2cd69422c720daaa531d1bdb9a6bde0e5 Mon Sep 17 00:00:00 2001
From: Teddy Wing
Date: Sun, 31 Jan 2021 19:50:38 +0100
Subject: Define a library for Go-style wait groups

Define a data structure and methods to represent a Go-style wait group,
similar to `sync.WaitGroup` (except single-threaded).

Implements a similar interface. Calling `wait` blocks until the wait
group's internal counter gets to 0.
---
 src/package.lisp    |  9 +++++++++
 src/wait-group.lisp | 27 +++++++++++++++++++++++++++
 wait-group.asd      |  6 ++++++
 3 files changed, 42 insertions(+)
 create mode 100644 src/package.lisp
 create mode 100644 src/wait-group.lisp
 create mode 100644 wait-group.asd

diff --git a/src/package.lisp b/src/package.lisp
new file mode 100644
index 0000000..c868d02
--- /dev/null
+++ b/src/package.lisp
@@ -0,0 +1,9 @@
+(defpackage :wait-group
+  (:nicknames :wg)
+
+  (:use :cl)
+
+  (:export #:make-wait-group
+           #:add
+           #:done
+           #:wait))
diff --git a/src/wait-group.lisp b/src/wait-group.lisp
new file mode 100644
index 0000000..35bb982
--- /dev/null
+++ b/src/wait-group.lisp
@@ -0,0 +1,27 @@
+(in-package :wait-group)
+
+(defclass wait-group ()
+  ((counter
+     :initform 0
+     :accessor counter)))
+
+(defun make-wait-group ()
+  (make-instance 'wait-group))
+
+(defgeneric add (wait-group &optional amount)
+  (:documentation ""))
+
+(defmethod add ((wait-group wait-group) &optional (amount 1))
+  (incf (counter wait-group) amount))
+
+(defgeneric done (wait-group)
+  (:documentation ""))
+
+(defmethod done ((wait-group wait-group))
+  (decf (counter wait-group)))
+
+(defgeneric wait (wait-group)
+  (:documentation ""))
+
+(defmethod wait ((wait-group wait-group))
+  (loop until (= (counter wait-group) 0)))
diff --git a/wait-group.asd b/wait-group.asd
new file mode 100644
index 0000000..ac7e280
--- /dev/null
+++ b/wait-group.asd
@@ -0,0 +1,6 @@
+(asdf:defsystem wait-group
+  :version "0.0.1"
+  :components ((:module "src"
+                :serial t
+                :components ((:file "package")
+                             (:file "wait-group")))))
-- 
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