From c8f4ef07c8ad76d934b720142f52e0a70e1ed273 Mon Sep 17 00:00:00 2001
From: Brendan Hansen <brendan.f.hansen@gmail.com>
Date: Sun, 17 Oct 2021 16:21:10 -0500
Subject: [PATCH] added a (maybe function) semaphore implementation

---
 core/sync/mutex.onyx     | 14 +++++++++++++-
 core/sync/semaphore.onyx | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 core/sync/semaphore.onyx

diff --git a/core/sync/mutex.onyx b/core/sync/mutex.onyx
index 4b8292b0..ecaf5518 100644
--- a/core/sync/mutex.onyx
+++ b/core/sync/mutex.onyx
@@ -2,6 +2,18 @@ package core.sync
 
 use package core.intrinsics.atomics
 
+// `lock` has two states: 0, and 1.
+//    0 means unlocked
+//    1 means locked
+//
+// To lock it:
+//    Try to store 1 if the value was already 0
+//    Otherwise, if it was already 1, wait until it goes to 0.
+//
+// To unlock it:
+//    Atomically set it to 0.
+//    Notify at most 1 other thread about this change.
+
 Mutex :: struct {
     lock  : i32;
     // owner : Thread_Id;
@@ -17,7 +29,7 @@ mutex_destroy :: (m: ^Mutex) {
 
 mutex_lock :: (m: ^Mutex) {
     while __atomic_cmpxchg(^m.lock, 0, 1) == 1 {
-        __atomic_wait(^m.lock, 0);
+        __atomic_wait(^m.lock, 1);
     }
 }
 
diff --git a/core/sync/semaphore.onyx b/core/sync/semaphore.onyx
new file mode 100644
index 00000000..7ecf5f40
--- /dev/null
+++ b/core/sync/semaphore.onyx
@@ -0,0 +1,37 @@
+package core.sync
+
+use package core.intrinsics.atomics
+
+Semaphore :: struct {
+    mutex   : Mutex;
+    counter : i32;
+}
+
+semaphore_init :: (s: ^Semaphore, value: i32) {
+    s.counter = value;
+
+    mutex_init(^s.mutex);
+}
+
+semaphore_post :: (s: ^Semaphore) {
+    scoped_mutex(^s.mutex);
+    s.counter += 1;
+    __atomic_notify(^s.counter, maximum = 1);
+}
+
+semaphore_wait :: (s: ^Semaphore) {
+    while true {
+        mutex_lock(^s.mutex);
+        if s.counter > 0 {
+            s.counter -= 1;
+
+            mutex_unlock(^s.mutex);
+            return;
+
+        } else {
+            mutex_unlock(^s.mutex);
+
+            __atomic_wait(^s.counter, 0);
+        }
+    }
+}
\ No newline at end of file
-- 
2.25.1