Add Bool.CAS for compare-and-swap semantics (#23)

atomic.go

@@ -232,6 +232,11 @@ func (b *Bool) Load() bool {
 	return truthy(atomic.LoadUint32(&b.v))
 }
 
+// CAS is an atomic compare-and-swap.
+func (b *Bool) CAS(old, new bool) bool {
+	return atomic.CompareAndSwapUint32(&b.v, boolToInt(old), boolToInt(new))
+}
+
 // Store atomically stores the passed value.
 func (b *Bool) Store(new bool) {
 	atomic.StoreUint32(&b.v, boolToInt(new))

atomic_test.go

@@ -108,6 +108,13 @@ func TestBool(t *testing.T) {
 	require.False(t, atom.Toggle(), "Expected swap to return previous value.")
 	require.True(t, atom.Load(), "Unexpected state after swap.")
 
+	require.True(t, atom.CAS(true, true), "CAS should swap when old matches")
+	require.True(t, atom.Load(), "CAS should have no effect")
+	require.True(t, atom.CAS(true, false), "CAS should swap when old matches")
+	require.False(t, atom.Load(), "CAS should have modified the value")
+	require.False(t, atom.CAS(true, false), "CAS should fail on old mismatch")
+	require.False(t, atom.Load(), "CAS should not have modified the value")
+
 	atom.Store(false)
 	require.False(t, atom.Load(), "Unexpected state after store.")
 

stress_test.go

@@ -122,6 +122,8 @@ func TestStressBool(t *testing.T) {
 		atom.Load()
 		atom.Store(false)
 		atom.Swap(true)
+		atom.CAS(true, false)
+		atom.CAS(true, false)
 		atom.Load()
 		atom.Toggle()
 		atom.Toggle()