From 40ae6a40a970ef4cdbffa7b24b280e316db8accc Mon Sep 17 00:00:00 2001
From: Prashant Varanasi <prashant@uber.com>
Date: Tue, 19 Nov 2019 13:33:40 -0800
Subject: [PATCH] Modify Bool to only use 1 or 0, rater than last bit (#62)

Most methods of Bool currently rely on the last bit, but `CAS`
generates the int value to match `old` based on the user's input
and so it assumes the value is either `1` or `0`.

The only reason we relid on the last bit is that `Toggle` was
implemented using an atomic add. We can instead implement
`Toggle` using a loop around `CAS`.

Fixes #61.
---
 CHANGELOG.md   | 7 ++++---
 atomic.go      | 9 +++++++--
 atomic_test.go | 4 +++-
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4d84d5a..6db846f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,8 +4,9 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
-- No changes yet.
+## [1.5.1] - 2019-11-19
+- Fix bug where `Bool.CAS` and `Bool.Toggle` do work correctly together
+  causing `CAS` to fail even though the old value matches.
 
 ## [1.5.0] - 2019-10-29
 ### Changed
@@ -47,7 +48,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Initial release.
 
-[Unreleased]: https://github.com/uber-go/atomic/compare/v1.5.0...HEAD
+[1.5.1]: https://github.com/uber-go/atomic/compare/v1.5.0...v1.5.1
 [1.5.0]: https://github.com/uber-go/atomic/compare/v1.4.0...v1.5.0
 [1.4.0]: https://github.com/uber-go/atomic/compare/v1.3.2...v1.4.0
 [1.3.2]: https://github.com/uber-go/atomic/compare/v1.3.1...v1.3.2
diff --git a/atomic.go b/atomic.go
index 1db6849..ad5fa09 100644
--- a/atomic.go
+++ b/atomic.go
@@ -250,11 +250,16 @@ func (b *Bool) Swap(new bool) bool {
 
 // Toggle atomically negates the Boolean and returns the previous value.
 func (b *Bool) Toggle() bool {
-	return truthy(atomic.AddUint32(&b.v, 1) - 1)
+	for {
+		old := b.Load()
+		if b.CAS(old, !old) {
+			return old
+		}
+	}
 }
 
 func truthy(n uint32) bool {
-	return n&1 == 1
+	return n == 1
 }
 
 func boolToInt(b bool) uint32 {
diff --git a/atomic_test.go b/atomic_test.go
index 6666f8a..55e220f 100644
--- a/atomic_test.go
+++ b/atomic_test.go
@@ -106,7 +106,9 @@ func TestUint64(t *testing.T) {
 
 func TestBool(t *testing.T) {
 	atom := NewBool(false)
-	require.False(t, atom.Toggle(), "Expected swap to return previous value.")
+	require.False(t, atom.Toggle(), "Expected Toggle to return previous value.")
+	require.True(t, atom.Toggle(), "Expected Toggle to return previous value.")
+	require.False(t, atom.Toggle(), "Expected Toggle 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")