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

@@ -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

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 {

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")