Add Swap for Float64 (#94)

Float64 wraps a Uint64, and since Uint64 supports Swap, Float64 can also
support Swap. Enable the Swap method in the generated code, and add
tests.

This also adds a note for why String doesn't support Swap (though it
will be possible after Go 1.17+).

CHANGELOG.md

@@ -4,6 +4,10 @@ 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
+### Added
+- Add `Float64.Swap` to match int atomic operations.
+
 ## [1.8.0] - 2021-06-09
 ### Added
 - Add `atomic.Uintptr` type for atomic operations on `uintptr` values.

float64.go

@@ -55,6 +55,12 @@ func (x *Float64) Store(v float64) {
 	x.v.Store(math.Float64bits(v))
 }
 
+// Swap atomically stores the given float64 and returns the old
+// value.
+func (x *Float64) Swap(o float64) float64 {
+	return math.Float64frombits(x.v.Swap(math.Float64bits(o)))
+}
+
 // MarshalJSON encodes the wrapped float64 into JSON.
 func (x *Float64) MarshalJSON() ([]byte, error) {
 	return json.Marshal(x.Load())

float64_ext.go

@@ -25,7 +25,7 @@ import (
 	"strconv"
 )
 
-//go:generate bin/gen-atomicwrapper -name=Float64 -type=float64 -wrapped=Uint64 -pack=math.Float64bits -unpack=math.Float64frombits -json -imports math -file=float64.go
+//go:generate bin/gen-atomicwrapper -name=Float64 -type=float64 -wrapped=Uint64 -pack=math.Float64bits -unpack=math.Float64frombits -swap -json -imports math -file=float64.go
 
 // Add atomically adds to the wrapped float64 and returns the new value.
 func (f *Float64) Add(s float64) float64 {

float64_test.go

@@ -42,6 +42,9 @@ func TestFloat64(t *testing.T) {
 	require.Equal(t, float64(42.5), atom.Add(0.5), "Add didn't work.")
 	require.Equal(t, float64(42.0), atom.Sub(0.5), "Sub didn't work.")
 
+	require.Equal(t, float64(42.0), atom.Swap(45.0), "Swap didn't return the old value.")
+	require.Equal(t, float64(45.0), atom.Load(), "Swap didn't set the correct value.")
+
 	t.Run("JSON/Marshal", func(t *testing.T) {
 		atom.Store(42.5)
 		bytes, err := json.Marshal(atom)

string_ext.go

@@ -21,6 +21,8 @@
 package atomic
 
 //go:generate bin/gen-atomicwrapper -name=String -type=string -wrapped=Value -file=string.go
+// Note: No Swap as String wraps Value, which wraps the stdlib sync/atomic.Value which
+// only supports Swap as of go1.17: https://github.com/golang/go/issues/39351
 
 // String returns the wrapped value.
 func (s *String) String() string {