Use easyjson to json marshalling

The JSON marshalling of Metadata now uses easyJSON to marshal this gives us a nice speed improvement and fixes a issue where marhshaling and unmarshaling json would  result is a different order.

```
BenchmarkMarshalJSON-4            500000              2553 ns/op             496 B/op          8 allocs/op
```

Author: boekkooi-fresh

metadata/metadata.go

@@ -3,19 +3,31 @@ package metadata
 import (
 	"encoding/json"
 
+	"github.com/mailru/easyjson"
 	"github.com/mailru/easyjson/jlexer"
+	"github.com/mailru/easyjson/jwriter"
+	"github.com/pkg/errors"
 )
 
-// Metadata is an immutable map[string]interface{} implementation
-type Metadata interface {
-	// Value returns the value associated with this context for key, or nil
-	// if no value is associated with key. Successive calls to Value with
-	// the same key returns the same result.
-	Value(key string) interface{}
+type (
+	// Metadata is an immutable map[string]interface{} implementation
+	Metadata interface {
+		// Value returns the value associated with this context for key, or nil
+		// if no value is associated with key. Successive calls to Value with
+		// the same key returns the same result.
+		Value(key string) interface{}
 
-	// AsMap return the Metadata as a map[string]interface{}
-	AsMap() map[string]interface{}
-}
+		// AsMap return the Metadata as a map[string]interface{}
+		AsMap() map[string]interface{}
+	}
+
+	// jsonMarshaler is an interface used to speed up the json marshalling process
+	jsonMarshaler interface {
+		// MarshalJSONKeyValue writes the Metadata Key and Value and all it's parent Key Value pairs into the writer
+		// If last is true it MUST omit the `,` from the last Key Value pair
+		MarshalJSONKeyValue(out *jwriter.Writer, last bool)
+	}
+)
 
 // New return a new Metadata instance without any information
 func New() Metadata {
@@ -43,8 +55,12 @@ type emptyData int
 var (
 	// Ensure emptyData implements the Metadata interface
 	_ Metadata = new(emptyData)
-	// Ensure valueData implements the json.Marshaler interface
+	// Ensure valueData implements the jsonMarshaler interface
+	_ jsonMarshaler = new(emptyData)
+	// Ensure emptyData implements the json.Marshaler interface
 	_ json.Marshaler = new(emptyData)
+	// Ensure emptyData implements the easyjson.Marshaler interface
+	_ easyjson.Marshaler = new(emptyData)
 )
 
 func (*emptyData) Value(key string) interface{} {
@@ -59,6 +75,14 @@ func (v *emptyData) MarshalJSON() ([]byte, error) {
 	return []byte("{}"), nil
 }
 
+func (v *emptyData) MarshalEasyJSON(w *jwriter.Writer) {
+	w.RawByte('{')
+	w.RawByte('}')
+}
+
+func (v *emptyData) MarshalJSONKeyValue(*jwriter.Writer, bool) {
+}
+
 // valueData represents a key, value pair in a metadata chain
 type valueData struct {
 	Metadata
@@ -69,8 +93,12 @@ type valueData struct {
 var (
 	// Ensure valueData implements the Metadata interface
 	_ Metadata = new(valueData)
+	// Ensure valueData implements the jsonMarshaler interface
+	_ jsonMarshaler = new(valueData)
 	// Ensure valueData implements the json.Marshaler interface
 	_ json.Marshaler = new(valueData)
+	// Ensure valueData implements the easyjson.Marshaler interface
+	_ easyjson.Marshaler = new(valueData)
 )
 
 func (v *valueData) Value(key string) interface{} {
@@ -95,7 +123,33 @@ func (v *valueData) AsMap() map[string]interface{} {
 }
 
 func (v *valueData) MarshalJSON() ([]byte, error) {
-	return json.Marshal(v.AsMap())
+	w := jwriter.Writer{}
+	v.MarshalEasyJSON(&w)
+	return w.Buffer.BuildBytes(), w.Error
+}
+
+func (v *valueData) MarshalEasyJSON(w *jwriter.Writer) {
+	w.RawByte('{')
+	v.MarshalJSONKeyValue(w, true)
+	w.RawByte('}')
+}
+
+func (v *valueData) MarshalJSONKeyValue(out *jwriter.Writer, last bool) {
+	marshalJSONKeyValues(out, v.Metadata)
+
+	out.String(v.key)
+	out.RawByte(':')
+	if vm, ok := v.val.(easyjson.Marshaler); ok {
+		vm.MarshalEasyJSON(out)
+	} else if vm, ok := v.val.(json.Marshaler); ok {
+		out.Raw(vm.MarshalJSON())
+	} else {
+		out.Raw(json.Marshal(v.val))
+	}
+
+	if !last {
+		out.RawByte(',')
+	}
 }
 
 // UnmarshalJSON unmarshals the provided json into a Metadata instance
@@ -127,3 +181,42 @@ func UnmarshalJSON(json []byte) (Metadata, error) {
 
 	return metadata, in.Error()
 }
+
+func marshalJSONKeyValues(out *jwriter.Writer, parent Metadata) {
+	if parent == nil {
+		return
+	}
+
+	if m, ok := parent.(jsonMarshaler); ok {
+		m.MarshalJSONKeyValue(out, false)
+		return
+	}
+
+	var (
+		parentJSON []byte
+		err        error
+	)
+	if vm, ok := parent.(easyjson.Marshaler); ok {
+		w := &jwriter.Writer{}
+		vm.MarshalEasyJSON(out)
+		parentJSON = w.Buffer.BuildBytes()
+		err = w.Error
+	} else if vm, ok := parent.(json.Marshaler); ok {
+		parentJSON, err = vm.MarshalJSON()
+	} else {
+		parentJSON, err = json.Marshal(parent)
+	}
+
+	if err != nil {
+		out.Raw(parentJSON, err)
+		return
+	}
+
+	plen := len(parentJSON)
+	if parentJSON[1] != '{' || parentJSON[plen-1] != '}' {
+		out.Raw(parentJSON, errors.Errorf("JSON unmarshal failed for Metadata of type %T", parent))
+		return
+	}
+
+	out.Raw(parentJSON[1:plen-2], nil)
+}

metadata/metadata_test.go

@@ -4,8 +4,10 @@ package metadata_test
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 	"time"
+	"unicode"
 
 	"github.com/hellofresh/goengine/metadata"
 	"github.com/stretchr/testify/assert"
@@ -201,11 +203,18 @@ var jsonTestCases = []struct {
 func TestMarshalJSON(t *testing.T) {
 	for _, testCase := range jsonTestCases {
 		t.Run(testCase.title, func(t *testing.T) {
+			expectedJSON := strings.Map(func(r rune) rune {
+				if unicode.IsSpace(r) {
+					return -1
+				}
+				return r
+			}, testCase.json)
+
 			m := testCase.metadata()
 
 			mJSON, err := json.Marshal(m)
 
-			assert.JSONEq(t, testCase.json, string(mJSON))
+			assert.Equal(t, expectedJSON, string(mJSON))
 			assert.NoError(t, err)
 		})
 	}
@@ -218,7 +227,7 @@ func TestUnmarshalJSON(t *testing.T) {
 
 			// Need to use AsMap otherwise we can have inconsistent tests results.
 			if assert.NoError(t, err) {
-				assert.Equal(t, testCase.metadata().AsMap(), m.AsMap())
+				assert.Equal(t, testCase.metadata(), m)
 			}
 		})
 	}