feat: DID JSON marshaller (#54)

Adds JSON marshal and unmarshal methods to DID type.

Author: alanshaw

did/did.go

@@ -1,6 +1,7 @@
 package did
 
 import (
+	"encoding/json"
 	"fmt"
 	"strings"
 
@@ -55,6 +56,30 @@ func (d DID) GoString() string {
 	return d.String()
 }
 
+func (d DID) MarshalJSON() ([]byte, error) {
+	if d == Undef {
+		return json.Marshal(nil)
+	}
+	return json.Marshal(d.String())
+}
+
+func (d *DID) UnmarshalJSON(b []byte) error {
+	var str string
+	err := json.Unmarshal(b, &str)
+	if err != nil {
+		return fmt.Errorf("parsing string: %w", err)
+	}
+	if str == "" {
+		return nil
+	}
+	parsed, err := Parse(str)
+	if err != nil {
+		return fmt.Errorf("parsing DID: %w", err)
+	}
+	*d = parsed
+	return nil
+}
+
 func Decode(bytes []byte) (DID, error) {
 	code, _, err := varint.FromUvarint(bytes)
 	if err != nil {

did/did_test.go

@@ -1,7 +1,10 @@
 package did
 
 import (
+	"encoding/json"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestParseDIDKey(t *testing.T) {
@@ -77,3 +80,35 @@ func TestEquivalence(t *testing.T) {
 		t.Fatalf("two equivalent DID not equivalent")
 	}
 }
+
+func TestRoundtripJSON(t *testing.T) {
+	id, err := Parse("did:key:z6Mkod5Jr3yd5SC7UDueqK4dAAw5xYJYjksy722tA9Boxc4z")
+	require.NoError(t, err)
+
+	type Object struct {
+		ID                DID  `json:"id"`
+		UndefID           DID  `json:"undef_id"`
+		OptionalPresentID *DID `json:"optional_present_id"`
+		OptionalAbsentID  *DID `json:"optional_absent_id"`
+	}
+	obj := Object{
+		ID:                id,
+		UndefID:           Undef,
+		OptionalPresentID: &id,
+		OptionalAbsentID:  nil,
+	}
+
+	data, err := json.Marshal(obj)
+	require.NoError(t, err)
+
+	t.Log(string(data))
+
+	var out Object
+	err = json.Unmarshal(data, &out)
+	require.NoError(t, err)
+
+	require.Equal(t, obj.ID, out.ID)
+	require.Equal(t, obj.UndefID, out.UndefID)
+	require.Equal(t, obj.OptionalPresentID.String(), out.OptionalPresentID.String())
+	require.Nil(t, out.OptionalAbsentID)
+}