Implements json.Marshaler and json.Unmarshaler

Uses simple JSON string (un)marshaling under the
hood and Parse() on validating unmarshaled content.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

json.go

@@ -0,0 +1,23 @@
+package semver
+
+import (
+	"encoding/json"
+)
+
+// MarshalJSON implements the encoding/json.Marshaler interface.
+func (v Version) MarshalJSON() ([]byte, error) {
+	return json.Marshal(v.String())
+}
+
+// UnmarshalJSON implements the encoding/json.Unmarshaler interface.
+func (v *Version) UnmarshalJSON(data []byte) (err error) {
+	var versionString string
+
+	if err = json.Unmarshal(data, &versionString); err != nil {
+		return
+	}
+
+	*v, err = Parse(versionString)
+
+	return
+}

json_test.go

@@ -0,0 +1,45 @@
+package semver
+
+import (
+	"encoding/json"
+	"strconv"
+	"testing"
+)
+
+func TestJSONMarshal(t *testing.T) {
+	versionString := "3.1.4-alpha.1.5.9+build.2.6.5"
+	v, err := Parse(versionString)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	versionJSON, err := json.Marshal(v)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	quotedVersionString := strconv.Quote(versionString)
+
+	if string(versionJSON) != quotedVersionString {
+		t.Fatalf("JSON marshaled semantic version not equal: expected %q, got %q", quotedVersionString, string(versionJSON))
+	}
+}
+
+func TestJSONUnmarshal(t *testing.T) {
+	versionString := "3.1.4-alpha.1.5.9+build.2.6.5"
+	quotedVersionString := strconv.Quote(versionString)
+
+	var v Version
+	if err := json.Unmarshal([]byte(quotedVersionString), &v); err != nil {
+		t.Fatal(err)
+	}
+
+	if v.String() != versionString {
+		t.Fatalf("JSON unmarshaled semantic version not equal: expected %q, got %q", versionString, v.String())
+	}
+
+	badVersionString := strconv.Quote("3.1.4.1.5.9.2.6.5-other-digits-of-pi")
+	if err := json.Unmarshal([]byte(badVersionString), &v); err == nil {
+		t.Fatal("expected JSON unmarshal error, got nil")
+	}
+}