fix(omitempty): must omit empty slices

The `omitempty` tag was omitting `nil` slices but not slices that were
allocated but had length zero. This has been fixes for consistency with
json.Marshal. Also empty structs are no longer omitted for consistency
with json.Marhsal.

marshal.go

@@ -44,7 +44,7 @@ func marshalStruct(ctx context.Context, in reflect.Value) (map[string]interface{
 
 		if public(f) {
 			p := path.ComputePath(f)
-			shouldSkip := p.OmitAlways || (p.OmitEmpty && in.Field(i).IsZero())
+			shouldSkip := p.OmitAlways || (p.OmitEmpty && isEmpty(in.Field(i)))
 
 			if !shouldSkip {
 				r, err := marshal(ctx.WithField(f.Name, f.Type), in.Field(i))
@@ -138,3 +138,17 @@ func marshalJSONMarshaler(ctx context.Context, in reflect.Value) (interface{}, e
 
 	return r, nil
 }
+
+func isEmpty(v reflect.Value) bool {
+	k := v.Kind()
+	switch {
+	case k == reflect.Interface, k == reflect.Ptr:
+		return v.IsZero() || v.IsNil()
+	case k == reflect.String, k == reflect.Map, k == reflect.Slice, k == reflect.Array:
+		return v.Len() == 0
+	case basicType(k):
+		return v.IsZero()
+	default:
+		return false
+	}
+}

marshal_test.go

@@ -1,6 +1,7 @@
 package jsonry_test
 
 import (
+	"encoding/json"
 	"errors"
 
 	"code.cloudfoundry.org/jsonry"
@@ -209,50 +210,102 @@ var _ = Describe("Marshal", func() {
 	})
 
 	Describe("omitempty", func() {
-		It("omits zero values of basic types", func() {
+		It("reads the `omitempty` field from JSON and JSONry tags with and without names", func() {
 			s := struct {
 				A string `json:",omitempty"`
 				B string `json:"bee,omitempty"`
 				C string `jsonry:",omitempty"`
 				D string `jsonry:"dee,omitempty"`
-				E string
 			}{}
-			expectToMarshal(s, `{"E":""}`)
+			expectToMarshal(s, `{}`)
+		})
+
+		//		and any empty array, slice, map, or string.
+
+		It("omits false", func() {
+			s := struct {
+				A bool `jsonry:",omitempty"`
+			}{}
+			expectToMarshal(s, `{}`)
+		})
+
+		It("omits 0", func() {
+			s := struct {
+				A int     `jsonry:",omitempty"`
+				B uint    `jsonry:",omitempty"`
+				C float64 `jsonry:",omitempty"`
+			}{}
+			expectToMarshal(s, `{}`)
 		})
 
-		It("omits zero value structs", func() {
+		It("omits nil pointers", func() {
 			type t struct{ A string }
 			s := struct {
-				B t `jsonry:",omitempty"`
+				A *string      `jsonry:",omitempty"`
+				B *t           `jsonry:",omitempty"`
+				C *[]string    `jsonry:",omitempty"`
+				D *map[int]int `jsonry:",omitempty"`
+				E *interface{} `jsonry:",omitempty"`
 			}{}
 			expectToMarshal(s, `{}`)
 		})
 
-		It("omits empty lists", func() {
+		It("omits nil interface values", func() {
 			s := struct {
-				A []string  `jsonry:",omitempty"`
-				D [0]string `jsonry:",omitempty"`
+				A interface{}    `jsonry:",omitempty"`
+				B json.Marshaler `jsonry:",omitempty"`
 			}{}
 			expectToMarshal(s, `{}`)
 		})
 
+		It("omits empty arrays", func() {
+			s := struct {
+				A [0]int `jsonry:",omitempty"`
+			}{A: [0]int{}}
+			expectToMarshal(s, `{}`)
+		})
+
+		It("omits empty slices", func() {
+			s := struct {
+				A []int `jsonry:",omitempty"`
+				B []int `jsonry:",omitempty"`
+				C []int `jsonry:",omitempty"`
+			}{
+				B: []int{},
+				C: make([]int, 0, 1),
+			}
+			expectToMarshal(s, `{}`)
+		})
+
 		It("omits empty maps", func() {
 			s := struct {
 				A map[interface{}]interface{} `jsonry:",omitempty"`
 				D map[int]int                 `jsonry:",omitempty"`
-			}{}
+			}{
+				D: make(map[int]int),
+			}
 			expectToMarshal(s, `{}`)
 		})
 
-		It("omits nil pointers", func() {
+		It("omits empty strings", func() {
 			s := struct {
-				A *string `json:",omitempty"`
-				B *string `json:"bee,omitempty"`
-				C *string `jsonry:",omitempty"`
-				D *string `jsonry:"dee,omitempty"`
-				E *string
+				A string `jsonry:",omitempty"`
+				B string `jsonry:",omitempty"`
+			}{
+				B: "",
+			}
+			expectToMarshal(s, `{}`)
+		})
+
+		// For consistency with json.Marshal, see https://github.com/golang/go/issues/11939
+		It("does not omit empty structs", func() {
+			type t struct {
+				A string `jsonry:",omitempty"`
+			}
+			s := struct {
+				B t `jsonry:",omitempty"`
 			}{}
-			expectToMarshal(s, `{"E":null}`)
+			expectToMarshal(s, `{"B":{}}`)
 		})
 	})