now marshaller respects options like 'omitempty'

bencode/struct.go

@@ -424,7 +424,7 @@ func bencodeKey(field reflect.StructField) (key string) {
 			}
 		}
 	}
-	return
+	return key
 }
 
 func writeStruct(w io.Writer, val reflect.Value) (err error) {
@@ -440,7 +440,11 @@ func writeStruct(w io.Writer, val reflect.Value) (err error) {
 
 	for i := 0; i < numFields; i++ {
 		field := typ.Field(i)
-		svList[i].key = bencodeKey(field)
+		key, opts := parseTag(bencodeKey(field))
+		if key == "-" || opts.Contains("omitempty") && isEmptyValue(val.Field(i)) {
+			continue
+		}
+		svList[i].key = key
 		svList[i].value = val.Field(i)
 	}
 
@@ -456,6 +460,59 @@ func writeStruct(w io.Writer, val reflect.Value) (err error) {
 	return
 }
 
+func isEmptyValue(v reflect.Value) bool {
+	switch v.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
+		return v.Len() == 0
+	case reflect.Bool:
+		return !v.Bool()
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return v.Int() == 0
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return v.Uint() == 0
+	case reflect.Float32, reflect.Float64:
+		return v.Float() == 0
+	case reflect.Interface, reflect.Ptr:
+		return v.IsNil()
+	}
+	return false
+}
+
+// tagOptions is the string following a comma in a struct field's "json"
+// tag, or the empty string. It does not include the leading comma.
+type tagOptions string
+
+// parseTag splits a struct field's json tag into its name and
+// comma-separated options.
+func parseTag(tag string) (string, tagOptions) {
+	if idx := strings.Index(tag, ","); idx != -1 {
+		return tag[:idx], tagOptions(tag[idx+1:])
+	}
+	return tag, tagOptions("")
+}
+
+// Contains reports whether a comma-separated list of options
+// contains a particular substr flag. substr must be surrounded by a
+// string boundary or commas.
+func (o tagOptions) Contains(optionName string) bool {
+	if len(o) == 0 {
+		return false
+	}
+	s := string(o)
+	for s != "" {
+		var next string
+		i := strings.Index(s, ",")
+		if i >= 0 {
+			s, next = s[:i], s[i+1:]
+		}
+		if s == optionName {
+			return true
+		}
+		s = next
+	}
+	return false
+}
+
 func writeValue(w io.Writer, val reflect.Value) (err error) {
 	if !val.IsValid() {
 		err = errors.New("Can't write null value")

bencode/struct_test.go

@@ -1,12 +1,14 @@
 package bencode
 
 import (
+	"bytes"
 	"testing"
 )
 
 type testStruct struct {
 	Field1 string `bencode:"my field1"`
 	Field2 int64  `bencode:"my field2"`
+	Field3 int64  `bencode:"my field3,omitempty"`
 }
 
 type testOldTag struct {
@@ -15,12 +17,16 @@ type testOldTag struct {
 }
 
 func TestMarshalling(t *testing.T) {
-	ts1 := testStruct{"foo", 123456}
+	ts1 := testStruct{Field1: "foo", Field2: 123456}
 	buf, err := Marshal(ts1)
 	if err != nil {
 		t.Fatal(err)
 	}
 
+	if bytes.Contains(buf, []byte("omitempty")) || bytes.Contains(buf, []byte("field3")) {
+		t.Errorf("should not have the string 'omitempty' or 'field3' in %q", buf)
+	}
+
 	ts2 := testStruct{}
 	err = Unmarshal(buf, &ts2)
 	if err != nil {

torrent/file_test.go

@@ -1,6 +1,7 @@
 package torrent
 
 import (
+	"bytes"
 	"github.com/vbatts/go-bt/bencode"
 	"testing"
 )
@@ -19,6 +20,10 @@ func TestFileMarshal(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if bytes.Contains(buf, []byte("omitempty")) || bytes.Contains(buf, []byte("created by")) {
+		t.Errorf("should not have the string 'omitempty' or 'created by' in %q", buf)
+	}
+
 	f2 := File{}
 	err = bencode.Unmarshal(buf, &f2)
 	if err != nil {