added another error check.

errors.go

@@ -8,4 +8,5 @@ var (
 	errInternalError = errors.New("Internal Error")
 	errInvalidUTF8   = internalInvalidUTF8Complainer{}
 	errNilReader     = internalNilReaderComplainer{}
+	errNilWriter     = internalNilWriterComplainer{}
 )

nilwriter.go

@@ -0,0 +1,16 @@
+package utf8s
+
+type NilWriterComplainer interface {
+	error
+	NilWriterComplainer()
+}
+
+type internalNilWriterComplainer struct{}
+
+func (complainer internalNilWriterComplainer) Error() string {
+	return "Nil Writer"
+}
+
+func (complainer internalNilWriterComplainer) NilWriterComplainer() {
+	// Nothing here.
+}

writerune.go

@@ -5,8 +5,10 @@ import (
 )
 
 // WriteRune writes a single UTF-8 encoded Unicode character and returns the number of bytes written.
-func WriteRune(w io.Writer, r rune) (int, error) {
+func WriteRune(writer io.Writer, r rune) (int, error) {
-
+	if nil == writer {
+		return 0, errNilWriter
+	}
 
 	switch {
 	case 127 >= r:
@@ -16,7 +18,7 @@ func WriteRune(w io.Writer, r rune) (int, error) {
 
 		p := buffer[:]
 
-		return w.Write(p)
+		return writer.Write(p)
 
 	case 0x7FF >= r:
 		var buffer [2]byte
@@ -26,7 +28,7 @@ func WriteRune(w io.Writer, r rune) (int, error) {
 
 		p := buffer[:]
 
-		return w.Write(p)
+		return writer.Write(p)
 
 	case 0xFFFF >= r:
 		var buffer [3]byte
@@ -37,7 +39,7 @@ func WriteRune(w io.Writer, r rune) (int, error) {
 
 		p := buffer[:]
 
-		return w.Write(p)
+		return writer.Write(p)
 
 	case 0x10FFFF >= r:
 		var buffer [4]byte
@@ -49,7 +51,7 @@ func WriteRune(w io.Writer, r rune) (int, error) {
 
 		p := buffer[:]
 
-		return w.Write(p)
+		return writer.Write(p)
 
 	default:
 		return 0, errInternalError