Issue #46: Fix semantics of `n` result

Per contract, the returned number of bytes written should be the number of bytes _from the input_.
Therefore, the added bytes (`\r` or `\n`) shall not count into that number.

2 files changed, 15 insertions, 9 deletions

diff --git a/pkg/rfc822/writer.go b/pkg/rfc822/writer.go
index 07751ea..dd96fbb 100644
--- a/pkg/rfc822/writer.go
+++ b/pkg/rfc822/writer.go
@@ -23,29 +23,31 @@ func (f rfc822Writer) Write(p []byte) (n int, err error) {
 	crFound := false
 	start := 0
 
-	write := func(str []byte) {
+	write := func(str []byte, count bool) {
 		var j int
 		j, err = f.w.Write(str)
-		n = n + j
+		if count {
+			n += j
+		}
 	}
 
 	for idx, b := range p {
 		if crFound && b != '\n' {
 			// insert '\n'
-			if write(p[start:idx]); err != nil {
+			if write(p[start:idx], true); err != nil {
 				return
 			}
-			if write(lf); err != nil {
+			if write(lf, false); err != nil {
 				return
 			}
 
 			start = idx
 		} else if !crFound && b == '\n' {
 			// insert '\r'
-			if write(p[start:idx]); err != nil {
+			if write(p[start:idx], true); err != nil {
 				return
 			}
-			if write(cr); err != nil {
+			if write(cr, false); err != nil {
 				return
 			}
 
@@ -55,12 +57,12 @@ func (f rfc822Writer) Write(p []byte) (n int, err error) {
 	}
 
 	// write the remainder
-	if write(p[start:]); err != nil {
+	if write(p[start:], true); err != nil {
 		return
 	}
 
 	if crFound { // dangling \r
-		write(lf)
+		write(lf, false)
 	}
 
 	return
diff --git a/pkg/rfc822/writer_test.go b/pkg/rfc822/writer_test.go
index 7beae8d..34c9d4a 100644
--- a/pkg/rfc822/writer_test.go
+++ b/pkg/rfc822/writer_test.go
@@ -34,10 +34,14 @@ func TestRfc822Writer_Write(t *testing.T) {
 		t.Run(tt.before, func(t *testing.T) {
 			b := bytes.Buffer{}
 			w := Writer(&b)
-			if _, err := io.WriteString(w, tt.before); err != nil {
+			n, err := io.WriteString(w, tt.before)
+			if err != nil {
 				t.Errorf("Error: %v", err)
 				return
 			}
+			if n != len(tt.before) {
+				t.Errorf("Unexpected number of bytes written: %d, expected: %d", n, len(tt.before))
+			}
 			res := b.String()
 			if tt.after != res {
 				t.Errorf("Expected: %q, got: %q", tt.after, res)