runes.Reader: simplify buffer handling

Signed-off-by: Alejandro Mery <amery@jpi.io>

runes/reader.go

@@ -87,12 +87,12 @@ func (b *Reader) needsBytes(n int) error {
 func (b *Reader) rebuffer(size int) {
 	var src, dst []byte
 
-	if size <= cap(b.buf) {
+	if size > cap(b.buf) {
-		// same buffer
-		dst = b.buf
-	} else {
 		// new buffer
 		dst = make([]byte, size)
+	} else {
+		// same buffer
+		dst = b.buf
 	}
 
 	src = b.buf[b.off:]
@@ -106,16 +106,7 @@ func (b *Reader) rebuffer(size int) {
 }
 
 func (b *Reader) prepareBuffer(n int) {
-	available := b.available()
+	if n > b.available() {
-
-	switch {
-	case n <= available:
-		// n or more already available
-	case n <= available+b.off:
-		// rebase is enough
-		b.rebuffer(0)
-	default:
-		// resize
 		needed := len(b.buf) + n - b.off
 		size := cap(b.buf)