runes.Reader: introduce String(), Emit() and Discard()

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

runes/reader.go

@@ -30,6 +30,32 @@ type Reader struct {
 	cursor int
 }
 
+// String returns what's already Read but not yet emitted or discarded
+func (b *Reader) String() string {
+	return string(b.buf[b.off:b.cursor])
+}
+
+// Emit returns what's already being Read and discards it afterwards
+func (b *Reader) Emit() string {
+	s := b.String()
+	b.Discard()
+	return s
+}
+
+// Discard removes from the buffer everything that has been Read
+func (b *Reader) Discard() {
+	switch {
+	case b.ready() == 0:
+		// reset
+		b.buf = b.buf[:0]
+		b.cursor = 0
+		b.off = 0
+	default:
+		// step
+		b.off = b.cursor
+	}
+}
+
 // ready tells how many bytes are ready to decode
 func (b *Reader) ready() int {
 	return len(b.buf) - b.cursor