lexer: introduce Position.Add()

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

lexer/position.go

@@ -64,3 +64,23 @@ func (p *Position) StepLine() {
 	p.Line++
 	p.Column = 1
 }
+
+// Add adds a relative position considering
+// potential new lines
+func (p *Position) Add(rel Position) {
+	if p.Line == 0 {
+		p.Reset()
+	}
+
+	switch {
+	case rel.Line == 0:
+		// nothing
+	case rel.Line > 1:
+		// includes new lines
+		p.Line += rel.Line - 1
+		p.Column = rel.Column
+	default:
+		// same line
+		p.Column += rel.Column - 1
+	}
+}