lexer: introduce a Position (Line, Column) handler

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

lexer/position.go

@@ -0,0 +1,66 @@
+package lexer
+
+import "fmt"
+
+// Position indicates a line and column pair on a file.
+// Counting starts at 1.
+type Position struct {
+	Line   int
+	Column int
+}
+
+// String generates a pretty "(Line, Column)"" representation of the Position
+func (p Position) String() string {
+	if p.Line == 0 {
+		p.Reset()
+	}
+
+	return fmt.Sprintf("(%v, %v)", p.Line, p.Column)
+}
+
+// GoString generates a string representation of the Position for %#v usage
+func (p Position) GoString() string {
+	if p.Line == 0 {
+		p.Reset()
+	}
+
+	return fmt.Sprintf("lexer.Position{%v, %v}", p.Line, p.Column)
+}
+
+// Reset places a position at (1,1)
+func (p *Position) Reset() {
+	p.Line, p.Column = 1, 1
+}
+
+// Step moves the column one place
+func (p *Position) Step() {
+	if p.Line == 0 {
+		p.Reset()
+	}
+
+	p.Column++
+}
+
+// StepN moves the column N places forward
+func (p *Position) StepN(n int) {
+	if p.Line == 0 {
+		p.Reset()
+	}
+
+	switch {
+	case n > 0:
+		p.Column += n
+	default:
+		panic(fmt.Errorf("invalid %v increment", n))
+	}
+}
+
+// StepLine moves position to the start of the next line
+func (p *Position) StepLine() {
+	if p.Line == 0 {
+		p.Reset()
+	}
+
+	p.Line++
+	p.Column = 1
+}