scanner.Scanner: initial struct and methods for the low level text scanner

Signed-off-by: Alejandro Mery <amery@geeks.cl>

scanner/scanner.go

@@ -1 +1,73 @@
 package scanner
+
+import (
+	"unicode/utf8"
+)
+
+const (
+	EOF = -1
+)
+
+type Position struct {
+	Offset       uint
+	Line, Column uint
+}
+
+type Scanner struct {
+	name  string
+	input string
+
+	base   Position
+	cursor Position
+	runes  uint
+}
+
+// Creates new Scanner to parse a given string, or nil if empty
+func NewScannerFromString(name, input string) *Scanner {
+	if len(input) == 0 {
+		return nil
+	}
+
+	return &Scanner{
+		name:   name,
+		input:  input,
+		base:   Position{0, 1, 1},
+		cursor: Position{0, 1, 1},
+		runes:  0,
+	}
+}
+
+// Current length of the upcomming Terminal
+func (l *Scanner) Length() (uint, uint) {
+	return l.cursor.Offset - l.base.Offset, l.runes
+}
+
+// Is the upcoming Terminal stil empty?
+func (l *Scanner) Empty() bool {
+	return l.runes == 0
+}
+
+// Move cursor forward
+func (l *Scanner) StepForth(runes, bytes uint) {
+	l.cursor.Offset += bytes
+	l.cursor.Column += runes
+	l.runes += runes
+}
+
+// Return the next rune but not moving the cursor
+func (l *Scanner) Peek() (rune, uint) {
+	if l.cursor.Offset == uint(len(l.input)) {
+		return EOF, 0
+	}
+	r, bytes := utf8.DecodeRuneInString(l.input[l.cursor.Offset:])
+	return r, uint(bytes)
+}
+
+// Return the next rune but moving the cursor
+func (l *Scanner) Next() (rune, uint) {
+	r, bytes := l.Peek()
+	if bytes > 0 {
+		l.StepForth(1, bytes)
+	}
+	return r, bytes
+}