lexer: introduce Accept()/AcceptAll() helpers

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

lexer/lexer.go

@@ -1,6 +1,8 @@
 // Package lexer provides basic helpers to implement parsers
 package lexer
 
+import "io"
+
 // StateFn is a State Function of the parser
 type StateFn func() (StateFn, error)
 
@@ -15,3 +17,38 @@ func Run(fn StateFn) error {
 
 	return err
 }
+
+// Accept consumes a rune from the source if it meets the condition.
+// it returns true if the condition was met and false if it wasn't.
+func Accept(src io.RuneScanner, cond func(r rune) bool) bool {
+	r, _, err := src.ReadRune()
+	switch {
+	case err != nil:
+		return false
+	case cond(r):
+		return true
+	default:
+		_ = src.UnreadRune()
+		return false
+	}
+}
+
+// AcceptAll consumes runes from the source as long as they meet the
+// condition. it returns true if the condition was met for at least one rune,
+// and false if it wasn't.
+func AcceptAll(src io.RuneScanner, cond func(r rune) bool) bool {
+	var accepted bool
+
+	for {
+		r, _, err := src.ReadRune()
+		switch {
+		case err != nil:
+			return accepted
+		case cond(r):
+			accepted = true
+		default:
+			_ = src.UnreadRune()
+			return accepted
+		}
+	}
+}