Initial commit

1 files changed, 92 insertions, 0 deletions

diff --git a/scanner.go b/scanner.go
new file mode 100644
index 0000000..a07981d
--- /dev/null
+++ b/scanner.go
@@ -0,0 +1,92 @@
+package main
+
+//go:generate stringer -type=Token
+type Token int
+
+const (
+	INVALID Token = iota 
+	EOF              
+	NUM             
+	ADD            
+	SUB           
+	MUL          
+	DIV         
+	EXP        
+)
+
+type scanner struct {
+	c   byte
+	i   int
+	buf []byte
+}
+
+func newScanner(s []byte) *scanner {
+	return &scanner{
+		buf: s,
+	}
+}
+
+func (s *scanner) read() bool {
+	if len(s.buf) <= s.i {
+		return false
+	}
+	s.c = s.buf[s.i]
+	s.i++
+	return true
+}
+
+func (s *scanner) unread() {
+	s.i--
+	s.c = s.buf[s.i]
+}
+
+// Pos returns the index of the next character
+func (s *scanner) Pos() int {
+	return s.i
+}
+
+func (s *scanner) Scan() (t Token, d []byte) {
+read:
+	ok := s.read()
+	if !ok {
+		return EOF, []byte{}
+	}
+	switch s.c {
+	case '+':
+		return ADD, nil
+	case '-':
+		return SUB, nil
+	case '*':
+		return MUL, nil
+	case '/':
+		return DIV, nil
+	case '^':
+		return EXP, nil
+	case ' ', '\n', '\t':
+		goto read
+	default:
+		if (s.c >= '0' && s.c <= '9') {
+			s.unread()
+			return s.scanNumber(false)
+		}
+		if s.c == '-' {
+			return s.scanNumber(true)
+		}
+	}
+	return INVALID, []byte{s.c}
+}
+
+func (s *scanner) scanNumber(neg bool) (t Token, d []byte) {
+	if neg {
+		d = append(d, '-')
+	}
+	for s.read() {
+		if (s.c >= '0' && s.c <= '9') {
+			d = append(d, s.c)
+		} else {
+			s.unread()
+			break
+		}
+	}
+	return NUM, d
+}