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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
}
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