1 files changed, 207 insertions, 0 deletions
diff --git a/exprparser.go b/exprparser.go
new file mode 100644
index 0000000..0d62dbc
--- /dev/null
+++ b/exprparser.go
@@ -0,0 +1,207 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+)
+
+type Token int
+
+const eof = rune(0)
+
+const (
+	ILLEGAL Token = iota
+	EOF
+	WS
+
+	IDENT
+
+	ASTERISK
+	COMMA
+	SEMICOLON
+	GT
+	LT
+	GET
+	LET
+	EQ
+	PLUS
+	MINUS
+	DIVISION
+	R_PAREN
+	L_PAREN
+	PERIOD
+)
+
+type Scanner struct {
+	r *bufio.Reader
+}
+
+func newScanner(r io.Reader) *Scanner {
+	return &Scanner{r: bufio.NewReader(r)}
+}
+
+func (s *Scanner) read() rune {
+	ch, _, err := s.r.ReadRune()
+	if err != nil {
+		return eof
+	}
+
+	return ch
+}
+
+func isWhiteSpace(ch rune) bool {
+	if ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r' {
+		return true
+	}
+	return false
+}
+
+func isLetter(ch rune) bool {
+	if (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') {
+		return true
+	}
+	return false
+}
+
+func isDigit(ch rune) bool {
+	if ch >= '0' && ch <= '9' {
+		return true
+	}
+	return false
+}
+
+func isNumber(chs []rune) bool {
+	var dotCounter int
+	for i := range chs {
+		if chs[i] == '.' {
+			dotCounter++
+			if dotCounter > 1 {
+				return false
+			}
+			continue
+		}
+
+		if !isDigit(chs[i]) {
+			return false
+		}
+	}
+
+	return true
+}
+
+func (s *Scanner) unread() { _ = s.r.UnreadRune() }
+
+func (s *Scanner) scanWhitespace() (tok Token, lit string) {
+	var buf bytes.Buffer
+	buf.WriteRune(s.read())
+
+	for {
+		if ch := s.read(); ch == eof {
+			break
+		} else if !isWhiteSpace(ch) {
+			s.unread()
+			break
+		} else {
+			buf.WriteRune(ch)
+		}
+	}
+
+	return WS, buf.String()
+}
+
+func (s *Scanner) scanIdent() (tok Token, lit string) {
+	var buf bytes.Buffer
+	buf.WriteRune(s.read())
+
+	for {
+		if ch := s.read(); ch == eof {
+			break
+		} else if !isLetter(ch) && !isDigit(ch) && ch != '_' {
+			s.unread()
+			break
+		} else {
+			_, _ = buf.WriteRune(ch)
+		}
+	}
+
+	return IDENT, buf.String()
+}
+
+func (s *Scanner) scan() (tok Token, lit string) {
+	ch := s.read()
+
+	if isWhiteSpace(ch) {
+		s.unread()
+		return s.scanWhitespace()
+	} else if isLetter(ch) {
+		s.unread()
+		return s.scanIdent()
+	}
+
+	switch ch {
+	case eof:
+		return EOF, string(ch)
+	case '*':
+		return ASTERISK, string(ch)
+	case '+':
+		return PLUS, string(ch)
+	case '-':
+		return MINUS, string(ch)
+	case '/':
+		return ASTERISK, string(ch)
+	case '(':
+		return R_PAREN, string(ch)
+	case ')':
+		return L_PAREN, string(ch)
+	case ',':
+		return ASTERISK, string(ch)
+	case ';':
+		return ASTERISK, string(ch)
+	case '<':
+		return LT, string(ch)
+	case '>':
+		return GT, string(ch)
+	case '.':
+		return PERIOD, string(ch)
+	}
+
+	return ILLEGAL, string(ch)
+}
+
+type Parser struct {
+	s   *Scanner
+	buf struct {
+		tok Token
+		lit string
+		n   int
+	}
+}
+
+func newParser(r io.Reader) *Parser {
+	return &Parser{s: newScanner(r)}
+}
+
+func (p *Parser) scan() (tok Token, lit string) {
+	if p.buf.n != 0 {
+		p.buf.n = 0
+		return p.buf.tok, p.buf.lit
+	}
+
+	tok, lit = p.s.scan()
+	p.buf.tok, p.buf.lit = tok, lit
+
+	return
+}
+
+func (p *Parser) unscan() { p.buf.n = 1 }
+
+type ASTNode struct {
+	isIdentifier bool
+	isLiteral    bool
+	isIllegal    bool
+	isEOF        bool
+	isWS         bool
+	children     []*ASTNode
+	value        interface{}
+}