Working with two character tokens

lexer/lexer.go

@@ -9,7 +9,14 @@ func (l *Lexer) NextToken() token.Token {
 
 	switch l.ch {
 	case '=':
-		tok = newToken(token.ASSIGN, l.ch)
+		if l.peekChar() == '=' {
+			ch := l.ch
+			l.readChar()
+			literal := string(ch) + string(l.ch)
+			tok = token.Token{Type: token.EQ, Literal: literal}
+		} else {
+			tok = newToken(token.ASSIGN, l.ch)
+		}
 	case ';':
 		tok = newToken(token.SEMICOLON, l.ch)
 	case '(':
@@ -35,7 +42,14 @@ func (l *Lexer) NextToken() token.Token {
 	case '}':
 		tok = newToken(token.RBRACE, l.ch)
 	case '!':
-		tok = newToken(token.BANG, l.ch)
+		if l.peekChar() == '=' {
+			ch := l.ch
+			l.readChar()
+			literal := string(ch) + string(l.ch)
+			tok = token.Token{Type: token.NOT_EQ, Literal: literal}
+		} else {
+			tok = newToken(token.BANG, l.ch)
+		}
 	case 0:
 		tok.Literal = ""
 		tok.Type = token.EOF
@@ -113,3 +127,11 @@ func (l *Lexer) readNumber() string {
 func isDigit(ch byte) bool {
 	return '0' <= ch && ch <= '9'
 }
+
+func (l *Lexer) peekChar() byte {
+	if l.readPosition >= len(l.input) {
+		return 0
+	} else {
+		return l.input[l.readPosition]
+	}
+}

lexer/lexer_test.go

@@ -22,7 +22,11 @@ if (5 > 10) {
 	return false;
 } else {
 	return true;
-}`
+}
+
+10 == 10;
+10 != 5;
+`
 
 	tests := []struct {
 		expectedType    token.TokenType
@@ -93,6 +97,14 @@ if (5 > 10) {
 		{token.TRUE, "true"},
 		{token.SEMICOLON, ";"},
 		{token.RBRACE, "}"},
+		{token.INT, "10"},
+		{token.EQ, "=="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+		{token.INT, "10"},
+		{token.NOT_EQ, "!="},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
 		{token.EOF, ""},
 	}
 

token/token.go

@@ -60,4 +60,6 @@ const (
 	IF       = "IF"
 	ELSE     = "ELSE"
 	RETURN   = "RETURN"
+	EQ       = "=="
+	NOT_EQ   = "!="
 )