From cebc052f322f18c06fdbb2148063ccb43aa5465d Mon Sep 17 00:00:00 2001
From: tijani <tijani@tijanilawal.com>
Date: Thu, 12 May 2022 22:04:29 +0000
Subject: [PATCH] - The lexer can now peek forward into the input stream. -
 Added the rest of the basic math operators. - Added > and <. - Test, Test,
 Test. - Added if, else, return, true, false. - Lexer can now differentiate
 between '=' and '==' and '!' and '!='.

git-svn-id: https://svn.tlawal.org/svn/monkey@2 f6afcba9-9ef1-4bdd-9b72-7484f5705bac
---
 lexer/lexer.go      | 42 +++++++++++++++++++++++++++++++----
 lexer/lexer_test.go | 54 +++++++++++++++++++++++++++++++++++++++++++++
 token/tokens.go     | 27 +++++++++++++++++++----
 3 files changed, 115 insertions(+), 8 deletions(-)

diff --git a/lexer/lexer.go b/lexer/lexer.go
index 7edab73..ecd1d3e 100644
--- a/lexer/lexer.go
+++ b/lexer/lexer.go
@@ -29,6 +29,14 @@ func (lexer *Lexer) read_char() {
 	lexer.read_position += 1
 }
 
+func (lexer *Lexer) peek_char() byte {
+	if lexer.read_position >= len(lexer.input) {
+		return 0
+	} else {
+		return lexer.input[lexer.read_position]
+	}
+}
+
 func (lexer *Lexer) read_identifier() string {
 	position := lexer.position
 	for is_letter(lexer.current_char) {
@@ -49,13 +57,13 @@ func (lexer *Lexer) skip_whitespace() {
 
 func (lexer *Lexer) read_number() string {
 	position := lexer.position
-	for is_digit(lexer.current_char){
+	for is_digit(lexer.current_char) {
 		lexer.read_char()
 	}
-	return lexer.input[position: lexer.position]
+	return lexer.input[position:lexer.position]
 }
 
-func is_digit(ch byte) bool{
+func is_digit(ch byte) bool {
 	return '0' <= ch && ch <= '9'
 }
 
@@ -65,7 +73,23 @@ func (lexer *Lexer) next_token() token.Token {
 
 	switch lexer.current_char {
 	case '=':
-		tok = new_token(token.ASSIGN, lexer.current_char)
+		if lexer.peek_char() == '=' {
+			ch := lexer.current_char
+			lexer.read_char()
+			literal := string(ch) + string(lexer.current_char)
+			tok = token.Token{Type: token.EQ, Literal: literal}
+		} else {
+			tok = new_token(token.ASSIGN, lexer.current_char)
+		}
+	case '!':
+		if lexer.peek_char() == '=' {
+			ch := lexer.current_char
+			lexer.read_char()
+			literal := string(ch) + string(lexer.current_char)
+			tok = token.Token{Type: token.NOT_EQ, Literal: literal}
+		} else {
+			tok = new_token(token.BANG, lexer.current_char)
+		}
 	case ';':
 		tok = new_token(token.SEMICOLON, lexer.current_char)
 	case '(':
@@ -80,6 +104,16 @@ func (lexer *Lexer) next_token() token.Token {
 		tok = new_token(token.COMMA, lexer.current_char)
 	case '+':
 		tok = new_token(token.PLUS, lexer.current_char)
+	case '-':
+		tok = new_token(token.MINUS, lexer.current_char)
+	case '/':
+		tok = new_token(token.SLASH, lexer.current_char)
+	case '*':
+		tok = new_token(token.ASTERISK, lexer.current_char)
+	case '<':
+		tok = new_token(token.LT, lexer.current_char)
+	case '>':
+		tok = new_token(token.GT, lexer.current_char)
 	case 0:
 		tok.Literal = ""
 		tok.Type = token.EOF
diff --git a/lexer/lexer_test.go b/lexer/lexer_test.go
index 910c888..7a41817 100644
--- a/lexer/lexer_test.go
+++ b/lexer/lexer_test.go
@@ -13,6 +13,18 @@ func TestNextToken(t *testing.T) {
 							x + y;
 						};
 						let result = add(five, ten);
+
+						!-/*5;
+						5 < 10 > 5;
+
+						if(5 < 10){
+							return true;
+						} else {
+							return false;
+						}
+
+						10 == 10;
+						10 != 9;
 	`
 	tests := []struct {
 		expectedType    token.TokenType
@@ -57,6 +69,48 @@ func TestNextToken(t *testing.T) {
 		{token.IDENT, "ten"},
 		{token.RPAREN, ")"},
 		{token.SEMICOLON, ";"},
+
+		{token.BANG, "!"},
+		{token.MINUS, "-"},
+		{token.SLASH, "/"},
+		{token.ASTERISK, "*"},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+
+		{token.INT, "5"},
+		{token.LT, "<"},
+		{token.INT, "10"},
+		{token.GT, ">"},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+
+		{token.IF, "if"},
+		{token.LPAREN, "("},
+		{token.INT, "5"},
+		{token.LT, "<"},
+		{token.INT, "10"},
+		{token.RPAREN, ")"},
+		{token.LBRACE, "{"},
+		{token.RETURN, "return"},
+		{token.TRUE, "true"},
+		{token.SEMICOLON, ";"},
+		{token.RBRACE, "}"},
+		{token.ELSE, "else"},
+		{token.LBRACE, "{"},
+		{token.RETURN, "return"},
+		{token.FALSE, "false"},
+		{token.SEMICOLON, ";"},
+		{token.RBRACE, "}"},
+
+		{token.INT, "10"},
+		{token.EQ, "=="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+		{token.INT, "10"},
+		{token.NOT_EQ, "!="},
+		{token.INT, "9"},
+		{token.SEMICOLON, ";"},
+
 		{token.EOF, ""},
 	}
 
diff --git a/token/tokens.go b/token/tokens.go
index e9c9921..cc9478e 100644
--- a/token/tokens.go
+++ b/token/tokens.go
@@ -16,8 +16,17 @@ const (
 	INT   = "INT"
 
 	// Operators
-	ASSIGN = "="
-	PLUS   = "+"
+	ASSIGN   = "="
+	PLUS     = "+"
+	MINUS    = "-"
+	BANG     = "!"
+	ASTERISK = "*"
+	SLASH    = "/"
+	EQ       = "=="
+	NOT_EQ   = "!="
+
+	LT = "<"
+	GT = ">"
 
 	// Delimiters
 	COMMA     = ","
@@ -30,11 +39,21 @@ const (
 	// Keywords
 	FUNCTION = "FUNCTION"
 	LET      = "LET"
+	IF       = "IF"
+	ELSE     = "ELSE"
+	TRUE     = "TRUE"
+	FALSE    = "FALSE"
+	RETURN   = "RETURN"
 )
 
 var keywords = map[string]TokenType{
-	"fn":  FUNCTION,
-	"let": LET,
+	"fn":     FUNCTION,
+	"let":    LET,
+	"if":     IF,
+	"else":   ELSE,
+	"true":   TRUE,
+	"false":  FALSE,
+	"return": RETURN,
 }
 
 func LookupIdentifier(ident string) TokenType {