2 files changed, 277 insertions, 0 deletions
diff --git a/users/fcuny/exp/monkey/pkg/lexer/lexer.go b/users/fcuny/exp/monkey/pkg/lexer/lexer.go
new file mode 100644
index 0000000..3e98cf0
--- /dev/null
+++ b/users/fcuny/exp/monkey/pkg/lexer/lexer.go
@@ -0,0 +1,152 @@
+// Package lexer provides a lexer to the monkey language.
+package lexer
+
+import "monkey/pkg/token"
+
+// Lexer represents the lexer
+type Lexer struct {
+	input string
+	// current position in input
+	position int
+	// current reading position in input (after a char)
+	readPosition int
+	// current character under examination
+	ch byte
+}
+
+// New returns a new lexer
+func New(input string) *Lexer {
+	l := &Lexer{input: input}
+	l.readChar()
+	return l
+}
+
+// Read the current character and advances our position in the input string.
+func (l *Lexer) readChar() {
+	// if we've reached the end of the input, we set the current character to 0,
+	// which is the ASCII code for NUL.
+	if l.readPosition >= len(l.input) {
+		l.ch = 0
+	} else {
+		l.ch = l.input[l.readPosition]
+	}
+	l.position = l.readPosition
+	l.readPosition++
+}
+
+func (l *Lexer) readIdentifier() string {
+	position := l.position
+	for isLetter(l.ch) {
+		l.readChar()
+	}
+	return l.input[position:l.position]
+}
+
+func (l *Lexer) readNumber() string {
+	position := l.position
+	for isDigit(l.ch) {
+		l.readChar()
+	}
+	return l.input[position:l.position]
+}
+
+// we don't care about white space characters, we skip them when we find them.
+func (l *Lexer) skipWhitespace() {
+	for l.ch == ' ' || l.ch == '\t' || l.ch == '\n' || l.ch == '\r' {
+		l.readChar()
+	}
+}
+
+// peekChar returns the character at position (which is the next charatecter),
+// but does not increment `readPosition` and `position`.
+// This is needed to read tokens that are composed of two characters (e.g. `==`).
+func (l *Lexer) peekChar() byte {
+	if l.readPosition >= len(l.input) {
+		return 0
+	}
+	return l.input[l.readPosition]
+}
+
+// NextToken reads the next token from the lexer and returns the current token.
+func (l *Lexer) NextToken() token.Token {
+	var tok token.Token
+
+	l.skipWhitespace()
+
+	switch l.ch {
+	case '=':
+		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.PLUS, l.ch)
+	case '-':
+		tok = newToken(token.MINUS, l.ch)
+	case '!':
+		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 '*':
+		tok = newToken(token.ASTERISK, l.ch)
+	case '/':
+		tok = newToken(token.SLASH, l.ch)
+	case '<':
+		tok = newToken(token.LT, l.ch)
+	case '>':
+		tok = newToken(token.GT, l.ch)
+
+	case ';':
+		tok = newToken(token.SEMICOLON, l.ch)
+	case ',':
+		tok = newToken(token.COMMA, l.ch)
+	case '(':
+		tok = newToken(token.LPAREN, l.ch)
+	case ')':
+		tok = newToken(token.RPAREN, l.ch)
+	case '{':
+		tok = newToken(token.LBRACE, l.ch)
+	case '}':
+		tok = newToken(token.RBRACE, l.ch)
+	case 0:
+		tok.Literal = ""
+		tok.Type = token.EOF
+	default:
+		if isLetter(l.ch) {
+			tok.Literal = l.readIdentifier()
+			tok.Type = token.LookupIdent(tok.Literal)
+			return tok
+		} else if isDigit(l.ch) {
+			tok.Type = token.INT
+			tok.Literal = l.readNumber()
+			return tok
+		} else {
+			tok = newToken(token.ILLEGAL, l.ch)
+		}
+
+	}
+
+	l.readChar()
+	return tok
+}
+
+func newToken(tokenType token.TokenType, ch byte) token.Token {
+	return token.Token{Type: tokenType, Literal: string(ch)}
+}
+
+func isLetter(ch byte) bool {
+	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_'
+}
+
+func isDigit(ch byte) bool {
+	return '0' <= ch && ch <= '9'
+}
diff --git a/users/fcuny/exp/monkey/pkg/lexer/lexer_test.go b/users/fcuny/exp/monkey/pkg/lexer/lexer_test.go
new file mode 100644
index 0000000..fdea1d3
--- /dev/null
+++ b/users/fcuny/exp/monkey/pkg/lexer/lexer_test.go
@@ -0,0 +1,125 @@
+package lexer
+
+import (
+	"monkey/pkg/token"
+	"testing"
+)
+
+func TestNextToken(t *testing.T) {
+	input := `let five = 5;
+let ten = 10;
+
+let add = fn(x, y) {
+  x + y
+};
+
+let result = add(five, ten);
+!-/*5;
+10 > 5;
+
+if (5 < 10) {
+  return true;
+} else {
+  return false;
+}
+
+10 == 10;
+10 != 9;
+`
+
+	tests := []struct {
+		expectedType    token.TokenType
+		expectedLiteral string
+	}{
+		{token.LET, "let"},
+		{token.IDENT, "five"},
+		{token.ASSIGN, "="},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+
+		{token.LET, "let"},
+		{token.IDENT, "ten"},
+		{token.ASSIGN, "="},
+		{token.INT, "10"},
+		{token.SEMICOLON, ";"},
+
+		{token.LET, "let"},
+		{token.IDENT, "add"},
+		{token.ASSIGN, "="},
+		{token.FUNCTION, "fn"},
+		{token.LPAREN, "("},
+		{token.IDENT, "x"},
+		{token.COMMA, ","},
+		{token.IDENT, "y"},
+		{token.RPAREN, ")"},
+		{token.LBRACE, "{"},
+		{token.IDENT, "x"},
+		{token.PLUS, "+"},
+		{token.IDENT, "y"},
+		{token.RBRACE, "}"},
+		{token.SEMICOLON, ";"},
+
+		{token.LET, "let"},
+		{token.IDENT, "result"},
+		{token.ASSIGN, "="},
+		{token.IDENT, "add"},
+		{token.LPAREN, "("},
+		{token.IDENT, "five"},
+		{token.COMMA, ","},
+		{token.IDENT, "ten"},
+		{token.RPAREN, ")"},
+		{token.SEMICOLON, ";"},
+
+		{token.BANG, "!"},
+		{token.MINUS, "-"},
+		{token.SLASH, "/"},
+		{token.ASTERISK, "*"},
+		{token.INT, "5"},
+		{token.SEMICOLON, ";"},
+
+		{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, ";"},
+	}
+
+	l := New(input)
+	for i, tt := range tests {
+		tok := l.NextToken()
+		if tok.Type != tt.expectedType {
+			t.Fatalf("tests[%d] - tokentype wrong. expected=%q, got=%q", i, tt.expectedType, tok.Type)
+		}
+
+		if tok.Literal != tt.expectedLiteral {
+			t.Fatalf("tests[%d] - tokenliteral wrong. expected=%q, got=%q", i, tt.expectedLiteral, tok.Literal)
+		}
+	}
+}