package lexer

import "monkey/token"

type Lexer struct {
	input         string
	position      int // current position in input (the current_char)
	read_position int // current reading position in input (after current_char)
	current_char  byte
}

func New(input string) *Lexer {
	l := &Lexer{input: input}
	l.read_char()
	return l
}

func (l_lexer *Lexer) NextToken() token.Token {
	var tok token.Token
	l_lexer.skip_whitespace()

	switch l_lexer.current_char {
	case '=':
		if l_lexer.peek_char() == '=' {
			ch := l_lexer.current_char
			l_lexer.read_char()
			literal := string(ch) + string(l_lexer.current_char)
			tok = token.Token{Type: token.EQ, Literal: literal}
		} else {
			tok = new_token(token.ASSIGN, l_lexer.current_char)
		}
	case '!':
		if l_lexer.peek_char() == '=' {
			ch := l_lexer.current_char
			l_lexer.read_char()
			literal := string(ch) + string(l_lexer.current_char)
			tok = token.Token{Type: token.NOT_EQ, Literal: literal}
		} else {
			tok = new_token(token.BANG, l_lexer.current_char)
		}
	case ';':
		tok = new_token(token.SEMICOLON, l_lexer.current_char)
	case '(':
		tok = new_token(token.LPAREN, l_lexer.current_char)
	case ')':
		tok = new_token(token.RPAREN, l_lexer.current_char)
	case '{':
		tok = new_token(token.LBRACE, l_lexer.current_char)
	case '}':
		tok = new_token(token.RBRACE, l_lexer.current_char)
	case ',':
		tok = new_token(token.COMMA, l_lexer.current_char)
	case '+':
		tok = new_token(token.PLUS, l_lexer.current_char)
	case '-':
		tok = new_token(token.MINUS, l_lexer.current_char)
	case '/':
		tok = new_token(token.SLASH, l_lexer.current_char)
	case '*':
		tok = new_token(token.ASTERISK, l_lexer.current_char)
	case '<':
		tok = new_token(token.LT, l_lexer.current_char)
	case '>':
		tok = new_token(token.GT, l_lexer.current_char)
	case '"':
		tok.Type = token.STRING
		tok.Literal = l_lexer.read_string()
	case '[':
		tok = new_token(token.LBRACKET, l_lexer.current_char)
	case ']':
		tok = new_token(token.RBRACKET, l_lexer.current_char)
	case 0:
		tok.Literal = ""
		tok.Type = token.EOF

	default:
		if is_letter(l_lexer.current_char) {
			tok.Literal = l_lexer.read_identifier()
			tok.Type = token.LookupIdentifier(tok.Literal)
			return tok
		} else if is_digit(l_lexer.current_char) {
			tok.Type = token.INT
			tok.Literal = l_lexer.read_number()
			return tok
		} else {
			tok = new_token(token.ILLEGAL, l_lexer.current_char)
		}
	}
	l_lexer.read_char()
	return tok
}

func new_token(TokenType token.TokenType, ch byte) token.Token {
	return token.Token{Type: TokenType, Literal: string(ch)}
}

func (l_lexer *Lexer) read_char() {
	if l_lexer.read_position >= len(l_lexer.input) {
		l_lexer.current_char = 0
	} else {
		l_lexer.current_char = l_lexer.input[l_lexer.read_position]
	}
	l_lexer.position = l_lexer.read_position
	l_lexer.read_position += 1
}

func (l_lexer *Lexer) peek_char() byte {
	if l_lexer.read_position >= len(l_lexer.input) {
		return 0
	} else {
		return l_lexer.input[l_lexer.read_position]
	}
}

func (l_lexer *Lexer) read_identifier() string {
	position := l_lexer.position
	for is_letter(l_lexer.current_char) {
		l_lexer.read_char()
	}
	return l_lexer.input[position:l_lexer.position]
}

func is_letter(ch byte) bool {
	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_'
}

func (l_lexer *Lexer) skip_whitespace() {
	for l_lexer.current_char == ' ' || l_lexer.current_char == '\t' || l_lexer.current_char == '\n' || l_lexer.current_char == '\r' {
		l_lexer.read_char()
	}
}

func (l_lexer *Lexer) read_number() string {
	position := l_lexer.position
	for is_digit(l_lexer.current_char) {
		l_lexer.read_char()
	}
	return l_lexer.input[position:l_lexer.position]
}

func is_digit(ch byte) bool {
	return '0' <= ch && ch <= '9'
}

/*
Read the current character until it encounters a closing '"' or end of input.
TODO: some additional thing that can be done at the lexer level with strings is to report an error when it
reaches the end of input without proper termination. Support for character escaping would be really neat.
*/
func (l_lexer *Lexer) read_string() string {
	position := l_lexer.position + 1
	for {
		l_lexer.read_char()
		if l_lexer.current_char == '"' || l_lexer.current_char == 0 {
			break
		}
	}
	return l_lexer.input[position:l_lexer.position]
}