package parser

// Lexer tokenizes input source code
type Lexer struct {
	input        string
	position     int
	readPosition int
	ch           byte
	line         int
	column       int
}

// NewLexer creates a new lexer instance
func NewLexer(input string) *Lexer {
	l := &Lexer{
		input:  input,
		line:   1,
		column: 0,
	}
	l.readChar()
	return l
}

// readChar reads the next character and advances position
func (l *Lexer) readChar() {
	if l.readPosition >= len(l.input) {
		l.ch = 0
	} else {
		l.ch = l.input[l.readPosition]
	}
	l.position = l.readPosition
	l.readPosition++

	if l.ch == '\n' {
		l.line++
		l.column = 0
	} else {
		l.column++
	}
}

// peekChar returns the next character without advancing position
func (l *Lexer) peekChar() byte {
	if l.readPosition >= len(l.input) {
		return 0
	}
	return l.input[l.readPosition]
}

// skipWhitespace skips whitespace characters
func (l *Lexer) skipWhitespace() {
	for l.ch == ' ' || l.ch == '\t' || l.ch == '\n' || l.ch == '\r' {
		l.readChar()
	}
}

// skipComment skips both line and block comments
func (l *Lexer) skipComment() {
	if l.ch == '/' && l.peekChar() == '/' {
		// Line comment
		for l.ch != '\n' && l.ch != 0 {
			l.readChar()
		}
	} else if l.ch == '/' && l.peekChar() == '*' {
		// Block comment
		l.readChar() // skip '/'
		l.readChar() // skip '*'

		for {
			if l.ch == 0 {
				break
			}
			if l.ch == '*' && l.peekChar() == '/' {
				l.readChar() // skip '*'
				l.readChar() // skip '/'
				break
			}
			l.readChar()
		}
	}
}

// readIdentifier reads an identifier
func (l *Lexer) readIdentifier() string {
	position := l.position
	for isLetter(l.ch) || isDigit(l.ch) {
		l.readChar()
	}
	return l.input[position:l.position]
}

// readNumber reads a number (including decimals)
func (l *Lexer) readNumber() string {
	position := l.position
	for isDigit(l.ch) {
		l.readChar()
	}

	// Handle decimal points
	if l.ch == '.' && isDigit(l.peekChar()) {
		l.readChar()
		for isDigit(l.ch) {
			l.readChar()
		}
	}

	return l.input[position:l.position]
}

// readString reads a string literal
func (l *Lexer) readString() string {
	position := l.position + 1
	for {
		l.readChar()
		if l.ch == '"' || l.ch == 0 {
			break
		}
	}
	return l.input[position:l.position]
}

// NextToken returns the next token from the input
func (l *Lexer) NextToken() Token {
	var tok Token

	l.skipWhitespace()

	// Handle comments
	if l.ch == '/' && (l.peekChar() == '/' || l.peekChar() == '*') {
		l.skipComment()
		l.skipWhitespace()
	}

	tok.Line = l.line
	tok.Column = l.column

	switch l.ch {
	case '=':
		tok = Token{Type: ASSIGN, Literal: string(l.ch), Line: l.line, Column: l.column}
	case '+':
		tok = Token{Type: PLUS, Literal: string(l.ch), Line: l.line, Column: l.column}
	case '-':
		tok = Token{Type: MINUS, Literal: string(l.ch), Line: l.line, Column: l.column}
	case '*':
		tok = Token{Type: STAR, Literal: string(l.ch), Line: l.line, Column: l.column}
	case '/':
		tok = Token{Type: SLASH, Literal: string(l.ch), Line: l.line, Column: l.column}
	case '(':
		tok = Token{Type: LPAREN, Literal: string(l.ch), Line: l.line, Column: l.column}
	case ')':
		tok = Token{Type: RPAREN, Literal: string(l.ch), Line: l.line, Column: l.column}
	case '"':
		tok.Type = STRING
		tok.Literal = l.readString()
	case 0:
		tok.Literal = ""
		tok.Type = EOF
	default:
		if isLetter(l.ch) {
			tok.Literal = l.readIdentifier()
			tok.Type = lookupIdent(tok.Literal)
			return tok
		} else if isDigit(l.ch) {
			tok.Type = NUMBER
			tok.Literal = l.readNumber()
			return tok
		} else {
			tok = Token{Type: ILLEGAL, Literal: string(l.ch), Line: l.line, Column: l.column}
		}
	}

	l.readChar()
	return tok
}

// Helper functions
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'
}