Fin/scanner.go

package config

import (
	"bufio"
	"errors"
	"fmt"
	"io"
)

// Pre-declared errors to reduce allocations
var (
	ErrUnterminatedString  = errors.New("unterminated string")
	ErrUnterminatedEscape  = errors.New("unterminated escape sequence")
	ErrUnterminatedComment = errors.New("unclosed block comment")
	ErrInvalidComment      = errors.New("invalid comment")
	ErrNameStartWithLetter = errors.New("name must start with letter")
)

// Scanner handles the low-level parsing of the configuration format
type Scanner struct {
	reader *bufio.Reader
	line   int // Current line number
	col    int // Current column position
	buffer []byte
	token  Token // Current token
}

// NewScanner creates a new scanner with the given reader
func NewScanner(r io.Reader) *Scanner {
	return &Scanner{
		reader: bufio.NewReader(r),
		line:   1, // Start at line 1
		col:    0,
		buffer: make([]byte, 0, 128), // Pre-allocate with reasonable capacity
	}
}

// ReadByte reads a single byte from the input
func (s *Scanner) ReadByte() (byte, error) {
	b, err := s.reader.ReadByte()
	if err == nil {
		if b == '\n' {
			s.line++
			s.col = 0
		} else {
			s.col++
		}
	}
	return b, err
}

// PeekByte looks at the next byte without consuming it
func (s *Scanner) PeekByte() (byte, error) {
	b, err := s.reader.Peek(1)
	if err != nil {
		return 0, err
	}
	return b[0], nil
}

// PeekBytes looks at the next n bytes without consuming them
func (s *Scanner) PeekBytes(n int) ([]byte, error) {
	return s.reader.Peek(n)
}

// UnreadByte pushes back a byte to the reader
func (s *Scanner) UnreadByte() error {
	err := s.reader.UnreadByte()
	if err == nil && s.col > 0 {
		s.col--
	}
	return err
}

// Error creates an error with line and column information
func (s *Scanner) Error(msg string) error {
	return fmt.Errorf("line %d, column %d: %s", s.line, s.col, msg)
}

// SkipWhitespace skips whitespace characters
func (s *Scanner) SkipWhitespace() error {
	for {
		b, err := s.PeekByte()
		if err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}

		// Fast check for common whitespace bytes
		if b != ' ' && b != '\t' && b != '\n' && b != '\r' {
			return nil
		}

		_, err = s.ReadByte()
		if err != nil {
			return err
		}
	}
}

// UnreadToken stores a token to be returned by the next call to NextToken
func (s *Scanner) UnreadToken(token Token) {
	s.token = token
}

// NextToken scans and returns the next token
func (s *Scanner) NextToken() (Token, error) {
	if s.token.Type != TokenError {
		// We have a stored token
		token := s.token
		s.token = Token{Type: TokenError} // Reset
		return token, nil
	}

	// Skip whitespace
	err := s.SkipWhitespace()
	if err == io.EOF {
		return Token{Type: TokenEOF}, nil
	}
	if err != nil {
		return Token{Type: TokenError, Value: []byte(err.Error())}, err
	}

	b, err := s.PeekByte()
	if err != nil {
		if err == io.EOF {
			return Token{Type: TokenEOF}, nil
		}
		return Token{Type: TokenError, Value: []byte(err.Error())}, err
	}

	// Record start position for error reporting
	startLine, startColumn := s.line, s.col

	// Process based on first character
	switch {
	case b == '=':
		_, _ = s.ReadByte() // consume equals
		return Token{Type: TokenEquals, Line: startLine, Column: startColumn}, nil

	case b == '{':
		_, _ = s.ReadByte() // consume open brace
		return Token{Type: TokenOpenBrace, Line: startLine, Column: startColumn}, nil

	case b == '}':
		_, _ = s.ReadByte() // consume close brace
		return Token{Type: TokenCloseBrace, Line: startLine, Column: startColumn}, nil

	case b == '-':
		// Could be a comment or a negative number
		peekBytes, err := s.PeekBytes(2)
		if err == nil && len(peekBytes) == 2 && peekBytes[1] == '-' {
			err = s.scanComment()
			if err != nil {
				return Token{Type: TokenError, Value: []byte(err.Error())}, err
			}
			return Token{Type: TokenComment, Line: startLine, Column: startColumn}, nil
		}

		// Check if it's a negative number
		if err == nil && len(peekBytes) == 2 && isDigit(peekBytes[1]) {
			return s.scanNumber(startLine, startColumn)
		}

		// Just a single dash
		_, _ = s.ReadByte() // consume dash
		return Token{Type: TokenError, Value: []byte("unexpected '-'")},
			s.Error("unexpected '-'")

	case b == '"':
		return s.scanString(startLine, startColumn)

	case isLetter(b):
		return s.scanName(startLine, startColumn)

	case isDigit(b):
		return s.scanNumber(startLine, startColumn)

	default:
		_, _ = s.ReadByte() // consume the unexpected character
		return Token{Type: TokenError, Value: []byte(fmt.Sprintf("unexpected character: %c", b)), Line: startLine, Column: startColumn},
			s.Error(fmt.Sprintf("unexpected character: %c", b))
	}
}

// scanComment processes a comment
func (s *Scanner) scanComment() error {
	// Consume the first dash
	_, err := s.ReadByte()
	if err != nil {
		return err
	}

	// Check for second dash
	b, err := s.ReadByte()
	if err != nil {
		return err
	}
	if b != '-' {
		return ErrInvalidComment
	}

	// Check for block comment [[
	if b1, err := s.PeekByte(); err == nil && b1 == '[' {
		_, _ = s.ReadByte() // consume first [
		if b2, err := s.PeekByte(); err == nil && b2 == '[' {
			_, _ = s.ReadByte() // consume second [
			// Process block comment
			for {
				b, err := s.ReadByte()
				if err != nil {
					return ErrUnterminatedComment
				}
				if b == ']' {
					if n, err := s.PeekByte(); err == nil && n == ']' {
						_, _ = s.ReadByte() // consume second ]
						return nil
					}
				}
			}
		}
	}

	// Line comment - consume until newline or EOF
	for {
		b, err := s.ReadByte()
		if err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}
		if b == '\n' {
			return nil
		}
	}
}

// scanString scans a quoted string
func (s *Scanner) scanString(startLine, startColumn int) (Token, error) {
	// Reset buffer
	s.buffer = s.buffer[:0]

	// Consume opening quote
	_, err := s.ReadByte()
	if err != nil {
		return Token{Type: TokenError, Value: []byte(err.Error())}, err
	}

	for {
		b, err := s.ReadByte()
		if err != nil {
			return Token{Type: TokenError, Value: []byte(ErrUnterminatedString.Error())}, ErrUnterminatedString
		}

		if b == '"' {
			break
		}

		// Handle escape sequences
		if b == '\\' {
			escaped, err := s.ReadByte()
			if err != nil {
				return Token{Type: TokenError, Value: []byte(ErrUnterminatedEscape.Error())}, ErrUnterminatedEscape
			}
			switch escaped {
			case '"':
				s.buffer = append(s.buffer, '"')
			case '\\':
				s.buffer = append(s.buffer, '\\')
			case 'n':
				s.buffer = append(s.buffer, '\n')
			case 't':
				s.buffer = append(s.buffer, '\t')
			default:
				s.buffer = append(s.buffer, '\\', escaped)
			}
		} else {
			s.buffer = append(s.buffer, b)
		}
	}

	// Use the buffer directly - consumer is responsible for copying if needed
	return Token{
		Type:   TokenString,
		Value:  s.buffer,
		Line:   startLine,
		Column: startColumn,
	}, nil
}

// scanName scans an identifier
func (s *Scanner) scanName(startLine, startColumn int) (Token, error) {
	// Reset buffer
	s.buffer = s.buffer[:0]

	// Read first character
	b, err := s.ReadByte()
	if err != nil {
		return Token{Type: TokenError, Value: []byte(err.Error())}, err
	}

	if !isLetter(b) {
		return Token{Type: TokenError, Value: []byte(ErrNameStartWithLetter.Error())}, ErrNameStartWithLetter
	}
	s.buffer = append(s.buffer, b)

	// Read rest of name
	for {
		b, err := s.PeekByte()
		if err == io.EOF {
			break
		}
		if err != nil {
			return Token{Type: TokenError, Value: []byte(err.Error())}, err
		}
		if !isLetter(b) && !isDigit(b) && b != '_' {
			break
		}
		s.buffer = append(s.buffer, b)
		_, _ = s.ReadByte()
	}

	// Check if it's a boolean - fixed comparison
	tokenType := TokenName
	if string(s.buffer) == "true" || string(s.buffer) == "false" {
		tokenType = TokenBoolean
	}

	return Token{
		Type:   tokenType,
		Value:  s.buffer,
		Line:   startLine,
		Column: startColumn,
	}, nil
}

// scanNumber scans a numeric value
func (s *Scanner) scanNumber(startLine, startColumn int) (Token, error) {
	// Reset buffer
	s.buffer = s.buffer[:0]

	// Read first character (might be a minus sign or digit)
	b, err := s.ReadByte()
	if err != nil {
		return Token{Type: TokenError, Value: []byte(err.Error())}, err
	}
	s.buffer = append(s.buffer, b)

	// Scan the rest of the number
	hasDot := false
	for {
		b, err := s.PeekByte()
		if err != nil {
			if err == io.EOF {
				break
			}
			return Token{Type: TokenError, Value: []byte(err.Error())}, err
		}

		if b == '.' && !hasDot {
			hasDot = true
			_, _ = s.ReadByte()
			s.buffer = append(s.buffer, b)
		} else if isDigit(b) {
			_, _ = s.ReadByte()
			s.buffer = append(s.buffer, b)
		} else {
			break
		}
	}

	return Token{
		Type:   TokenNumber,
		Value:  s.buffer,
		Line:   startLine,
		Column: startColumn,
	}, nil
}