Fin/scanner.go

package fin

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

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")
)

type Scanner struct {
	reader    *bufio.Reader
	line      int
	col       int
	buffer    []byte
	bufferRef *[]byte
	token     Token
}

var scannerPool = sync.Pool{
	New: func() any {
		bufferRef := GetByteSlice()
		return &Scanner{
			line:      1,
			col:       0,
			bufferRef: bufferRef,
			buffer:    (*bufferRef)[:0],
		}
	},
}

func NewScanner(r io.Reader) *Scanner {
	s := scannerPool.Get().(*Scanner)
	s.reader = bufio.NewReaderSize(r, 1024)
	s.line = 1
	s.col = 0
	s.resetBuffer()
	s.token = Token{Type: TokenError}
	return s
}

func ReleaseScanner(s *Scanner) {
	if s != nil {
		s.reader = nil
		s.resetBuffer()
		scannerPool.Put(s)
	}
}

// Helper to reset buffer consistently
func (s *Scanner) resetBuffer() {
	s.buffer = (*s.bufferRef)[:0]
}

// Helper for creating error tokens
func (s *Scanner) errorToken(msg string, line, col int) (Token, error) {
	err := fmt.Errorf("line %d, column %d: %s", line, col, msg)
	return Token{Type: TokenError, Value: []byte(msg), Line: line, Column: col}, err
}

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
}

func (s *Scanner) PeekByte() (byte, error) {
	b, err := s.reader.Peek(1)
	if err != nil {
		return 0, err
	}
	return b[0], nil
}

func (s *Scanner) PeekBytes(n int) ([]byte, error) {
	return s.reader.Peek(n)
}

func (s *Scanner) UnreadByte() error {
	err := s.reader.UnreadByte()
	if err == nil && s.col > 0 {
		s.col--
	}
	return err
}

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

func (s *Scanner) SkipWhitespace() error {
	for {
		b, err := s.PeekByte()
		if err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}
		if b != ' ' && b != '\t' && b != '\n' && b != '\r' {
			return nil
		}
		_, err = s.ReadByte()
		if err != nil {
			return err
		}
	}
}

func (s *Scanner) UnreadToken(token Token) {
	s.token = token
}

func (s *Scanner) NextToken() (Token, error) {
	if s.token.Type != TokenError {
		token := s.token
		s.token = Token{Type: TokenError}
		return token, nil
	}

	if err := s.SkipWhitespace(); err != nil {
		if err == io.EOF {
			return Token{Type: TokenEOF, Line: s.line, Column: s.col}, nil
		}
		return s.errorToken(err.Error(), s.line, s.col)
	}

	b, err := s.PeekByte()
	if err != nil {
		if err == io.EOF {
			return Token{Type: TokenEOF, Line: s.line, Column: s.col}, nil
		}
		return s.errorToken(err.Error(), s.line, s.col)
	}

	startLine, startColumn := s.line, s.col

	switch {
	case b == '{':
		_, _ = s.ReadByte()
		return Token{Type: TokenOpenBrace, Line: startLine, Column: startColumn}, nil
	case b == '}':
		_, _ = s.ReadByte()
		return Token{Type: TokenCloseBrace, Line: startLine, Column: startColumn}, nil
	case b == '-':
		peekBytes, err := s.PeekBytes(2)
		if err == nil && len(peekBytes) == 2 && peekBytes[1] == '-' {
			if err := s.scanComment(); err != nil {
				return s.errorToken(err.Error(), startLine, startColumn)
			}
			return Token{Type: TokenComment, Line: startLine, Column: startColumn}, nil
		}
		if err == nil && len(peekBytes) == 2 && isDigit(peekBytes[1]) {
			return s.scanNumber(startLine, startColumn)
		}
		_, _ = s.ReadByte()
		return s.errorToken("unexpected '-'", startLine, startColumn)
	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()
		msg := fmt.Sprintf("unexpected character: %c", b)
		return s.errorToken(msg, startLine, startColumn)
	}
}

func (s *Scanner) scanComment() error {
	_, err := s.ReadByte() // consume first dash
	if err != nil {
		return err
	}
	b, err := s.ReadByte() // consume second dash
	if err != nil {
		return err
	}
	if b != '-' {
		return ErrInvalidComment
	}

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

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

func (s *Scanner) scanString(startLine, startColumn int) (Token, error) {
	s.resetBuffer()
	_, err := s.ReadByte() // consume opening quote
	if err != nil {
		return s.errorToken(err.Error(), startLine, startColumn)
	}

	for {
		b, err := s.ReadByte()
		if err != nil {
			return s.errorToken(ErrUnterminatedString.Error(), startLine, startColumn)
		}
		if b == '"' {
			break
		}
		if b == '\\' {
			escaped, err := s.ReadByte()
			if err != nil {
				return s.errorToken(ErrUnterminatedEscape.Error(), startLine, startColumn)
			}
			switch escaped {
			case '"', '\\':
				s.buffer = append(s.buffer, escaped)
			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)
		}
	}

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

func (s *Scanner) scanName(startLine, startColumn int) (Token, error) {
	s.resetBuffer()
	b, err := s.ReadByte()
	if err != nil {
		return s.errorToken(err.Error(), startLine, startColumn)
	}
	if !isLetter(b) {
		return s.errorToken(ErrNameStartWithLetter.Error(), startLine, startColumn)
	}
	s.buffer = append(s.buffer, b)

	for {
		b, err := s.PeekByte()
		if err == io.EOF {
			break
		}
		if err != nil {
			return s.errorToken(err.Error(), startLine, startColumn)
		}
		if !isLetter(b) && !isDigit(b) && b != '_' {
			break
		}
		s.buffer = append(s.buffer, b)
		_, _ = s.ReadByte()
	}

	tokenType := TokenName
	if bytesEqual(s.buffer, []byte("true")) || bytesEqual(s.buffer, []byte("false")) {
		tokenType = TokenBoolean
	}

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

func (s *Scanner) scanNumber(startLine, startColumn int) (Token, error) {
	s.resetBuffer()
	b, err := s.ReadByte()
	if err != nil {
		return s.errorToken(err.Error(), startLine, startColumn)
	}
	s.buffer = append(s.buffer, b)

	hasDot := false
	for {
		b, err := s.PeekByte()
		if err != nil {
			if err == io.EOF {
				break
			}
			return s.errorToken(err.Error(), startLine, startColumn)
		}
		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
}