Fin/scanner.go

package config

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"strconv"
	"strings"
	"unicode"
)

// 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 []rune
}

// 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([]rune, 0, 64),
	}
}

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

// PeekRune looks at the next rune without consuming it
func (s *Scanner) PeekRune() (rune, error) {
	r, _, err := s.reader.ReadRune()
	if err != nil {
		return 0, err
	}
	s.reader.UnreadRune()
	return r, nil
}

// 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 {
		r, err := s.PeekRune()
		if err == io.EOF {
			return nil
		}
		if err != nil {
			return err
		}
		if !unicode.IsSpace(r) {
			return nil
		}
		_, _, err = s.ReadRune()
		if err != nil {
			return err
		}
	}
}

// peekAndCheckRune checks if the next rune matches expected without consuming it
func (s *Scanner) peekAndCheckRune(expected rune) (bool, error) {
	r, err := s.PeekRune()
	if err != nil {
		return false, err
	}
	return r == expected, nil
}

// consumeIfMatch consumes the next rune if it matches expected
func (s *Scanner) consumeIfMatch(expected rune) (bool, error) {
	matches, err := s.peekAndCheckRune(expected)
	if err != nil || !matches {
		return false, err
	}

	_, _, err = s.ReadRune() // consume the rune
	return err == nil, err
}

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

	// Check for second dash
	r, _, err := s.ReadRune()
	if err != nil {
		return err
	}
	if r != '-' {
		return s.Error("invalid comment")
	}

	// Check for block comment [[
	r, err = s.PeekRune()
	if err == nil && r == '[' {
		_, _, _ = s.ReadRune() // consume first [
		r, err = s.PeekRune()
		if err == nil && r == '[' {
			_, _, _ = s.ReadRune() // consume second [
			return s.scanBlockComment()
		}
	}

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

// scanBlockComment processes a block comment
func (s *Scanner) scanBlockComment() error {
	for {
		r, _, err := s.ReadRune()
		if err != nil {
			return s.Error("unclosed block comment")
		}

		if r == ']' {
			r, err = s.PeekRune()
			if err == nil && r == ']' {
				_, _, _ = s.ReadRune() // consume second ]
				return nil
			}
		}
	}
}

// ScanName reads a name identifier
func (s *Scanner) ScanName() (string, error) {
	s.buffer = s.buffer[:0] // Reset buffer

	// Read first character
	r, _, err := s.ReadRune()
	if err != nil {
		return "", err
	}

	if !unicode.IsLetter(r) {
		return "", s.Error("name must start with letter")
	}
	s.buffer = append(s.buffer, r)

	// Read rest of name
	for {
		r, err := s.PeekRune()
		if err == io.EOF {
			break
		}
		if err != nil {
			return "", err
		}
		if !unicode.IsLetter(r) && !unicode.IsDigit(r) && r != '_' {
			break
		}
		s.buffer = append(s.buffer, r)
		_, _, _ = s.ReadRune()
	}

	return string(s.buffer), nil
}

// processArrayElement processes a single array element
func (s *Scanner) processArrayElement() (any, error) {
	r, err := s.PeekRune()
	if err != nil {
		return nil, err
	}

	// Handle identifier-like elements
	if unicode.IsLetter(r) {
		name, err := s.ScanName()
		if err != nil {
			return nil, err
		}

		// Try to convert to appropriate type
		convertedValue, err := s.ConvertValue(name)
		if err == nil {
			return convertedValue, nil
		}
		return name, nil
	}

	// Handle other element types (strings, numbers, etc.)
	return s.ScanValue()
}

// processMapEntry processes a key-value pair in a map
func (s *Scanner) processMapEntry() (string, any, bool, error) {
	name, err := s.ScanName()
	if err != nil {
		return "", nil, false, err
	}

	err = s.SkipWhitespace()
	if err != nil {
		return "", nil, false, err
	}

	// Check for equals sign
	isEquals, err := s.consumeIfMatch('=')
	if err != nil && err != io.EOF {
		return "", nil, false, err
	}

	if isEquals {
		value, err := s.ScanValue()
		if err != nil {
			return "", nil, false, err
		}
		return name, value, true, nil // true indicates this is a map entry
	}

	// Check for opening brace (nested map/array)
	isBrace, err := s.peekAndCheckRune('{')
	if err != nil && err != io.EOF {
		return "", nil, false, err
	}

	if isBrace {
		value, err := s.ScanValue()
		if err != nil {
			return "", nil, false, err
		}
		return name, value, true, nil // true indicates this is a map entry
	}

	// If neither equals nor brace, it's an array element (name as string)
	return name, name, false, nil // false indicates this is not a map entry
}

// ScanValue processes and returns a value from the config
func (s *Scanner) ScanValue() (any, error) {
	err := s.SkipWhitespace()
	if err != nil {
		return nil, err
	}

	r, err := s.PeekRune()
	if err != nil {
		return nil, err
	}

	// Check if it's an array/map
	if r == '{' {
		return s.ScanArrayOrMap()
	}

	// Check if it's a quoted string
	if r == '"' {
		return s.ScanString()
	}

	// Otherwise, treat it as a simple value
	var value []rune
	for {
		r, err := s.PeekRune()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, err
		}
		if unicode.IsSpace(r) || r == '}' {
			break
		}
		value = append(value, r)
		_, _, _ = s.ReadRune()
	}

	if len(value) == 0 {
		return nil, s.Error("empty value")
	}

	// Convert value to appropriate type
	return s.ConvertValue(string(value))
}

// ScanArrayOrMap processes arrays and maps
func (s *Scanner) ScanArrayOrMap() (any, error) {
	// Consume opening brace
	_, _, err := s.ReadRune()
	if err != nil {
		return nil, err
	}

	// Initialize a new map to store contents
	contents := make(map[string]any)
	// And a slice to track array elements
	var arrayElements []any
	isArray := true

	for {
		err := s.SkipWhitespace()
		if err != nil {
			return nil, err
		}

		r, err := s.PeekRune()
		if err == io.EOF {
			return nil, s.Error("unclosed array/map")
		}
		if err != nil {
			return nil, err
		}

		// Check for closing brace
		if r == '}' {
			_, _, _ = s.ReadRune() // consume the closing brace
			break
		}

		// Handle comments inside arrays/maps
		if r == '-' {
			err = s.ScanComment()
			if err != nil {
				return nil, err
			}
			continue
		}

		// If we find a named property, it might be a map entry
		if unicode.IsLetter(r) {
			name, value, isMapEntry, err := s.processMapEntry()
			if err != nil {
				return nil, err
			}

			if isMapEntry {
				// It's a key-value pair for a map
				isArray = false
				contents[name] = value
			} else {
				// It's an array element
				arrayElements = append(arrayElements, value)
			}
			continue
		}

		// Handle array elements that start with quotes, numbers, etc.
		value, err := s.processArrayElement()
		if err != nil {
			return nil, err
		}
		arrayElements = append(arrayElements, value)
	}

	// Check for array/map distinction and return appropriate result
	if isArray && len(contents) == 0 {
		return arrayElements, nil
	}
	return contents, nil
}

// ScanString reads a quoted string
func (s *Scanner) ScanString() (any, error) {
	// Consume opening quote
	_, _, err := s.ReadRune()
	if err != nil {
		return nil, err
	}

	var builder strings.Builder
	builder.Grow(64) // Preallocate with reasonable capacity

	for {
		r, _, err := s.ReadRune()
		if err != nil {
			return nil, s.Error("unterminated string")
		}

		if r == '"' {
			break
		}

		// Handle escape sequences
		if r == '\\' {
			escaped, _, err := s.ReadRune()
			if err != nil {
				return nil, err
			}
			switch escaped {
			case '"':
				builder.WriteRune('"')
			case '\\':
				builder.WriteRune('\\')
			case 'n':
				builder.WriteRune('\n')
			case 't':
				builder.WriteRune('\t')
			default:
				builder.WriteRune('\\')
				builder.WriteRune(escaped)
			}
		} else {
			builder.WriteRune(r)
		}
	}

	return builder.String(), nil
}

// ConvertValue converts string values to their appropriate types
func (s *Scanner) ConvertValue(value string) (any, error) {
	// Fast path for booleans
	if value == "true" {
		return true, nil
	}
	if value == "false" {
		return false, nil
	}

	// Early exit for empty values
	if len(value) == 0 {
		return nil, errors.New("empty value")
	}

	// Check for number type in one pass
	isNegative := value[0] == '-'
	startIdx := 0
	if isNegative {
		if len(value) == 1 {
			return nil, errors.New("invalid value: -")
		}
		startIdx = 1
	}

	hasDot := false
	for i := startIdx; i < len(value); i++ {
		if value[i] == '.' {
			if hasDot {
				return nil, errors.New("invalid number format")
			}
			hasDot = true
		} else if value[i] < '0' || value[i] > '9' {
			return nil, errors.New("invalid value format: " + value)
		}
	}

	// Process as integer or float based on presence of decimal
	if !hasDot {
		return strconv.ParseInt(value, 10, 64)
	}

	// Float (ensure not ending with dot)
	if value[len(value)-1] != '.' {
		return strconv.ParseFloat(value, 64)
	}

	return nil, errors.New("invalid value format: " + value)
}