// Parsing keys handling both bare and quoted keys.

package toml

import (

	"errors"

	"fmt"

)

// Convert the bare key group string to an array.

// The input supports double quotation and single quotation,

// but escape sequences are not supported. Lexers must unescape them beforehand.

func parseKey(key string) ([]string, error) {

	runes := []rune(key)

	var groups []string

	if len(key) == 0 {

		return nil, errors.New("empty key")

	}

	idx := 0

	for idx < len(runes) {

		for ; idx < len(runes) && isSpace(runes[idx]); idx++ {

			// skip leading whitespace

		}

		if idx >= len(runes) {

			break

		}

		r := runes[idx]

		if isValidBareChar(r) {

			// parse bare key

			startIdx := idx

			endIdx := -1

			idx++

			for idx < len(runes) {

				r = runes[idx]

				if isValidBareChar(r) {

					idx++

				} else if r == '.' {

					endIdx = idx

					break

				} else if isSpace(r) {

					endIdx = idx

					for ; idx < len(runes) && isSpace(runes[idx]); idx++ {

						// skip trailing whitespace

					}

					if idx < len(runes) && runes[idx] != '.' {

						return nil, fmt.Errorf("invalid key character after whitespace: %c", runes[idx])

					}

					break

				} else {

					return nil, fmt.Errorf("invalid bare key character: %c", r)

				}

			}

			if endIdx == -1 {

				endIdx = idx

			}

			groups = append(groups, string(runes[startIdx:endIdx]))

		} else if r == '\'' {

			// parse single quoted key

			idx++

			startIdx := idx

			for {

				if idx >= len(runes) {

					return nil, fmt.Errorf("unclosed single-quoted key")

				}

				r = runes[idx]

				if r == '\'' {

					groups = append(groups, string(runes[startIdx:idx]))

					idx++

					break

				}

				idx++

			}

		} else if r == '"' {

			// parse double quoted key

			idx++

			startIdx := idx

			for {

				if idx >= len(runes) {

					return nil, fmt.Errorf("unclosed double-quoted key")

				}

				r = runes[idx]

				if r == '"' {

					groups = append(groups, string(runes[startIdx:idx]))

					idx++

					break

				}

				idx++

			}

		} else if r == '.' {

			idx++

			if idx >= len(runes) {

				return nil, fmt.Errorf("unexpected end of key")

			}

			r = runes[idx]

			if !isValidBareChar(r) && r != '\'' && r != '"' && r != ' ' {

				return nil, fmt.Errorf("expecting key part after dot")

			}

		} else {

			return nil, fmt.Errorf("invalid key character: %c", r)

		}

	}

	if len(groups) == 0 {

		return nil, fmt.Errorf("empty key")

	}

	return groups, nil

}

func isValidBareChar(r rune) bool {

}