package stringunpack

import (
	"errors"
	"strconv"
	"strings"
)

var ErrIncorrectString = errors.New("incorrect string")

func Unpack(s string) (string, error) {
	if s == "" {
		return "", nil
	}

	var sb strings.Builder
	sb.Grow(len(s))

	var curSymbol rune
	for _, r := range s {
		isNumber := false
		number := 0
		if n, err := strconv.Atoi(string(r)); err == nil {
			number = n
			isNumber = true
		}

		if curSymbol == rune(0) {
			switch {
			case isNumber:
				return "", ErrIncorrectString
			case !isNumber:
				curSymbol = r
			}
		} else {
			switch {
			case !isNumber:
				sb.WriteRune(curSymbol)
				curSymbol = r
			case isNumber:
				for i := 0; i < number; i++ {
					sb.WriteRune(curSymbol)
				}
				curSymbol = rune(0)
			}
		}
	}

	if curSymbol != rune(0) {
		sb.WriteRune(curSymbol)
	}

	return sb.String(), nil
}