using System;
using System.Threading.Channels;

namespace StringVerification
{
    public static class IsbnVerifier
    {
        /// <summary>
        /// Verifies if the string representation of number is a valid ISBN-10 identification number of book.
        /// </summary>
        /// <param name="number">The string representation of book's number.</param>
        /// <returns>true if number is a valid ISBN-10 identification number of book, false otherwise.</returns>
        /// <exception cref="ArgumentException">Thrown if number is null or empty or whitespace.</exception>
        public static bool IsValid(string number)
        {
            int checkSum = 0;

            if (CharToInt(number[0]) == -1 || CharToInt(number[0]) == 10)
            {
                return false;
            }

            checkSum += CharToInt(number[0]);

            if (number.Length < 10 || number.Length > 13)
            {
                return false;
            }

            byte countDefis = 0;

            if (number[1] == '-')
            {
                countDefis++;
            }

            for (int i = 1 + countDefis; i < 4 + countDefis; i++)
            {
                if (CharToInt(number[i]) == -1 || CharToInt(number[i]) == 10)
                {
                    return false;
                }

                checkSum += CharToInt(number[i]) * (10 - i - countDefis);
            }

            if (number[4 + countDefis] == '-')
            {
                countDefis++;
            }

            for (int i = 5 + countDefis; i < number.Length - 2; i++)
            {
                if (CharToInt(number[i]) == -1 || CharToInt(number[i]) == 10)
                {
                    return false;
                }

                checkSum += CharToInt(number[i]) * (10 - i - countDefis);
            }

            if (CharToInt(number[^1]) == -1)
            {
                return false;
            }

            checkSum += CharToInt(number[^1]) * 10;

            return checkSum % 11 == 0;
        }

        private static int CharToInt(char digit) =>
            digit switch
            {
                '0' => 0,
                '1' => 1,
                '2' => 2,
                '3' => 3,
                '4' => 4,
                '5' => 5,
                '6' => 6,
                '7' => 7,
                '8' => 8,
                '9' => 9,
                'X' => 10,
                _ => -1
            };
    }
}