using System;

namespace Algorithms.Numeric
{
    /// <summary>
    ///  In number theory, a Keith number or repfigit number is a natural number n in a given number base b with k digits such that
    ///  when a sequence is created such that the first k terms are the k digits of n and each subsequent term is the sum of the
    ///  previous k terms, n is part of the sequence.
    /// </summary>
    public static class KeithNumberChecker
    {
        /// <summary>
        ///     Checks if a number is a Keith number or not.
        /// </summary>
        /// <param name="number">Number to check.</param>
        /// <returns>True if it is a Keith number; False otherwise.</returns>
        public static bool IsKeithNumber(int number)
        {
            if (number < 0)
            {
                throw new ArgumentException($"{nameof(number)} cannot be negative");
            }

            var tempNumber = number;

            var stringNumber = number.ToString();

            var digitsInNumber = stringNumber.Length;

            /* storing the terms of the series */
            var termsArray = new int[number];

            for (var i = digitsInNumber - 1; i >= 0; i--)
            {
                termsArray[i] = tempNumber % 10;
                tempNumber /= 10;
            }

            var sum = 0;
            var k = digitsInNumber;
            while (sum < number)
            {
                sum = 0;

                for (var j = 1; j <= digitsInNumber; j++)
                {
                    sum += termsArray[k - j];
                }

                termsArray[k] = sum;
                k++;
            }

            return sum == number;
        }
    }
}