using System;

namespace Algorithms.Numeric;

/// <summary>
///     In number theory, a perfect number is a positive integer that is equal to the sum of its positive
///     divisors, excluding the number itself.For instance, 6 has divisors 1, 2 and 3 (excluding
///     itself), and 1 + 2 + 3 = 6, so 6 is a perfect number.
/// </summary>
public static class PerfectNumberChecker
{
    /// <summary>
    ///     Checks if a number is a perfect number or not.
    /// </summary>
    /// <param name="number">Number to check.</param>
    /// <returns>True if is a perfect number; False otherwise.</returns>
    /// <exception cref="ArgumentException">Error number is not on interval (0.0; int.MaxValue).</exception>
    public static bool IsPerfectNumber(int number)
    {
        if (number < 0)
        {
            throw new ArgumentException($"{nameof(number)} cannot be negative");
        }

        var sum = 0; /* sum of its positive divisors */
        for (var i = 1; i < number; ++i)
        {
            if (number % i == 0)
            {
                sum += i;
            }
        }

        return sum == number;
    }
}