/// <summary>
/// Run solution for problem 12.
/// </summary>
/// <param name="divisorCountThreshold">Divisor count threshold.</param>
/// <returns>
/// First triangle number to have equal or greater than
/// <paramref name="divisorCountThreshold"/>.
/// </returns>
public static int Run(int divisorCountThreshold)
{
var answer = 0;
var divisors = Array.Empty <int>();
for (var i = 1; divisors.Length + 1 < divisorCountThreshold; i += 1)
{
answer += i;
divisors = GeneralMath.GetProperDivisors(answer).ToArray();
}
return(answer);
}
/// <summary>
/// Run solution for problem 23.
/// </summary>
/// <remarks>
/// Abundant Number:
/// Number where the sum of its proper divisors is greater than the number itself.
///
/// All numbers greater than 28,123 can be written as the sum of two abundant numbers.
/// </remarks>
/// <returns>
/// Sum of all numbers that cannot be written as the sum of two abundant numbers.
/// </returns>
public static int Run()
{
var nonAbundantNumbersThreshold = 28123;
var abundantNumbers = new List <int>();
var abundantSums = new List <int>();
var nonAbundantSum = 0;
for (var i = 1; i <= nonAbundantNumbersThreshold; i += 1)
{
if (GeneralMath.GetProperDivisors(i).Aggregate(0, (sum, divisor) => sum + divisor) > i)
{
abundantNumbers.Add(i);
}
nonAbundantSum += i;
}
var abundantNumbersSums = new HashSet <int>();
for (var i = 0; i < abundantNumbers.Count; i += 1)
{
for (var j = i; j < abundantNumbers.Count; j += 1)
{
var abundantNumbersSum = abundantNumbers[i] + abundantNumbers[j];
if (abundantNumbersSum <= nonAbundantNumbersThreshold)
{
abundantNumbersSums.Add(abundantNumbersSum);
}
else
{
break;
}
}
}
nonAbundantSum -= abundantNumbersSums.Aggregate(0, (sum, abundantNumbersSum) => sum + abundantNumbersSum);
return(nonAbundantSum);
}
public void GetProperDivisors_Returns_Input_Numbers_Proper_Divisors()
{
GeneralMath.GetProperDivisors(220).Should().BeEquivalentTo(
new[] { 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110 });
}
