PrimeTester.cs

using System;

namespace Advanced.Algorithms.Numerical;

/// <summary>
///     Tests for Prime in School method optimized.
/// </summary>
public class PrimeTester
{
    /// <summary>
    ///     Check if given number is prime.
    /// </summary>
    public static bool IsPrime(int number)
    {
        if (number <= 1) return false;

        if (number <= 3) return true;

        //number can be divided by 2 or 3
        if (number % 2 == 0 || number % 3 == 0) return false;

        //skip six numbers in each step
        //since we don't need to check for 3 even numbers 
        //and one number divisible by 3
        //inside the loop
        //check until square root of number
        var sqrt = Math.Sqrt(number);
        for (var i = 5; i <= sqrt; i = i + 6)
            //check for two potential primes
            if (number % i == 0 || number % (i + 2) == 0)
                return false;

        return true;
    }
}