Eratostene Sieve

eratostene.cs

using System.Collections;
using System.Collections.Generic;

namespace Eratostene
{
    /**
    * Compute Prime Numbers 
    *
    *  Example:
    <code>

            var eratostene = new Eratostene(1000);
            eratostene.Crivello();
            var primes = eratostene.GetPrimes();
            var howMany = eratostene.GetNumberOfPrimes();
            Console.WriteLine($"Found {howMany} primes.");
            foreach (var prime in primes)
            {
                Console.WriteLine($"> {prime}");
            }

    </code>
    **/
    public class Eratostene
    {
        private readonly int _maxPrime;
        private readonly BitArray _sieve;

        public Eratostene(int count)
        {
            this._maxPrime = count;
            this._sieve = new BitArray(count, true)
            {
                [0] = false,
                [1] = false
            };
        }

        public bool IsPrime(int number)
        {
            return this._sieve[number];
        }

        public List<int> GetPrimes()
        {
            var retPrimes = new List<int>();
            for (var k = 0; k < this._maxPrime; k++)
            {
                if (this._sieve[k])
                {
                    retPrimes.Add(k);
                }
            }
            return retPrimes;
        }

        public int GetNumberOfPrimes()
        {
            var count = 0;
            for (var k = 0; k < this._maxPrime; k++)
            {
                if (this._sieve[k])
                    count++;
            }
            return count;
        }

        public void Crivello()
        {
            var currPrime = this.NextPrime(0);

            while (currPrime*currPrime < this._maxPrime)
            {
                var i = currPrime*currPrime;
                while (i < this._maxPrime)
                {
                    this._sieve[i] = false;
                    i += currPrime;
                }
                currPrime = this.NextPrime(currPrime);
            }
        }

        private int NextPrime(int lastPrime)
        {
            for (var j = lastPrime + 1; j < this._maxPrime; j++)
            {
                if (this._sieve[j])
                {
                    return j;
                }
            }
            return 0;
        }
    }
}