Fix Sieve of Eratosthenes (TheAlgorithms#377)

Author: deljohnson1

Algorithms.Tests/Other/SieveOfEratosthenesTests.cs

@@ -1,12 +1,13 @@
-using System.Numerics;
+using System.Numerics;
 using Algorithms.Other;
+using FluentAssertions;
 using NUnit.Framework;
 
 namespace Algorithms.Tests.Other
 {
     public static class SieveOfEratosthenesTests
     {
-        private static readonly BigInteger[] First10000PrimeNumbers =
+        private static readonly long[] First10000PrimeNumbers =
         {
             2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103,
             107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223,
@@ -667,7 +668,17 @@ public static class SieveOfEratosthenesTests
         };
 
         [Test]
-        public static void First10_000PrimesCorrect() =>
-            Assert.AreEqual(First10000PrimeNumbers, SieveOfEratosthenes.GetPrimeNumbers(10_000));
+        public static void First10_000PrimesCorrect() => 
+            Assert.AreEqual(First10000PrimeNumbers, new SieveOfEratosthenes(104729).GetPrimes());
+
+        [Test]
+        public static void TestMaxNumber() => Assert.AreEqual(new SieveOfEratosthenes(69).MaximumNumber, 69);
+
+        [TestCase(13, true)]
+        [TestCase(10, false)]
+        public static void TestIsPrime(int input, bool expected)
+        {
+            Assert.AreEqual(new SieveOfEratosthenes(100).IsPrime(input), expected);
+        }
     }
 }

Algorithms/Other/SieveOfEratosthenes.cs

@@ -1,31 +1,71 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Numerics;
+using System.Runtime.CompilerServices;
 
 namespace Algorithms.Other
 {
     /// <summary>
-    ///     TODO.
+    ///     Implements the Sieve of Eratosthenes.
     /// </summary>
-    public static class SieveOfEratosthenes
+    public class SieveOfEratosthenes
     {
+        private readonly bool[] primes;
+
         /// <summary>
-        ///     TODO.
+        /// Initializes a new instance of the <see cref="SieveOfEratosthenes"/> class.
+        /// Uses the Sieve of Eratosthenes to precalculate the primes from 0 up to maximumNumberToCheck.
+        /// Requires enough memory to allocate maximumNumberToCheck bytes.
+        /// https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes .
         /// </summary>
-        /// <param name="count">TODO. 2.</param>
-        /// <returns>TODO. 3.</returns>
-        public static List<BigInteger> GetPrimeNumbers(int count)
+        /// <param name="maximumNumberToCheck">long which specifies the largest number you wish to know if it is prime.</param>
+        public SieveOfEratosthenes(long maximumNumberToCheck)
         {
-            var output = new List<BigInteger>();
-            for (BigInteger n = 2; output.Count < count; n++)
+            primes = new bool[maximumNumberToCheck + 1];
+
+            // initialize primes array
+            Array.Fill(this.primes, true, 2, primes.Length - 2);
+
+            for(long i = 2; i * i <= maximumNumberToCheck; i++)
             {
-                if (output.All(x => n % x != 0))
+                if (!primes[i])
+                {
+                    continue;
+                }
+
+                for(long composite = i * i; composite <= maximumNumberToCheck; composite += i)
                 {
-                    output.Add(n);
+                    primes[composite] = false;
                 }
             }
+        }
+
+        /// <summary>
+        /// Gets the maximumNumberToCheck the class was instantiated with.
+        /// </summary>
+        public long MaximumNumber => primes.Length - 1;
 
-            return output;
+        /// <summary>
+        /// Returns a boolean indicating whether the number is prime.
+        /// </summary>
+        /// <param name="numberToCheck">The number you desire to know if it is prime or not.</param>
+        /// <returns>A boolean indicating whether the number is prime or not.</returns>
+        public bool IsPrime(long numberToCheck) => primes[numberToCheck];
+
+        /// <summary>
+        /// Returns an IEnumerable of long primes in asending order.
+        /// </summary>
+        /// <returns>Primes in ascending order.</returns>
+        public IEnumerable<long> GetPrimes()
+        {
+            for(long i = 2; i < primes.Length; i++)
+            {
+                if (primes[i])
+                {
+                    yield return i;
+                }
+            }
         }
     }
 }