Initial Commit

.gitignore

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+obj/
+bin/

Eratosthenes.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace EratosthenesPrimes
+{
+    static class Eratosthenes
+    {
+        public static List<long> FindPrimes(long maxNum, int sampleSize = 1000)
+        {
+            if (maxNum < 2)
+            {
+                return new List<long>();
+            }
+            List<long> foundPrimes = new List<long> { 2 };
+            long sampleStart = 3;
+            long largestSample = 2;
+
+            while (largestSample < maxNum)
+            {
+                var realSampleSize = maxNum - largestSample < sampleSize ? (int)(maxNum - largestSample) : sampleSize;
+                var sample = Enumerable.Repeat(true, realSampleSize).ToArray();
+
+                largestSample = sampleStart + realSampleSize;
+                var sqrt = Math.Sqrt(largestSample);
+
+                foreach (var prime in foundPrimes)
+                {
+                    if (prime > sqrt)
+                    {
+                        // None of the composite numbers in our current sample will be divisible by a prime > sqrt.
+                        break;
+                    }
+
+                    var multiple = prime * (sampleStart / prime); // The largest multiple less than or equal to sampleStart.
+                    multiple += multiple < sampleStart ? prime : 0;
+                    while (multiple < largestSample)
+                    {
+                        sample[multiple - sampleStart] = false;
+                        multiple += prime;
+                    }
+                }
+                for (int i = 0; i < sample.Length; i++)
+                {
+                    if (sample[i])
+                    {
+                        var prime = sampleStart + i;
+                        foundPrimes.Add(prime);
+
+                        var multiple = prime * 2;
+                        while (multiple < largestSample)
+                        {
+                            sample[multiple - sampleStart] = false;
+                            multiple += prime;
+                        }
+                    }
+                }
+
+                sampleStart = largestSample;
+            }
+
+            return foundPrimes;
+        }
+
+        public static bool IsPrime(long prime)
+        {
+            if(prime < 2)
+            {
+                return false;
+            }
+
+            long sqrt = (long)Math.Sqrt(prime);
+            foreach(var i in FindPrimes(sqrt))
+            {
+                if(prime % i == 0)
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+    }
+}

EratosthenesPrimes.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+  </PropertyGroup>
+
+</Project>

Program.cs

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+using System;
+using System.Diagnostics;
+
+namespace EratosthenesPrimes
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            long limit = 1000;
+            int size = 1000;
+
+            if(args.Length >= 1)
+            {
+                if(!Int64.TryParse(args[0], out limit))
+                {
+                    limit = 1000;
+                }
+            }
+            if(args.Length >= 2)
+            {
+                if(!Int32.TryParse(args[1], out size))
+                {
+                    size = 1000;
+                }
+            }
+
+            var sw = new Stopwatch();
+            sw.Start();
+            var primes = Eratosthenes.FindPrimes(limit, size);
+            sw.Stop();
+            Console.WriteLine($"{primes.Count} primes found in {sw.Elapsed.Seconds}.{sw.ElapsedMilliseconds % 1000, 3:D3} seconds");
+
+            Console.WriteLine("Press Enter to print the primes or Ctrl-C to quit.");
+            Console.ReadLine();
+
+            foreach(var p in primes)
+            {
+                Console.WriteLine(p);
+            }
+        }
+    }
+}