Data commit

Task/Emirp-primes/C-sharp/emirp-primes.cs

@@ -0,0 +1,70 @@
+using static System.Console;
+using System;
+using System.Linq;
+using System.Collections.Generic;
+
+public class Program
+{
+    public static void Main() {
+        const int limit = 1_000_000;
+        WriteLine("First 20:");
+        WriteLine(FindEmirpPrimes(limit).Take(20).Delimit());
+        WriteLine();
+		
+        WriteLine("Between 7700 and 8000:");
+        WriteLine(FindEmirpPrimes(limit).SkipWhile(p => p < 7700).TakeWhile(p => p < 8000).Delimit());
+        WriteLine();
+		
+        WriteLine("10000th:");
+        WriteLine(FindEmirpPrimes(limit).ElementAt(9999));
+    }
+	
+    private static IEnumerable<int> FindEmirpPrimes(int limit)
+    {
+        var primes = Primes(limit).ToHashSet();
+		
+        foreach (int prime in primes) {
+            int reverse = prime.Reverse();
+            if (reverse != prime && primes.Contains(reverse)) yield return prime;
+	}
+    }
+	
+    private static IEnumerable<int> Primes(int bound) {
+        if (bound < 2) yield break;
+        yield return 2;
+		
+        BitArray composite = new BitArray((bound - 1) / 2);
+        int limit = ((int)(Math.Sqrt(bound)) - 1) / 2;
+        for (int i = 0; i < limit; i++) {
+            if (composite[i]) continue;
+	    int prime = 2 * i + 3;
+	    yield return prime;
+			
+	    for (int j = (prime * prime - 2) / 2; j < composite.Count; j += prime)
+	        composite[j] = true;
+        }
+	for (int i = limit; i < composite.Count; i++)
+	    if (!composite[i]) yield return 2 * i + 3;
+    }
+}
+
+public static class Extensions
+{
+    public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source) => new HashSet<T>(source);
+
+    private const string defaultSeparator = " ";
+    public static string Delimit<T>(this IEnumerable<T> source, string separator = defaultSeparator) =>
+        string.Join(separator ?? defaultSeparator, source);
+
+    public static int Reverse(this int number)
+    {
+	if (number < 0) return -Reverse(-number);
+	if (number < 10) return number;
+	int reverse = 0;
+	while (number > 0) {
+	    reverse = reverse * 10 + number % 10;
+	    number /= 10;
+	}
+	return reverse;
+    }
+}