Data commit

Task/Twin-primes/C-sharp/twin-primes-1.cs

@@ -0,0 +1,58 @@
+using System;
+
+class Program {
+
+    static uint[] res = new uint[10];
+    static uint ri = 1, p = 10, count = 0;
+
+    static void TabulateTwinPrimes(uint bound) {
+        if (bound < 5) return; count++;
+        uint cl = (bound - 1) >> 1, i = 1, j,
+             limit = (uint)(Math.Sqrt(bound) - 1) >> 1;
+        var comp = new bool[cl]; bool lp;
+        for (j = 3; j < cl; j += 3) comp[j] = true;
+        while (i < limit) {
+            if (lp = !comp[i]) {
+                uint pr = (i << 1) + 3;
+                for (j = (pr * pr - 2) >> 1; j < cl; j += pr)
+                    comp[j] = true;
+            }
+            if (!comp[++i]) {
+                uint pr = (i << 1) + 3;
+                if (lp) {
+                    if (pr > p) {
+                        res[ri++] = count;
+                        p *= 10;
+                    }
+                    count++;
+                    i++;
+                }
+                for (j = (pr * pr - 2) >> 1; j < cl; j += pr)
+                    comp[j] = true;
+            }
+        }
+        cl--;
+        while (i < cl) {
+            lp = !comp[i++];
+            if (!comp[i] && lp) {
+                if ((i++ << 1) + 3 > p) {
+                    res[ri++] = count;
+                    p *= 10;
+                }
+                count++;
+            }
+        }
+        res[ri] = count;
+    }
+
+    static void Main(string[] args) {
+        var sw = System.Diagnostics.Stopwatch.StartNew();
+        string fmt = "{0,9:n0} twin primes below {1,-13:n0}";
+        TabulateTwinPrimes(1_000_000_000);
+        sw.Stop();
+        p = 1;
+        for (var j = 1; j <= ri; j++)
+            Console.WriteLine(fmt, res[j], p *= 10);
+        Console.Write("{0} sec", sw.Elapsed.TotalSeconds);
+    }
+}

Task/Twin-primes/C-sharp/twin-primes-2.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Linq;
+using System.Collections;
+using System.Collections.Generic;
+
+public static class TwinPrimes
+{
+    public static void Main()
+    {
+        CountTwinPrimes(Enumerable.Range(1, 9).Select(i => (int)Math.Pow(10, i)).ToArray());
+    }
+
+    private static void CountTwinPrimes(params int[] bounds)
+    {
+        Array.Sort(bounds);
+        int b = 0;
+        int count = 0;
+        string format = "There are {0:N0} twin primes below {1:N0}";
+        foreach (var twin in FindTwinPrimes(bounds[^1])) {
+            if (twin.p2 >= bounds[b]) {
+                Console.WriteLine(format, count, bounds[b]);
+                b++;
+            }
+            count++;
+        }
+        Console.WriteLine(format, count, bounds[b]);
+    }
+
+    private static IEnumerable<(int p1, int p2)> FindTwinPrimes(int bound) =>
+        PrimeSieve(bound).Pairwise().Where(pair => pair.p1 + 2 == pair.p2);
+
+    private static IEnumerable<int> PrimeSieve(int bound)
+    {
+        if (bound < 2) yield break;
+        yield return 2;
+
+        var 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;
+        }
+    }
+
+    private static IEnumerable<(T p1, T p2)> Pairwise<T>(this IEnumerable<T> source)
+    {
+        using var e = numbers.GetEnumerator();
+        if (!e.MoveNext()) yield break;
+        T p1 = e.Current;
+        while (e.MoveNext()) {
+            T p2 = e.Current;
+            yield return (p1, p2);
+            p1 = p2;
+        }
+    }
+}