Data commit

Task/Weird-numbers/C-sharp/weird-numbers.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace WeirdNumbers {
+    class Program {
+        static List<int> Divisors(int n) {
+            List<int> divs = new List<int> { 1 };
+            List<int> divs2 = new List<int>();
+
+            for (int i = 2; i * i <= n; i++) {
+                if (n % i == 0) {
+                    int j = n / i;
+                    divs.Add(i);
+                    if (i != j) {
+                        divs2.Add(j);
+                    }
+                }
+            }
+
+            divs.Reverse();
+            divs2.AddRange(divs);
+            return divs2;
+        }
+
+        static bool Abundant(int n, List<int> divs) {
+            return divs.Sum() > n;
+        }
+
+        static bool Semiperfect(int n, List<int> divs) {
+            if (divs.Count > 0) {
+                var h = divs[0];
+                var t = divs.Skip(1).ToList();
+                if (n < h) {
+                    return Semiperfect(n, t);
+                } else {
+                    return n == h
+                        || Semiperfect(n - h, t)
+                        || Semiperfect(n, t);
+                }
+            } else {
+                return false;
+            }
+        }
+
+        static List<bool> Sieve(int limit) {
+            // false denotes abundant and not semi-perfect.
+            // Only interested in even numbers >= 2
+            bool[] w = new bool[limit];
+            for (int i = 2; i < limit; i += 2) {
+                if (w[i]) continue;
+                var divs = Divisors(i);
+                if (!Abundant(i, divs)) {
+                    w[i] = true;
+                } else if (Semiperfect(i, divs)) {
+                    for (int j = i; j < limit; j += i) {
+                        w[j] = true;
+                    }
+                }
+            }
+            return w.ToList();
+        }
+
+        static void Main() {
+            var w = Sieve(17_000);
+            int count = 0;
+            int max = 25;
+            Console.WriteLine("The first 25 weird numbers:");
+            for (int n = 2; count < max; n += 2) {
+                if (!w[n]) {
+                    Console.Write("{0} ", n);
+                    count++;
+                }
+            }
+            Console.WriteLine();
+        }
+    }
+}