Data commit

Task/De-Bruijn-sequences/C-sharp/de-bruijn-sequences.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace DeBruijn {
+    class Program {
+        const string digits = "0123456789";
+
+        static string DeBruijn(int k, int n) {
+            var alphabet = digits.Substring(0, k);
+            var a = new byte[k * n];
+            var seq = new List<byte>();
+            void db(int t, int p) {
+                if (t > n) {
+                    if (n % p == 0) {
+                        seq.AddRange(new ArraySegment<byte>(a, 1, p));
+                    }
+                } else {
+                    a[t] = a[t - p];
+                    db(t + 1, p);
+                    var j = a[t - p] + 1;
+                    while (j < k) {
+                        a[t] = (byte)j;
+                        db(t + 1, t);
+                        j++;
+                    }
+                }
+            }
+            db(1, 1);
+            var buf = new StringBuilder();
+            foreach (var i in seq) {
+                buf.Append(alphabet[i]);
+            }
+            var b = buf.ToString();
+            return b + b.Substring(0, n - 1);
+        }
+
+        static bool AllDigits(string s) {
+            foreach (var c in s) {
+                if (c < '0' || '9' < c) {
+                    return false;
+                }
+            }
+            return true;
+        }
+
+        static void Validate(string db) {
+            var le = db.Length;
+            var found = new int[10_000];
+            var errs = new List<string>();
+            // Check all strings of 4 consecutive digits within 'db'
+            // to see if all 10,000 combinations occur without duplication.
+            for (int i = 0; i < le - 3; i++) {
+                var s = db.Substring(i, 4);
+                if (AllDigits(s)) {
+                    int.TryParse(s, out int n);
+                    found[n]++;
+                }
+            }
+            for (int i = 0; i < 10_000; i++) {
+                if (found[i] == 0) {
+                    errs.Add(string.Format("    PIN number {0,4} missing", i));
+                } else if (found[i] > 1) {
+                    errs.Add(string.Format("    PIN number {0,4} occurs {1} times", i, found[i]));
+                }
+            }
+            var lerr = errs.Count;
+            if (lerr == 0) {
+                Console.WriteLine("  No errors found");
+            } else {
+                var pl = lerr == 1 ? "" : "s";
+                Console.WriteLine("  {0} error{1} found:", lerr, pl);
+                errs.ForEach(Console.WriteLine);
+            }
+        }
+
+        static string Reverse(string s) {
+            char[] arr = s.ToCharArray();
+            Array.Reverse(arr);
+            return new string(arr);
+        }
+
+        static void Main() {
+            var db = DeBruijn(10, 4);
+            var le = db.Length;
+
+            Console.WriteLine("The length of the de Bruijn sequence is {0}", le);
+            Console.WriteLine("\nThe first 130 digits of the de Bruijn sequence are: {0}", db.Substring(0, 130));
+            Console.WriteLine("\nThe last 130 digits of the de Bruijn sequence are: {0}", db.Substring(le - 130, 130));
+
+            Console.WriteLine("\nValidating the deBruijn sequence:");
+            Validate(db);
+
+            Console.WriteLine("\nValidating the reversed deBruijn sequence:");
+            Validate(Reverse(db));
+
+            var bytes = db.ToCharArray();
+            bytes[4443] = '.';
+            db = new string(bytes);
+            Console.WriteLine("\nValidating the overlaid deBruijn sequence:");
+            Validate(db);
+        }
+    }
+}