Optimized anagrams count computation

dotnet/WhiteRabbit/PrecomputedPermutationsGenerator.cs

@@ -5,22 +5,13 @@
 
     internal static class PrecomputedPermutationsGenerator
     {
-        private static PermutationsGenerator.Permutation[][] Permutations { get; } = new[]
-        {
-            GeneratePermutations(0),
-            GeneratePermutations(1),
-            GeneratePermutations(2),
-            GeneratePermutations(3),
-            GeneratePermutations(4),
-            GeneratePermutations(5),
-            GeneratePermutations(6),
-            GeneratePermutations(7),
-        };
-
-        public static PermutationsGenerator.Permutation[] HamiltonianPermutations(int n)
-        {
-            return Permutations[n];
-        }
+        private static PermutationsGenerator.Permutation[][] Permutations { get; } = Enumerable.Range(0, 8).Select(GeneratePermutations).ToArray();
+
+        private static long[] PermutationsNumbers { get; } = GeneratePermutationsNumbers().Take(19).ToArray();
+
+        public static PermutationsGenerator.Permutation[] HamiltonianPermutations(int n) => Permutations[n];
+
+        public static long GetPermutationsNumber(int n) => PermutationsNumbers[n];
 
         private static PermutationsGenerator.Permutation[] GeneratePermutations(int n)
         {
@@ -32,5 +23,19 @@
 
             return result.Concat(Enumerable.Repeat(result[0], Constants.PhrasesPerSet - (result.Length % Constants.PhrasesPerSet))).ToArray();
         }
+
+        private static IEnumerable<long> GeneratePermutationsNumbers()
+        {
+            long result = 1;
+            yield return result;
+
+            var i = 1;
+            while (true)
+            {
+                result *= i;
+                yield return result;
+                i++;
+            }
+        }
     }
 }

dotnet/WhiteRabbit/StringsProcessor.cs

@@ -68,9 +68,8 @@
         public long GetPhrasesCount()
         {
             return this.VectorsProcessor.GenerateSequences()
-                .Select(this.ConvertVectorsToWords)
-                .SelectMany(Flattener.Flatten)
-                .Sum(words => (long)PrecomputedPermutationsGenerator.HamiltonianPermutations(words.Length).Count());
+                .Select(this.ConvertVectorsToWordsNumber)
+                .Sum(tuple => tuple.Item2 * PrecomputedPermutationsGenerator.GetPermutationsNumber(tuple.Item1));
         }
 
         private byte[][][] ConvertVectorsToWords(int[] vectors)
@@ -85,6 +84,17 @@
             return words;
         }
 
+        private Tuple<int, long> ConvertVectorsToWordsNumber(int[] vectors)
+        {
+            long result = 1;
+            for (var i = 0; i < vectors.Length; i++)
+            {
+                result *= this.WordsDictionary[vectors[i]].Length;
+            }
+
+            return Tuple.Create(vectors.Length, result);
+        }
+
         private IEnumerable<PhraseSet> ConvertWordsToPhrases(byte[][] words)
         {
             var permutations = PrecomputedPermutationsGenerator.HamiltonianPermutations(words.Length);