TrustPilotChallenge/WhiteRabbit/Processor.cs

namespace WhiteRabbit
{
    using System;
    using System.Collections.Generic;
    using System.Collections.Immutable;
    using System.Linq;

    internal class Processor
    {
        private const int DifferentChars = 12;
        private const int ArraySize = DifferentChars * sizeof(int);

        public Processor(string sourceString, int maxWordsCount)
        {
            var rawNumberOfOccurrences = sourceString.Where(ch => ch != ' ').GroupBy(ch => ch).ToDictionary(group => group.Key, group => group.Count());
            this.IntToChar = rawNumberOfOccurrences.Select(kvp => kvp.Key).OrderBy(ch => ch).ToArray();
            if (this.IntToChar.Length != DifferentChars)
            {
                throw new ArgumentException("Unsupported phrase", nameof(sourceString));
            }
            this.CharToInt = Enumerable.Range(0, DifferentChars).ToDictionary(i => this.IntToChar[i], i => i);
            this.NumberOfOccurrences = Enumerable.Range(0, DifferentChars).Select(i => this.IntToChar[i]).Select(ch => rawNumberOfOccurrences.ContainsKey(ch) ? rawNumberOfOccurrences[ch] : 0).ToArray();
            this.MaxWordsCount = maxWordsCount;
        }

        private Dictionary<char, int> CharToInt { get; }

        private char[] IntToChar { get; }

        private int[] NumberOfOccurrences { get; }

        private int TotalCharsNumber { get; }

        private int MaxWordsCount { get; }

        private long Iterations { get; set; } = 0;

        public IEnumerable<string> GeneratePhrases(IEnumerable<string> words)
        {
            var filtered = FilterWords(words);
            var formattedWords = FormatWords(filtered);

            var dictionary = ImmutableStack.Create(formattedWords.Keys.ToArray());
            var anagrams = GenerateOrderedPhrases(this.NumberOfOccurrences, ImmutableStack.Create<int[]>(), dictionary);
            var anagramsWords = anagrams
                .Select(list => ImmutableStack.Create(list.Select(wordArray => formattedWords[wordArray]).ToArray()))
                .SelectMany(Flatten)
                .Select(stack => stack.ToArray());

            return anagramsWords.SelectMany(GeneratePermutations).Select(list => string.Join(" ", list));
        }

        private IEnumerable<string> FilterWords(IEnumerable<string> words)
        {
            return words
                .Where(word => word.All(this.CharToInt.ContainsKey))
                .OrderBy(word => word)
                .Distinct()
                .Where(word => word.GroupBy(ch => this.CharToInt[ch]).All(group => group.Count() <= this.NumberOfOccurrences[group.Key]));
        }

        private Dictionary<int[], string[]> FormatWords(IEnumerable<string> filteredWords)
        {
            return filteredWords
                .GroupBy(word => new string(word.OrderBy(ch => ch).ToArray()))
                .ToDictionary(
                    group => Enumerable.Range(0, DifferentChars).Select(i => group.Key.Count(ch => ch == IntToChar[i])).ToArray(),
                    group => group.ToArray());
        }

        private int[] GetStatus(int[] originalState, int[] newWord, out int status)
        {
            var tmpArray = new int[DifferentChars];
            tmpArray[0] = originalState[0] - newWord[0];
            tmpArray[1] = originalState[1] - newWord[1];
            tmpArray[2] = originalState[2] - newWord[2];
            tmpArray[3] = originalState[3] - newWord[3];
            tmpArray[4] = originalState[4] - newWord[4];
            tmpArray[5] = originalState[5] - newWord[5];
            tmpArray[6] = originalState[6] - newWord[6];
            tmpArray[7] = originalState[7] - newWord[7];
            tmpArray[8] = originalState[8] - newWord[8];
            tmpArray[9] = originalState[9] - newWord[9];
            tmpArray[10] = originalState[10] - newWord[10];
            tmpArray[11] = originalState[11] - newWord[11];

            // Negative if at least one element is negative; zero if all elements are zero; positive if all elements are non-negative and at least one element is positive
            status = tmpArray[0] | tmpArray[1] | tmpArray[2] | tmpArray[3] | tmpArray[4] | tmpArray[5] | tmpArray[6] | tmpArray[7] | tmpArray[8] | tmpArray[9] | tmpArray[10] | tmpArray[11];
            return tmpArray;
        }

        // This method takes most of the time, so everything related to it must be optimized
        private IEnumerable<int[][]> GenerateOrderedPhrases(int[] currentState, ImmutableStack<int[]> phraseStack, ImmutableStack<int[]> dictionaryStack)
        {
            var remainder = dictionaryStack;
            var count = phraseStack.Count() + 1;
            while (!remainder.IsEmpty)
            {
                int[] currentWord;
                var nextRemainder = remainder.Pop(out currentWord);

                this.Iterations++;
                if (this.Iterations % 1000000 == 0)
                {
                    Console.WriteLine($"Iteration #{this.Iterations}: {string.Join(" ", phraseStack.Push(currentWord).Reverse().Select(word => new string(Enumerable.Range(0, DifferentChars).SelectMany(i => Enumerable.Repeat(IntToChar[i], word[i])).ToArray())))}");
                }

                int status;
                var state = GetStatus(currentState, currentWord, out status);
                if (status > 0 && count < this.MaxWordsCount)
                {
                    foreach (var result in GenerateOrderedPhrases(state, phraseStack.Push(currentWord), remainder))
                    {
                        yield return result;
                    }
                }
                else if (status == 0)
                {
                    yield return phraseStack.Push(currentWord).Reverse().ToArray();
                }

                remainder = nextRemainder;
            }
        }

        private IEnumerable<string[]> GeneratePermutations(string[] original)
        {
            foreach (var permutation in PermutationsGenerator.HamiltonianPermutations(original.Length))
            {
                yield return permutation.Select(i => original[i]).ToArray();
            }
        }

        private IEnumerable<ImmutableStack<string>> Flatten(ImmutableStack<string[]> phrase)
        {
            if (phrase.IsEmpty)
            {
                return new[] { ImmutableStack.Create<string>() };
            }

            string[] wordVariants;
            var newStack = phrase.Pop(out wordVariants);
            return Flatten(newStack).SelectMany(remainder => wordVariants.Select(word => remainder.Push(word)));
        }
    }
}