using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var index = new GinIndex();

        index.Add("Hello World Bogdan", 1);

        index.Add("Hello Test World", 2);

        index.Add("Test World", 3);

        index.Add("Wadf Hello Test World", 4);

        var result = index.Search("Hello Test");

        foreach (var item in result) {

            Console.WriteLine(item);    

        }

    }

}

​

public class GinIndex

{

    private static Dictionary<string, HashSet<int>> _index = new Dictionary<string, HashSet<int>>();

    public void Add(string text, int identifier) {

        var words = text

            .Split(' ')

            .Select(x => x.ToLowerInvariant())

            .ToArray();

        foreach(var word in words) {

            if (_index.ContainsKey(word)) {

                var records = _index[word];

                records.Add(identifier);

            } else {

                var records = new HashSet<int>();

                records.Add(identifier);

                _index.Add(word, records);

            }

        }

    }

    public int[] Search(string text) {

        var words = text

            .Split(' ')

            .Select(x => x.ToLowerInvariant())

            .ToArray();

        var resultSets = new List<HashSet<int>>(words.Length);

        foreach(var word in words) {

            if (!_index.ContainsKey(word)) {

                return new int[0];

            }

            resultSets.Add(_index[word]);

        }

        var result = resultSets[0];

        for(int i = 1; i < resultSets.Count; i++) {

            result.IntersectWith(resultSets[i]);

        }

        return result.ToArray();

    }

}