InvertedIndex.cs

using System.Collections.Generic;
using System.Linq;

namespace DataStructures
{
    /// <summary>
    /// Inverted index is the simplest form of document indexing,
    /// allowing performing boolean queries on text data.
    ///
    /// This realization is just simplified for better understanding the process of indexing
    /// and working on straightforward string inputs.
    /// </summary>
    public class InvertedIndex
    {
        private readonly Dictionary<string, List<string>> invertedIndex = new();

        /// <summary>
        /// Build inverted index with source name and source content.
        /// </summary>
        /// <param name="sourceName">Name of the source.</param>
        /// <param name="sourceContent">Content of the source.</param>
        public void AddToIndex(string sourceName, string sourceContent)
        {
            var context = sourceContent.Split(' ').Distinct();
            foreach (var word in context)
            {
                if (!invertedIndex.ContainsKey(word))
                {
                    invertedIndex.Add(word, new List<string> { sourceName });
                }
                else
                {
                    invertedIndex[word].Add(sourceName);
                }
            }
        }

        /// <summary>
        /// Returns the source names contains ALL terms inside at same time.
        /// </summary>
        /// <param name="terms">List of terms.</param>
        /// <returns>Source names.</returns>
        public IEnumerable<string> And(IEnumerable<string> terms)
        {
            var entries = terms
                .Select(term => invertedIndex
                    .Where(x => x.Key.Equals(term))
                    .SelectMany(x => x.Value))
                .ToList();

            var intersection = entries
                .Skip(1)
                .Aggregate(new HashSet<string>(entries.First()), (hashSet, enumerable) =>
                {
                    hashSet.IntersectWith(enumerable);
                    return hashSet;
                });

            return intersection;
        }

        /// <summary>
        /// Returns the source names contains AT LEAST ONE from terms inside.
        /// </summary>
        /// <param name="terms">List of terms.</param>
        /// <returns>Source names.</returns>
        public IEnumerable<string> Or(IEnumerable<string> terms)
        {
            var sources = new List<string>();
            foreach (var term in terms)
            {
                var source = invertedIndex
                    .Where(x => x.Key.Equals(term))
                    .SelectMany(x => x.Value);

                sources.AddRange(source);
            }

            return sources.Distinct();
        }
    }
}