Add initial inverted index implementation. It's untested though.

SearchBox/Index.cs

@@ -31,6 +31,7 @@ namespace SearchBox
 			}
 
 		}
+
 		public Index(string inSource, IndexOptions options)
 			: this(inSource, EmbeddedFiles.EnumerateLines("SearchBox.EmbeddedFiles.Stopwords.txt"), options)
 		{
@@ -55,6 +56,10 @@ namespace SearchBox
 
 
 
+		public IEnumerable<string> Tokens()
+		{
+			return index.Keys;
+		}
 		public IEnumerable<KeyValuePair<string, List<int>>> IterateItems()
 		{
 			foreach(KeyValuePair<string, List<int>> item in index)

SearchBox/InvertedIndex.cs

@@ -1,23 +1,41 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 
 namespace SearchBox
 {
 	public class InvertedIndex
 	{
-		private Dictionary<string, Dictionary<int, List<int>>> invertedIndex = new Dictionary<string, Dictionary<int, List<int>>>();
+		private ConcurrentDictionary<string, ConcurrentDictionary<int, List<int>>> invertedIndex = new Dictionary<string, Dictionary<int, List<int>>>();
 
 		public InvertedIndex()
 		{
 		}
 
-		public bool AddIndex(Index newIndex)
+		public bool AddIndex(int pageId, Index newIndex)
 		{
 			foreach (KeyValuePair<string, List<int>> token in newIndex)
 			{
+				if (!invertedIndex.ContainsKey(token.Key) &&
+					!invertedIndex.TryAdd(token.Key, new ConcurrentDictionary<int, List<int>>()))
+					return false;
 
+				if (!invertedIndex[token.Key].TryAdd(pageId, token.Value))
+					return false;
 			}
 			return true;
 		}
+
+		public bool RemoveIndex(int pageId, Index newIndex)
+		{
+			foreach (string token in newIndex.Tokens())
+			{
+				if (!invertedIndex.ContainsKey(token) || !invertedIndex[token].ContainsKey(pageId)) continue;
+
+				if (!invertedIndex[token].TryRemove(pageId, out List<int> noop))
+					return false;
+			}
+			return false;
+		}
 	}
 }