Reverse in index

[JoshClose]

In the documentation for full-text search with wildcards, it states:

Searching with a wildcard as the prefix of the term (e.g. *text) is less recommended,
as it will cause the server to perform a full index scan.

Instead, consider using a static-index that indexes the field in reverse order
and then query with a wildcard as the postfix, which is much faster.

If I put a Reverse call in my index, it errors out.

I've tried:

text.Reverse().ToString() ?? string.Empty
text.ToArray().Reverse().ToString() ?? string.Empty
text.ToCharArray().Reverse().ToString() ?? string.Empty

All of these will give me a variation of Reverse doesn't exist.

Failed to execute mapping function on sessions/196-A. Exception: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'string' does not contain a definition for 'Reverse'
   at CallSite.Target(Closure, CallSite, Object)
   at Raven.Server.Documents.Indexes.Static.Generated.Index_Sessions_ByConnection.<>c.<.ctor>b__0_5(Object this2)
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Raven.Server.Documents.Indexes.Static.DynamicArray.DynamicArrayIterator.InterfaceMoveNext(DynamicArrayIterator& iterator) in C:\Builds\RavenDB-Stable-5.4\54038\src\Raven.Server\Documents\Indexes\Static\DynamicArray.cs:line 799
   at Raven.Server.Documents.Indexes.Static.TimeCountingEnumerable.Enumerator.MoveNext() in C:\Builds\RavenDB-Stable-5.4\54038\src\Raven.Server\Documents\Indexes\Static\TimeCountingEnumerable.cs:line 42
   at Raven.Server.Documents.Indexes.MapIndexBase`2.UpdateIndexEntriesLucene(IndexItem indexItem, IEnumerable mapResults, Lazy`1 writer, TransactionOperationContext indexContext, IndexingStatsScope stats) in C:\Builds\RavenDB-Stable-5.4\54038\src\Raven.Server\Documents\Indexes\MapIndexBase.cs:line 142
   at Raven.Server.Documents.Indexes.MapIndexBase`2.HandleMap(IndexItem indexItem, IEnumerable mapResults, Lazy`1 writer, TransactionOperationContext indexContext, IndexingStatsScope stats) in C:\Builds\RavenDB-Stable-5.4\54038\src\Raven.Server\Documents\Indexes\MapIndexBase.cs:line 69
   at Raven.Server.Documents.Indexes.Workers.MapItems.Execute(QueryOperationContext queryContext, TransactionOperationContext indexContext, Lazy`1 writeOperation, IndexingStatsScope stats, CancellationToken token) in C:\Builds\RavenDB-Stable-5.4\54038\src\Raven.Server\Documents\Indexes\Workers\MapItems.cs:line 169

What is the correct way to do this?

The documentation even shows this. https://ravendb.net/docs/article-page/5.4/csharp/indexes/indexing-linq-extensions#reverse I looked at the source code and that isn't one of the extensions in IndexingLinqExtensions.

Here is code to reproduce.

#nullable enable
void Main()
{
	using var store = new DocumentStore
	{
		Database = "Test1",
		Urls = new string[] { "http://localhost:8080" }
	};
	store.Initialize();
	
	IndexCreation.CreateIndexes(typeof(Foo_BySomething).Assembly, store);
	
	using var session = store.OpenSession();

	session.Store(new Foo { IpAddress = "1.2.3.4" });
	session.SaveChanges();
}

public class Foo
{
	public string? Id { get; set; }

	public string IpAddress { get; set; } = string.Empty;

	public List<Bar> Bars { get; set; } = new List<Bar>();
}

public class Bar
{
	public string IpAddress { get; set; } = string.Empty;
}

public class Foo_BySomething : AbstractIndexCreationTask<Foo, Foo_BySomething.Result>
{
	public class Result
	{
		public string FooIp { get; set; } = string.Empty;

		public string FooIpReverse { get; set; } = string.Empty;

		public string BarIp { get; set; } = string.Empty;

		public string BarIpReverse { get; set; } = string.Empty;
	}
	
	public Foo_BySomething()
	{
		Map = foos =>
			from f in foos
			from b in f.Bars.DefaultIfEmpty()
			let fooIp = Regex.Replace(f.IpAddress, "[^0-9a-fA-F]", "").ToUpper()
			let barIp = Regex.Replace(b.IpAddress ?? string.Empty, "[^0-9a-fA-F]", "").ToUpper()
			select new Result
			{
				FooIp = fooIp,
				FooIpReverse = fooIp.Reverse().ToString() ?? string.Empty,
				BarIp = barIp,
				BarIpReverse = barIp.Reverse().ToString() ?? string.Empty
			};
	}
}

[grisha-kotler]

@JoshClose
This works correctly for me (text.Reverse().ToString() ?? string.Empty)
Can you provide a failing test?

[JoshClose]

I added code to reproduce to the question.

[JoshClose]

It seems like it's specifically related to barIp.Reverse(). If I remove that, f.IpAddress.Reverse() works fine.

How do you do this with a nullable fanout?

[JoshClose]

I updated the code to be more like my actual code.

[JoshClose]

I've found I can cheat by adding a needless reduce in. This works but it would be better to have a solution that doesn't require reduce.

public class Foo_BySomething : AbstractIndexCreationTask<Foo, Foo_BySomething.Result>
{
	public class Result
	{
		public string FooIp { get; set; } = string.Empty;

		public string FooIpReverse { get; set; } = string.Empty;

		public string BarIp { get; set; } = string.Empty;

		public string BarIpReverse { get; set; } = string.Empty;
		
		public Guid MapReduceKey { get; set; }
	}

	public Foo_BySomething()
	{
		Map = foos =>
			from f in foos
			from b in f.Bars.DefaultIfEmpty()
			let fooIp = Regex.Replace(f.IpAddress, "[^0-9a-fA-F]", string.Empty)
			let fooIpReverse = string.Empty
			let barIp = Regex.Replace(b.IpAddress.ToString() ?? string.Empty, "[^0-9a-fA-F]", string.Empty)
			let barIpReverse = string.Empty
			select new Result
			{
				FooIp = fooIp,
				FooIpReverse = fooIpReverse,
				BarIp = barIp,
				BarIpReverse = barIpReverse,
				MapReduceKey = Guid.NewGuid(),
			};
			
		Reduce = results =>
			from r in results
			group r by r.MapReduceKey into g
			select new Result
			{
				FooIp = g.First().FooIp,
				FooIpReverse = g.First().FooIp.Reverse().ToString() ?? string.Empty,
				BarIp = g.First().BarIp,
				BarIpReverse = g.First().BarIp.Reverse().ToString() ?? string.Empty,
				MapReduceKey = g.Key,
			};
	}
}

[grisha-kotler]

A workaround for now would be to do the reverse before you use Regex.Replace -
Regex.Replace(new string(f.IpAddress.Reverse().ToArray()), "[^0-9a-fA-F]", string.Empty)

[grisha-kotler]

You can track the status of this issue here:
https://issues.hibernatingrhinos.com/issue/RavenDB-21559/string-does-not-contain-a-definition-for-Reverse

[JoshClose]

That doesn't work when using it with b because of the DefaultIfEmpty. You will want to make sure it works in that scenario also.