New: Author folder hint when selecting a root folder while adding a new author

(cherry picked from commit dd09f31abb4dd3f699bcff0a47577075300c70ee)

Fix AuthorFolderAsRootFolderValidator

(cherry picked from commit 0ce81e1ab69d43fde382cc4ae22cd46fe626dea7)

src/Readarr.Api.V1/Author/AuthorFolderAsRootFolderValidator.cs

@@ -0,0 +1,49 @@
+using System;
+using System.IO;
+using FluentValidation.Validators;
+using NzbDrone.Common.Extensions;
+using NzbDrone.Core.Organizer;
+
+namespace Readarr.Api.V1.Author
+{
+    public class AuthorFolderAsRootFolderValidator : PropertyValidator
+    {
+        private readonly IBuildFileNames _fileNameBuilder;
+
+        public AuthorFolderAsRootFolderValidator(IBuildFileNames fileNameBuilder)
+        {
+            _fileNameBuilder = fileNameBuilder;
+        }
+
+        protected override string GetDefaultMessageTemplate() => "Root folder path contains author folder";
+
+        protected override bool IsValid(PropertyValidatorContext context)
+        {
+            if (context.PropertyValue == null)
+            {
+                return true;
+            }
+
+            var authorResource = context.ParentContext.InstanceToValidate as AuthorResource;
+
+            if (authorResource == null)
+            {
+                return true;
+            }
+
+            var rootFolderPath = context.PropertyValue.ToString();
+            var rootFolder = new DirectoryInfo(rootFolderPath).Name;
+            var author = authorResource.ToModel();
+            var authorFolder = _fileNameBuilder.GetAuthorFolder(author);
+
+            if (authorFolder == rootFolder)
+            {
+                return false;
+            }
+
+            var distance = authorFolder.LevenshteinDistance(rootFolder);
+
+            return distance >= Math.Max(1, authorFolder.Length * 0.2);
+        }
+    }
+}