Enough already. I'm obviously not incentivized/shamed into writing unit tests for things in UNTESTED dir.s. It's just making a mess of the file tree

FileManager/AudibleFileStorage.cs

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+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text.RegularExpressions;
+using Dinah.Core;
+using Dinah.Core.Collections.Generic;
+
+namespace FileManager
+{
+    // could add images here, but for now images are stored in a well-known location
+    public enum FileType { Unknown, Audio, AAX, PDF }
+
+	/// <summary>
+	/// Files are large. File contents are never read by app.
+	/// Paths are varied.
+	/// Files are written during download/decrypt/backup/liberate.
+	/// Paths are read at app launch and during download/decrypt/backup/liberate.
+	/// Many files are often looked up at once
+	/// </summary>
+	public abstract class AudibleFileStorage : Enumeration<AudibleFileStorage>
+	{
+		public abstract string[] Extensions { get; }
+		public abstract string StorageDirectory { get; }
+
+		#region static
+		public static AudioFileStorage Audio { get; } = new AudioFileStorage();
+		public static AudibleFileStorage AAX { get; } = new AaxFileStorage();
+		public static AudibleFileStorage PDF { get; } = new PdfFileStorage();
+
+		public static string DownloadsInProgress
+        {
+            get
+            {
+                if (!Configuration.Instance.DownloadsInProgressEnum.In("WinTemp", "LibationFiles"))
+                    Configuration.Instance.DownloadsInProgressEnum = "WinTemp";
+                var AaxRootDir
+                    = Configuration.Instance.DownloadsInProgressEnum == "WinTemp"
+                    ? Configuration.WinTemp
+                    : Configuration.Instance.LibationFiles;
+
+                return Directory.CreateDirectory(Path.Combine(AaxRootDir, "DownloadsInProgress")).FullName;
+            }
+        }
+
+        public static string DecryptInProgress
+        {
+            get
+            {
+                if (!Configuration.Instance.DecryptInProgressEnum.In("WinTemp", "LibationFiles"))
+                    Configuration.Instance.DecryptInProgressEnum = "WinTemp";
+
+                var M4bRootDir
+                    = Configuration.Instance.DecryptInProgressEnum == "WinTemp"
+                    ? Configuration.WinTemp
+                    : Configuration.Instance.LibationFiles;
+
+                return Directory.CreateDirectory(Path.Combine(M4bRootDir, "DecryptInProgress")).FullName;
+            }
+        }
+
+        // not customizable. don't move to config
+        public static string DownloadsFinal => new DirectoryInfo(Configuration.Instance.LibationFiles).CreateSubdirectory("DownloadsFinal").FullName;
+
+        public static string BooksDirectory
+        {
+            get
+            {
+                if (string.IsNullOrWhiteSpace(Configuration.Instance.Books))
+                    Configuration.Instance.Books = Path.Combine(Configuration.Instance.LibationFiles, "Books");
+                return Directory.CreateDirectory(Configuration.Instance.Books).FullName;
+            }
+        }
+        #endregion
+
+        #region instance
+        public FileType FileType => (FileType)Value;
+
+		private IEnumerable<string> extensions_noDots { get; }
+		private string extAggr { get; }
+
+        protected AudibleFileStorage(FileType fileType) : base((int)fileType, fileType.ToString())
+		{
+			extensions_noDots = Extensions.Select(ext => ext.Trim('.')).ToList();
+			extAggr = extensions_noDots.Aggregate((a, b) => $"{a}|{b}");
+		}
+
+        /// <summary>
+        /// Example for full books:
+        /// Search recursively in _books directory. Full book exists if either are true
+        /// - a directory name has the product id and an audio file is immediately inside
+        /// - any audio filename contains the product id
+        /// </summary>
+        public bool Exists(string productId) => GetPath(productId) != null;
+
+		public string GetPath(string productId)
+        {
+            var cachedFile = FilePathCache.GetPath(productId, FileType);
+            if (cachedFile != null)
+                return cachedFile;
+
+            var firstOrNull =
+				Directory
+				.EnumerateFiles(StorageDirectory, "*.*", SearchOption.AllDirectories)
+				.FirstOrDefault(s => Regex.IsMatch(s, $@"{productId}.*?\.({extAggr})$", RegexOptions.IgnoreCase));
+
+			if (firstOrNull is null)
+				return null;
+
+			FilePathCache.Upsert(productId, FileType, firstOrNull);
+			return firstOrNull;
+        }
+
+        public string GetDestDir(string title, string asin)
+        {
+            // to prevent the paths from getting too long, we don't need after the 1st ":" for the folder
+            var underscoreIndex = title.IndexOf(':');
+            var titleDir
+                = underscoreIndex < 4
+                ? title
+                : title.Substring(0, underscoreIndex);
+            var finalDir = FileUtility.GetValidFilename(StorageDirectory, titleDir, null, asin);
+            return finalDir;
+        }
+
+        public bool IsFileTypeMatch(FileInfo fileInfo)
+            => extensions_noDots.ContainsInsensative(fileInfo.Extension.Trim('.'));
+        #endregion
+    }
+
+    public class AudioFileStorage : AudibleFileStorage
+    {
+        public const string SKIP_FILE_EXT = "libhack";
+
+		public override string[] Extensions { get; } = new[] { "m4b", "mp3", "aac", "mp4", "m4a", "ogg", "flac", SKIP_FILE_EXT };
+
+        // we always want to use the latest config value, therefore
+        // - DO use 'get' arrow "=>"
+        // - do NOT use assign "="
+        public override string StorageDirectory => BooksDirectory;
+
+        public AudioFileStorage() : base(FileType.Audio) { }
+
+        public string CreateSkipFile(string title, string asin, string contents = null)
+        {
+            var destinationDir = GetDestDir(title, asin);
+            Directory.CreateDirectory(destinationDir);
+
+            var path = FileUtility.GetValidFilename(destinationDir, title, SKIP_FILE_EXT, asin);
+            File.WriteAllText(path, contents ?? string.Empty);
+
+            return path;
+        }
+    }
+
+    public class AaxFileStorage : AudibleFileStorage
+    {
+        public override string[] Extensions { get; } = new[] { "aax" };
+
+        // we always want to use the latest config value, therefore
+        // - DO use 'get' arrow "=>"
+        // - do NOT use assign "="
+        public override string StorageDirectory => DownloadsFinal;
+
+        public AaxFileStorage() : base(FileType.AAX) { }
+    }
+
+    public class PdfFileStorage : AudibleFileStorage
+    {
+		public override string[] Extensions { get; } = new[] { "pdf", "zip" };
+
+        // we always want to use the latest config value, therefore
+        // - DO use 'get' arrow "=>"
+        // - do NOT use assign "="
+        public override string StorageDirectory => BooksDirectory;
+
+        public PdfFileStorage() : base(FileType.PDF) { }
+    }
+}