fix(scan): group chapter files indexed as "N of M" into one unmatched-scan item

listenarr.infrastructure/Library/Scanning/UnmatchedScanProcessor.Grouping.cs

@@ -405,15 +405,27 @@ private static void ApplyEmbeddedTags(PathParsedMetadata target, PathParsedMetad
 
         /// <summary>
         /// Extracts a normalized title stem from a filename for grouping purposes.
-        /// Strips leading track numbers, trailing Part/CD/Disc numbers, year and
-        /// series decorations in brackets. Files that resolve to the same stem are
-        /// treated as parts of the same audiobook. Returns the folder name as fallback
-        /// when the stem would otherwise be empty (for example purely numeric filenames).
+        /// Strips a trailing "N of M" index, leading track numbers, trailing
+        /// Part/CD/Disc numbers, year and series decorations in brackets. Files that
+        /// resolve to the same stem are treated as parts of the same audiobook. Returns
+        /// the folder name as fallback when the stem would otherwise be empty (for
+        /// example purely numeric filenames).
         /// </summary>
         private static string ExtractTitleStem(string filePath, string folderPath)
         {
             var name = Path.GetFileNameWithoutExtension(filePath);
 
+            // Strip a trailing "N of M" index: "Title 001 of 498", "Title - 3 of 12".
+            //
+            // This runs BEFORE the leading-track strip on purpose. A file named "001 of 498"
+            // with no title in it would otherwise lose its leading "001 " first, leaving
+            // "of 498" — a different stem in every file, and no longer numeric, so the
+            // folder-name fallback at the end never gets the chance to group them.
+            //
+            // Unlike a bare "(N)", this form is not ambiguous with a series marker: it
+            // carries its own total, so it says the file is one part of a set rather than
+            // one entry among separate works. Nothing is titled "Book 1 of 12".
+            name = Regex.Replace(name, @"[\s\-_]*\d+\s*of\s*\d+$", "", RegexOptions.IgnoreCase);
             // Strip leading track/disc number prefix: "01 - ", "Track 01 - ", "1. "
             name = Regex.Replace(name, @"^(track\s*)?\d+[\s\-_\.]+", "", RegexOptions.IgnoreCase);
             // Strip trailing Part/CD/Disc/Chapter number: "- Part 1", "CD2", "Disc 2", "pt00"

tests/Features/Api/Services/UnmatchedScanBackgroundServiceTests.cs

@@ -119,5 +119,63 @@ public void BuildGroupedFilesForFolder_UsesAuthorToKeepSameTitleSeparated()
             Assert.Contains(groups, group => group.Single() == fileA);
             Assert.Contains(groups, group => group.Single() == fileB);
         }
+
+        [Fact]
+        public void BuildGroupedFilesForFolder_GroupsChapterFilesIndexedAsNumberOfTotal()
+        {
+            // A chapter-per-file rip that numbers its parts "N of M". The index carries its
+            // own total, so it describes a set rather than distinct works, and every file
+            // belongs to the one book the folder names.
+            var folder = @"D:\test\Jack of Shadows";
+            var files = Enumerable.Range(1, 4)
+                .Select(n => Path.Join(folder, $"Jack of Shadows {n:000} of 004.mp3"))
+                .ToArray();
+
+            var groups = UnmatchedScanBackgroundService.BuildGroupedFilesForFolder(
+                files,
+                folder,
+                FileSystemPathSemantics.CurrentHostDefault);
+
+            var group = Assert.Single(groups);
+            Assert.Equal(4, group.Count);
+        }
+
+        [Fact]
+        public void BuildGroupedFilesForFolder_GroupsBareNumberOfTotalFilenames()
+        {
+            // The same convention with no title in the filename at all. Stripping the index
+            // empties the stem, which is what lets the folder-name fallback gather them.
+            var folder = @"D:\test\Jack of Shadows";
+            var files = Enumerable.Range(1, 3)
+                .Select(n => Path.Join(folder, $"{n:000} of 003.mp3"))
+                .ToArray();
+
+            var groups = UnmatchedScanBackgroundService.BuildGroupedFilesForFolder(
+                files,
+                folder,
+                FileSystemPathSemantics.CurrentHostDefault);
+
+            var group = Assert.Single(groups);
+            Assert.Equal(3, group.Count);
+        }
+
+        [Fact]
+        public void BuildGroupedFilesForFolder_KeepsTitlesWhoseOwnWordsReadLikeAnIndex()
+        {
+            // "of" between two words is not an index, and a trailing number is not a total.
+            // Two separate works in one author folder must stay separate.
+            var folder = @"D:\test\Roger Zelazny";
+            var jack = Path.Join(folder, "Jack of Shadows.mp3");
+            var nine = Path.Join(folder, "Nine Princes in Amber 2.mp3");
+
+            var groups = UnmatchedScanBackgroundService.BuildGroupedFilesForFolder(
+                new[] { jack, nine },
+                folder,
+                FileSystemPathSemantics.CurrentHostDefault);
+
+            Assert.Equal(2, groups.Count);
+            Assert.Contains(groups, group => group.Single() == jack);
+            Assert.Contains(groups, group => group.Single() == nine);
+        }
     }
 }