Fixed: Prefer 13 character ISBN from epub metadata

Fixes #1210

src/NzbDrone.Core.Test/MediaFiles/EbookTagServiceFixture.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using System.Linq;
+using FizzWare.NBuilder;
+using FluentAssertions;
+using NUnit.Framework;
+using NzbDrone.Core.MediaFiles;
+using NzbDrone.Core.Test.Framework;
+using NzbDrone.Test.Common;
+using VersOne.Epub.Schema;
+
+namespace NzbDrone.Core.Test.MediaFiles.AudioTagServiceFixture
+{
+    [TestFixture]
+    public class EbookTagServiceFixture : CoreTest<EBookTagService>
+    {
+        [Test]
+        public void should_prefer_isbn13()
+        {
+            var ids = Builder<EpubMetadataIdentifier>
+                .CreateListOfSize(2)
+                .TheFirst(1)
+                .With(x => x.Identifier = "4087738574")
+                .TheNext(1)
+                .With(x => x.Identifier = "9781455546176")
+                .Build()
+                .ToList();
+
+            Subject.GetIsbn(ids).Should().Be("9781455546176");
+        }
+    }
+}

src/NzbDrone.Core/MediaFiles/EbookTagService.cs

@@ -377,18 +377,15 @@ namespace NzbDrone.Core.MediaFiles
             return result;
         }
 
-        private string GetIsbn(IEnumerable<EpubMetadataIdentifier> ids)
+        public string GetIsbn(IEnumerable<EpubMetadataIdentifier> ids)
         {
-            foreach (var id in ids)
-            {
-                var isbn = StripIsbn(id?.Identifier);
-                if (isbn != null)
-                {
-                    return isbn;
-                }
-            }
+            var candidates = ids.Select(x => StripIsbn(x?.Identifier))
+                .Where(x => x != null)
+                .OrderByDescending(x => x.Length);
 
-            return null;
+            return candidates.FirstOrDefault(x => x.StartsWith("978"))
+                ?? candidates.FirstOrDefault(x => x.StartsWith("979"))
+                ?? candidates.FirstOrDefault();
         }
 
         private string GetIsbnChars(string input)