Resolve RAR companion volumes with unambiguous case-insensitive fallback

docs/FORMATS.md

@@ -44,6 +44,7 @@
 ### Rar Format Notes
 
 - RAR5 encrypted headers and data accept KDF counts from 0 through 24 (the binary logarithm of the PBKDF2 iteration count), matching UnRAR's acceptance limit. Larger counts are rejected before key derivation.
+- RAR companion filenames use the same exact-match-first, case-insensitive fallback as ZIP, with ambiguous fallback matches rejected.
 - Encrypted ciphertext must contain complete AES blocks. Truncation throws `InvalidFormatException` on synchronous reads or `IncompleteArchiveException` on asynchronous reads.
 
 ### 7Zip Format Notes

src/SharpCompress/Archives/ArchiveVolumeFactory.cs

@@ -28,11 +28,32 @@ internal abstract class ArchiveVolumeFactory
             );
         }
 
-        if (item != null && item.Exists)
+        return item is null ? null : ResolveFilePart(item);
+    }
+
+    internal static FileInfo? ResolveFilePart(FileInfo item)
+    {
+        if (item.Exists)
         {
             return item;
         }
-
-        return null;
+        if (!item.Directory!.Exists)
+        {
+            return null;
+        }
+        FileInfo? match = null;
+        foreach (var file in item.Directory.EnumerateFiles())
+        {
+            if (!string.Equals(file.Name, item.Name, StringComparison.OrdinalIgnoreCase))
+            {
+                continue;
+            }
+            if (match is not null)
+            {
+                throw new InvalidFormatException($"Ambiguous archive volume name: {item.Name}");
+            }
+            match = file;
+        }
+        return match;
     }
 }

src/SharpCompress/Archives/Rar/RarArchiveVolumeFactory.cs

@@ -61,11 +61,6 @@ internal static class RarArchiveVolumeFactory
             }
         }
 
-        if (item != null && item.Exists)
-        {
-            return item;
-        }
-
-        return null; //no more items
+        return item is null ? null : ArchiveVolumeFactory.ResolveFilePart(item);
     }
 }

src/SharpCompress/Archives/Zip/ZipArchiveVolumeFactory.cs

@@ -32,31 +32,6 @@ internal static class ZipArchiveVolumeFactory
             return ArchiveVolumeFactory.GetFilePart(index, part1);
         }
 
-        if (item != null && item.Exists)
-        {
-            return item;
-        }
-
-        // Generated suffixes are lowercase, but split archives can use any on-disk casing.
-        // Prefer the exact path above and never choose arbitrarily between fallback matches.
-        if (item is not null && item.Directory!.Exists)
-        {
-            FileInfo? match = null;
-            foreach (var file in item.Directory!.EnumerateFiles())
-            {
-                if (!string.Equals(file.Name, item.Name, StringComparison.OrdinalIgnoreCase))
-                {
-                    continue;
-                }
-                if (match is not null)
-                {
-                    throw new InvalidFormatException($"Ambiguous ZIP volume name: {item.Name}");
-                }
-                match = file;
-            }
-            return match;
-        }
-
-        return null; //no more items
+        return item is null ? null : ArchiveVolumeFactory.ResolveFilePart(item);
     }
 }

tests/SharpCompress.Test/Rar/RarVolumeCasingTests.cs

@@ -0,0 +1,45 @@
+using System;
+using System.IO;
+using SharpCompress.Archives.Rar;
+using SharpCompress.Common;
+using Xunit;
+
+namespace SharpCompress.Test.Rar;
+
+public class RarVolumeCasingTests : TestBase
+{
+    [Theory]
+    [InlineData("sample.RAR", "SAMPLE.R00")]
+    [InlineData("sample.part01.RAR", "SAMPLE.PART02.rar")]
+    [InlineData("sample.rar.001", "SAMPLE.RAR.002")]
+    public void Companion_Lookup_Is_Case_Insensitive(string firstName, string nextName)
+    {
+        var first = new FileInfo(Path.Combine(SCRATCH_FILES_PATH, firstName));
+        var next = Path.Combine(SCRATCH_FILES_PATH, nextName);
+        File.WriteAllBytes(next, []);
+        Assert.Equal(
+            next,
+            RarArchiveVolumeFactory.GetFilePart(1, first)!.FullName,
+            ignoreCase: true
+        );
+        Assert.Null(RarArchiveVolumeFactory.GetFilePart(2, first));
+    }
+
+    [Fact]
+    public void Companion_Lookup_Prefers_Exact_Path_And_Rejects_Ambiguity()
+    {
+        var first = new FileInfo(Path.Combine(SCRATCH_FILES_PATH, "sample.rar"));
+        var upper = Path.Combine(SCRATCH_FILES_PATH, "SAMPLE.R00");
+        var mixed = Path.Combine(SCRATCH_FILES_PATH, "Sample.r00");
+        File.WriteAllBytes(upper, []);
+        if (File.Exists(mixed))
+        {
+            return; // Case-insensitive filesystems cannot represent ambiguous companion names.
+        }
+        File.WriteAllBytes(mixed, []);
+        Assert.Throws<InvalidFormatException>(() => RarArchiveVolumeFactory.GetFilePart(1, first));
+        var exact = Path.Combine(SCRATCH_FILES_PATH, "sample.r00");
+        File.WriteAllBytes(exact, []);
+        Assert.Equal(exact, RarArchiveVolumeFactory.GetFilePart(1, first)!.FullName);
+    }
+}