Improve alternate parsing

src/NerdBank.GitVersioning.Tests/ManagedGit/GitRepositoryTests.cs

@@ -286,6 +286,38 @@ public void GetMissingObjectByShaTest()
             }
         }
 
+        [Fact]
+        public void ParseAlternates_SingleValue_Test()
+        {
+            var alternates = GitRepository.ParseAlternates(Encoding.UTF8.GetBytes("/home/git/nbgv/.git/objects\n"));
+            Assert.Collection(
+                alternates,
+                a => Assert.Equal("/home/git/nbgv/.git/objects", a));
+        }
+
+        [Fact]
+        public void ParseAlternates_TwoValues_Test()
+        {
+            var alternates = GitRepository.ParseAlternates(Encoding.UTF8.GetBytes("/home/git/nbgv/.git/objects:../../clone/.git/objects\n"));
+            Assert.Collection(
+                alternates,
+                a => Assert.Equal("/home/git/nbgv/.git/objects", a),
+                a => Assert.Equal("../../clone/.git/objects", a));
+        }
+
+        [Fact]
+        public void ParseAlternates_PathWithColon_Test()
+        {
+            var alternates = GitRepository.ParseAlternates(
+                Encoding.UTF8.GetBytes("C:/Users/nbgv/objects:C:/Users/nbgv2/objects/:../../clone/.git/objects\n"),
+                2);
+            Assert.Collection(
+                alternates,
+                a => Assert.Equal("C:/Users/nbgv/objects", a),
+                a => Assert.Equal("C:/Users/nbgv2/objects/", a),
+                a => Assert.Equal("../../clone/.git/objects", a));
+        }
+
         private static void AssertPath(string expected, string actual)
         {
             Assert.Equal(

src/NerdBank.GitVersioning/ManagedGit/GitRepository.cs

@@ -6,6 +6,7 @@
 using System.Globalization;
 using System.IO;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Text;
 
 namespace Nerdbank.GitVersioning.ManagedGit
@@ -110,16 +111,14 @@ public GitRepository(string workingDirectory, string gitDirectory, string common
                 var length = alternateStream!.Read(alternates);
                 alternates = alternates.Slice(0, length);
 
-                int index = 0;
-
-                while ((index = alternates.IndexOf((byte)':')) > 0)
+                foreach (var alternate in ParseAlternates(alternates))
                 {
-                    var alternate = GetString(alternates.Slice(0, index));
-                    alternate = Path.GetFullPath(Path.Combine(this.ObjectDirectory, alternate));
-
-                    this.alternates.Add(GitRepository.Create(workingDirectory, gitDirectory, commonDirectory, alternate));
-
-                    alternates = alternates.Slice(index + 1);
+                    this.alternates.Add(
+                        GitRepository.Create(
+                            workingDirectory,
+                            gitDirectory,
+                            commonDirectory,
+                            objectDirectory: Path.GetFullPath(Path.Combine(this.ObjectDirectory, alternate))));
                 }
             }
 
@@ -714,5 +713,47 @@ public static unsafe string GetString(ReadOnlySpan<byte> bytes)
                 return Encoding.GetString(pBytes, bytes.Length);
             }
         }
+
+        /// <summary>
+        /// Parses the contents of the alternates file, and returns a list of (relative) paths to the alternate object directories.
+        /// </summary>
+        /// <param name="alternates">
+        /// The contents of the alternates files.
+        /// </param>
+        /// <returns>
+        /// A list of (relative) paths to the alternate object directories.
+        /// </returns>
+        public static List<string> ParseAlternates(ReadOnlySpan<byte> alternates)
+            => ParseAlternates(alternates, RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? 2 : 0);
+
+        /// <summary>
+        /// Parses the contents of the alternates file, and returns a list of (relative) paths to the alternate object directories.
+        /// </summary>
+        /// <param name="alternates">
+        /// The contents of the alternates files.
+        /// </param>
+        /// <param name="skipCount">
+        /// The number of bytes to skip in the span when looking for a delimiter.
+        /// </param>
+        /// <returns>
+        /// A list of (relative) paths to the alternate object directories.
+        /// </returns>
+        public static List<string> ParseAlternates(ReadOnlySpan<byte> alternates, int skipCount)
+        {
+            List<string> values = new List<string>();
+
+            int index;
+
+            // The alternates path is colon (:)-separated. On Windows, there may be full paths, such as
+            // C:/Users/username/source/repos/nbgv/.git, which also contain a colon. Because the colon
+            // can only appear at the second position, we skip the first two characters (e.g. C:) on Windows.
+            while (alternates.Length > skipCount && (index = alternates.Slice(skipCount).IndexOfAny((byte)':', (byte)'\n')) > 0)
+            {
+                values.Add(GetString(alternates.Slice(0, skipCount + index)));
+                alternates = alternates.Slice(skipCount + index + 1);
+            }
+
+            return values;
+        }
     }
 }