Fix handling of odd-length partial commit IDs

Author: AArnott

src/NerdBank.GitVersioning.Tests/GitContextTests.cs

@@ -81,7 +81,7 @@ public void SelectCommitByPartialId(bool fromPack, bool oddLength)
             this.Context = this.CreateGitContext(this.RepoPath, null);
         }
 
-        Assert.True(this.Context.TrySelectCommit(this.Context.GitCommitId.Substring(0, oddLength ? 11 : 13)));
+        Assert.True(this.Context.TrySelectCommit(this.Context.GitCommitId.Substring(0, oddLength ? 11 : 12)));
         Assert.Equal(this.LibGit2Repository.Head.Tip.Sha, this.Context.GitCommitId);
     }
 
@@ -92,7 +92,7 @@ public void SelectCommitByPartialId(bool fromPack, bool oddLength)
     [InlineData(11)]
     public void GetShortUniqueCommitId(int length)
     {
-        Skip.If(length < 7 && this.Context is Nerdbank.GitVersioning.LibGit2.LibGit2Context);
+        Skip.If(length < 7 && this.Context is Nerdbank.GitVersioning.LibGit2.LibGit2Context, "LibGit2Sharp never returns commit IDs with fewer than 7 characters.");
         Assert.Equal(this.Context.GitCommitId.Substring(0, length), this.Context.GetShortUniqueCommitId(length));
     }
 

src/NerdBank.GitVersioning/ManagedGit/GitPack.cs

@@ -130,13 +130,14 @@ public GitPack(GetObjectFromRepositoryDelegate getObjectFromRepositoryDelegate,
         /// <param name="objectId">
         /// A partial object ID.
         /// </param>
+        /// <param name="endsWithHalfByte"><inheritdoc cref="GitPackIndexReader.GetOffset(Span{byte}, bool)" path="/param[@name='endsWithHalfByte']"/></param>
         /// <returns>
         /// If found, a full object ID which matches the partial object ID.
         /// Otherwise, <see langword="false"/>.
         /// </returns>
-        public GitObjectId? Lookup(Span<byte> objectId)
+        public GitObjectId? Lookup(Span<byte> objectId, bool endsWithHalfByte = false)
         {
-            (var _, var actualObjectId) = this.indexReader.Value.GetOffset(objectId);
+            (var _, var actualObjectId) = this.indexReader.Value.GetOffset(objectId, endsWithHalfByte);
             return actualObjectId;
         }
 

src/NerdBank.GitVersioning/ManagedGit/GitPackIndexMappedReader.cs

@@ -42,21 +42,19 @@ public GitPackIndexMappedReader(FileStream stream)
             this.accessor.SafeMemoryMappedViewHandle.AcquirePointer(ref this.ptr);
         }
 
-        private Span<byte> Value
+        private ReadOnlySpan<byte> Value
         {
             get
             {
-                return new Span<byte>(this.ptr, (int)this.accessor.Capacity);
+                return new ReadOnlySpan<byte>(this.ptr, (int)this.accessor.Capacity);
             }
         }
 
         /// <inheritdoc/>
-        public override (long?, GitObjectId?) GetOffset(Span<byte> objectName)
+        public override (long?, GitObjectId?) GetOffset(Span<byte> objectName, bool endsWithHalfByte = false)
         {
             this.Initialize();
 
-            Span<byte> buffer = stackalloc byte[4];
-
             var packStart = this.fanoutTable[objectName[0]];
             var packEnd = this.fanoutTable[objectName[0] + 1];
             var objectCount = this.fanoutTable[256];
@@ -82,7 +80,20 @@ public override (long?, GitObjectId?) GetOffset(Span<byte> objectName)
             {
                 i = (packStart + packEnd) / 2;
 
-                order = table.Slice(20 * i, objectName.Length).SequenceCompareTo(objectName);
+                ReadOnlySpan<byte> comparand = table.Slice(20 * i, objectName.Length);
+                if (endsWithHalfByte)
+                {
+                    // Copy out the value to be checked so we can zero out the last four bits,
+                    // so that it matches the last 4 bits of the objectName that isn't supposed to be compared.
+                    Span<byte> buffer = stackalloc byte[20];
+                    comparand.CopyTo(buffer);
+                    buffer[objectName.Length - 1] &= 0xf0;
+                    order = buffer.Slice(0, objectName.Length).SequenceCompareTo(objectName);
+                }
+                else
+                {
+                    order = comparand.SequenceCompareTo(objectName);
+                }
 
                 if (order < 0)
                 {

src/NerdBank.GitVersioning/ManagedGit/GitPackIndexReader.cs

@@ -37,11 +37,12 @@ public abstract class GitPackIndexReader : IDisposable
         /// <param name="objectId">
         /// A partial or full Git object id, in its binary representation.
         /// </param>
+        /// <param name="endsWithHalfByte"><see langword="true"/> if <paramref name="objectId"/> ends with a byte whose last 4 bits are all zeros and not intended for inclusion in the search; <see langword="false"/> otherwise.</param>
         /// <returns>
         /// If found, the offset of the Git object in the index file; otherwise,
         /// <see langword="null"/>.
         /// </returns>
-        public abstract (long?, GitObjectId?) GetOffset(Span<byte> objectId);
+        public abstract (long?, GitObjectId?) GetOffset(Span<byte> objectId, bool endsWithHalfByte = false);
 
         /// <inheritdoc/>
         public abstract void Dispose();

src/NerdBank.GitVersioning/ManagedGit/GitRepository.cs

@@ -403,11 +403,19 @@ public GitCommit GetCommit(GitObjectId sha, bool readAuthor = false)
                 }
 
                 // Search for _any_ object whose id starts with objectish in the packfile
+                bool endsWithHalfByte = objectish.Length % 2 == 1;
+                if (endsWithHalfByte)
+                {
+                    // Add one more character so hex can be converted to bytes.
+                    // The bit length to be compared will not consider the last four bits.
+                    objectish += "0";
+                }
+
                 var hex = ConvertHexStringToByteArray(objectish);
 
                 foreach (var pack in this.packs.Value)
                 {
-                    var objectId = pack.Lookup(hex);
+                    var objectId = pack.Lookup(hex, endsWithHalfByte);
 
                     // It's possible for the same object to be present in both the object database and the pack files,
                     // or in multiple pack files.