Introduce ObjectId.StartsWith()

LibGit2Sharp.Tests/ObjectIdFixture.cs

@@ -136,5 +136,25 @@ namespace LibGit2Sharp.Tests
             Assert.True(maybeSha.StartsWith(parsedObjectId.ToString(3)));
             Assert.Equal(maybeSha, parsedObjectId.ToString(42));
         }
+
+        [Theory]
+        [InlineData(new byte[] { 0xde, 0xad, 0xbe }, 6, true)]
+        [InlineData(new byte[] { 0xde, 0xad }, 4, true)]
+        [InlineData(new byte[] { 0xde, 0xad }, 3, true)]
+        [InlineData(new byte[] { 0xde, 0xad }, 2, true)]
+        [InlineData(new byte[] { 0xde, 0xad }, 1, true)]
+        [InlineData(new byte[] { 0xde, 0xaf }, 3, true)]
+        [InlineData(new byte[] { 0xde, 0xff }, 2, true)]
+        [InlineData(new byte[] { 0xdf, 0xff }, 1, true)]
+        [InlineData(new byte[] { 0x98, 0x76 }, 4, false)]
+        [InlineData(new byte[] { 0x98, 0x76 }, 3, false)]
+        [InlineData(new byte[] { 0x98, 0x76 }, 2, false)]
+        [InlineData(new byte[] { 0x98, 0x76 }, 1, false)]
+        public void StartsWith(byte[] rawId, int len, bool expected)
+        {
+            var id = new ObjectId("deadbeef84650f067bd5703b6a59a8b3b3c99a09");
+
+            Assert.Equal(expected, id.StartsWith(rawId, len));
+        }
     }
 }

LibGit2Sharp.Tests/OdbBackendFixture.cs

@@ -198,30 +198,7 @@ namespace LibGit2Sharp.Tests
 
                 foreach (ObjectId objectId in m_objectIdToContent.Keys)
                 {
-                    bool match = true;
-
-                    int length = len >> 1;
-                    for (int i = 0; i < length; i++)
-                    {
-                        if (objectId.RawId[i] != shortOid[i])
-                        {
-                            match = false;
-                            break;
-                        }
-                    }
-
-                    if (match && ((len & 1) == 1))
-                    {
-                        var a = objectId.RawId[length] >> 4;
-                        var b = shortOid[length] >> 4;
-
-                        if (a != b)
-                        {
-                            match = false;
-                        }
-                    }
-
-                    if (!match)
+                    if (!objectId.StartsWith(shortOid, len))
                     {
                         continue;
                     }

LibGit2Sharp/ObjectId.cs

@@ -297,5 +297,52 @@ namespace LibGit2Sharp
 
             return objectId.All(c => hexDigits.Contains(c.ToString(CultureInfo.InvariantCulture)));
         }
+
+        /// <summary>
+        /// Determine whether the beginning of this instance matches the
+        /// <paramref name="len"/> first nibbles of <paramref name="rawId"/>.
+        /// </summary>
+        /// <param name="rawId">The byte array to compare the <see cref="ObjectId"/> against.</param>
+        /// <param name="len">The number of nibbles from <paramref name="rawId"/> </param>
+        /// <returns></returns>
+        public bool StartsWith(byte[] rawId, int len)
+        {
+            Ensure.ArgumentNotNull(rawId, "rawId");
+
+            if (len < 1 || len > HexSize)
+            {
+                throw new ArgumentOutOfRangeException("len");
+            }
+
+            if (len > rawId.Length * 2)
+            {
+                throw new ArgumentOutOfRangeException("len", "len exceeds the size of rawId");
+            }
+
+            bool match = true;
+
+            int length = len >> 1;
+            for (int i = 0; i < length; i++)
+            {
+                if (RawId[i] != rawId[i])
+                {
+                    match = false;
+                    break;
+                }
+            }
+
+            if (match && ((len & 1) == 1))
+            {
+                var a = RawId[length] >> 4;
+                var b = rawId[length] >> 4;
+
+                if (a != b)
+                {
+                    match = false;
+                }
+            }
+
+            return match;
+        }
     }
 }