Add ObjectId getByte for random access

Processing git notes requires random access to part of the raw data
of each ObjectId... which isn't easy because ObjectIds are stored
with an internal representation of 5 ints.  Expose random access
to the individual data bytes through new methods, avoiding the
need to convert first to a byte[20].

Change-Id: I99e64700b27fc0c95aa14ef8ad46a0e8832d4441
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

1 files changed, 48 insertions, 2 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/AnyObjectId.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/AnyObjectId.java
index 7b30cec46f..61cc15dec0 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/AnyObjectId.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/AnyObjectId.java
@@ -97,15 +97,61 @@ public abstract class AnyObjectId implements Comparable {
 	int w5;
 
 	/**
-	 * For ObjectIdMap
+	 * Get the first 8 bits of the ObjectId.
 	 *
-	 * @return a discriminator usable for a fan-out style map
+	 * This is a faster version of {@code getByte(0)}.
+	 *
+	 * @return a discriminator usable for a fan-out style map. Returned values
+	 *         are unsigned and thus are in the range [0,255] rather than the
+	 *         signed byte range of [-128, 127].
 	 */
 	public final int getFirstByte() {
 		return w1 >>> 24;
 	}
 
 	/**
+	 * Get any byte from the ObjectId.
+	 *
+	 * Callers hard-coding {@code getByte(0)} should instead use the much faster
+	 * special case variant {@link #getFirstByte()}.
+	 *
+	 * @param index
+	 *            index of the byte to obtain from the raw form of the ObjectId.
+	 *            Must be in range [0, {@link Constants#OBJECT_ID_LENGTH}).
+	 * @return the value of the requested byte at {@code index}. Returned values
+	 *         are unsigned and thus are in the range [0,255] rather than the
+	 *         signed byte range of [-128, 127].
+	 * @throws ArrayIndexOutOfBoundsException
+	 *             {@code index} is less than 0, equal to
+	 *             {@link Constants#OBJECT_ID_LENGTH}, or greater than
+	 *             {@link Constants#OBJECT_ID_LENGTH}.
+	 */
+	public final int getByte(int index) {
+		int w;
+		switch (index >> 2) {
+		case 0:
+			w = w1;
+			break;
+		case 1:
+			w = w2;
+			break;
+		case 2:
+			w = w3;
+			break;
+		case 3:
+			w = w4;
+			break;
+		case 4:
+			w = w5;
+			break;
+		default:
+			throw new ArrayIndexOutOfBoundsException(index);
+		}
+
+		return (w >>> (8 * (3 - (index & 3)))) & 0xff;
+	}
+
+	/**
 	 * Compare this ObjectId to another and obtain a sort ordering.
 	 *
 	 * @param other