From b22a4e84886e7388d509376be9afce31833de054 Mon Sep 17 00:00:00 2001
From: "Shawn O. Pearce" <spearce@spearce.org>
Date: Tue, 2 Nov 2010 16:29:39 -0700
Subject: [PATCH] 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>
---
 ...{T0001_ObjectId.java => ObjectIdTest.java} | 16 +++++-
 .../src/org/eclipse/jgit/lib/AnyObjectId.java | 50 ++++++++++++++++++-
 2 files changed, 63 insertions(+), 3 deletions(-)
 rename org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/{T0001_ObjectId.java => ObjectIdTest.java} (89%)

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/T0001_ObjectId.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java
similarity index 89%
rename from org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/T0001_ObjectId.java
rename to org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java
index 03176cb8f..2eb1e6b9e 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/T0001_ObjectId.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdTest.java
@@ -47,7 +47,7 @@ package org.eclipse.jgit.lib;
 
 import junit.framework.TestCase;
 
-public class T0001_ObjectId extends TestCase {
+public class ObjectIdTest extends TestCase {
 	public void test001_toString() {
 		final String x = "def4c620bc3713bb1bb26b808ec9312548e73946";
 		final ObjectId oid = ObjectId.fromString(x);
@@ -108,4 +108,18 @@ public class T0001_ObjectId extends TestCase {
 		final ObjectId oid = ObjectId.fromString(x);
 		assertEquals(x.toLowerCase(), oid.name());
 	}
+
+	public void testGetByte() {
+		byte[] raw = new byte[20];
+		for (int i = 0; i < 20; i++)
+			raw[i] = (byte) (0xa0 + i);
+		ObjectId id = ObjectId.fromRaw(raw);
+
+		assertEquals(raw[0] & 0xff, id.getFirstByte());
+		assertEquals(raw[0] & 0xff, id.getByte(0));
+		assertEquals(raw[1] & 0xff, id.getByte(1));
+
+		for (int i = 2; i < 20; i++)
+			assertEquals("index " + i, raw[i] & 0xff, id.getByte(i));
+	}
 }
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 7b30cec46..61cc15dec 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/AnyObjectId.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/AnyObjectId.java
@@ -97,14 +97,60 @@ 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.
 	 *