Define a subsequence utility type

A diff algorithm may find this type useful if it wants to delegate a
particular range of elements to another algorithm, without changing
the underlying sequence types.

Change-Id: I4544467781233e21ac8b35081304b2bad7db00f6
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/diff/Subsequence.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright (C) 2010, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.diff;
+
+/**
+ * Wraps a {@link Sequence} to have a narrower range of elements.
+ *
+ * This sequence acts as a proxy for the real sequence, translating element
+ * indexes on the fly by adding {@code begin} to them. Sequences of this type
+ * must be used with a {@link SubsequenceComparator}.
+ *
+ * @param <S>
+ *            the base sequence type.
+ */
+public final class Subsequence<S extends Sequence> extends Sequence {
+	final S base;
+
+	final int begin;
+
+	private final int size;
+
+	/**
+	 * Construct a subset of another sequence.
+	 *
+	 * The size of the subsequence will be {@code end - begin}.
+	 *
+	 * @param base
+	 *            the real sequence.
+	 * @param begin
+	 *            First element index of {@code base} that will be part of this
+	 *            new subsequence. The element at {@code begin} will be this
+	 *            sequence's element 0.
+	 * @param end
+	 *            One past the last element index of {@code base} that will be
+	 *            part of this new subsequence.
+	 */
+	public Subsequence(S base, int begin, int end) {
+		this.base = base;
+		this.begin = begin;
+		this.size = end - begin;
+	}
+
+	@Override
+	public int size() {
+		return size;
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/diff/SubsequenceComparator.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2010, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.diff;
+
+/**
+ * Wrap another comparator for use with {@link Subsequence}.
+ *
+ * This comparator acts as a proxy for the real comparator, translating element
+ * indexes on the fly by adding the subsequence's begin offset to them.
+ * Comparators of this type must be used with a {@link Subsequence}.
+ *
+ * @param <S>
+ *            the base sequence type.
+ */
+public final class SubsequenceComparator<S extends Sequence> extends
+		SequenceComparator<Subsequence<S>> {
+	private final SequenceComparator<? super S> cmp;
+
+	/**
+	 * Construct a comparator wrapping another comparator.
+	 *
+	 * @param cmp
+	 *            the real comparator.
+	 */
+	public SubsequenceComparator(SequenceComparator<? super S> cmp) {
+		this.cmp = cmp;
+	}
+
+	@Override
+	public boolean equals(Subsequence<S> a, int ai, Subsequence<S> b, int bi) {
+		return cmp.equals(a.base, ai + a.begin, b.base, bi + b.begin);
+	}
+
+	@Override
+	public int hash(Subsequence<S> seq, int ptr) {
+		return cmp.hash(seq.base, ptr + seq.begin);
+	}
+}