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

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package org.eclipse.jgit.diff;

/**
 * Equivalence function for a {@link Sequence} compared by difference algorithm.
 *
 * Difference algorithms can use a comparator to compare portions of two
 * sequences and discover the minimal edits required to transform from one
 * sequence to the other sequence.
 *
 * Indexes within a sequence are zero-based.
 *
 * @param <S>
 *            type of sequence the comparator supports.
 */
public abstract class SequenceComparator<S extends Sequence> {
	/**
	 * Compare two items to determine if they are equivalent.
	 *
	 * It is permissible to compare sequence {@code a} with itself (by passing
	 * {@code a} again in position {@code b}).
	 *
	 * @param a
	 *            the first sequence.
	 * @param ai
	 *            item of {@code ai} to compare.
	 * @param b
	 *            the second sequence.
	 * @param bi
	 *            item of {@code bi} to compare.
	 * @return true if the two items are identical according to this function's
	 *         equivalence rule.
	 */
	public abstract boolean equals(S a, int ai, S b, int bi);

	/**
	 * Get a hash value for an item in a sequence.
	 *
	 * If two items are equal according to this comparator's
	 * {@link #equals(Sequence, int, Sequence, int)} method, then this hash
	 * method must produce the same integer result for both items.
	 *
	 * It is not required for two items to have different hash values if they
	 * are are unequal according to the {@code equals()} method.
	 *
	 * @param seq
	 *            the sequence.
	 * @param ptr
	 *            the item to obtain the hash for.
	 * @return hash the hash value.
	 */
	public abstract int hash(S seq, int ptr);

	/**
	 * Modify the edit to remove common leading and trailing items.
	 *
	 * The supplied edit {@code e} is reduced in size by moving the beginning A
	 * and B points so the edit does not cover any items that are in common
	 * between the two sequences. The ending A and B points are also shifted to
	 * remove common items from the end of the region.
	 *
	 * @param a
	 *            the first sequence.
	 * @param b
	 *            the second sequence.
	 * @param e
	 *            the edit to start with and update.
	 * @return {@code e} if it was updated in-place, otherwise a new edit
	 *         containing the reduced region.
	 */
	public Edit reduceCommonStartEnd(S a, S b, Edit e) {
		// Skip over items that are common at the start.
		//
		while (e.beginA < e.endA && e.beginB < e.endB
				&& equals(a, e.beginA, b, e.beginB)) {
			e.beginA++;
			e.beginB++;
		}

		// Skip over items that are common at the end.
		//
		while (e.beginA < e.endA && e.beginB < e.endB
				&& equals(a, e.endA - 1, b, e.endB - 1)) {
			e.endA--;
			e.endB--;
		}

		return e;
	}
}