path: root/bundles/org.eclipse.compare/compare/org/eclipse/compare/rangedifferencer/RangeDifferencer.java

/*
 * Licensed Materials - Property of IBM,
 * WebSphere Studio Workbench
 * (c) Copyright IBM Corp 2000, 2001
 */
package org.eclipse.compare.rangedifferencer;

import java.util.ArrayList;
import java.util.List;

import org.eclipse.jface.util.Assert;

/**
 * For finding the differences between two or three <code>IRangeComparator</code>s.
 * <p>
 * To use the differencer, clients provide an <code>IRangeComparator</code>
 * that breaks their input data into a sequence of comparable entities. The differencer
 * returns the differences among these sequences as an array of <code>RangeDifference</code> objects
 * (<code>findDifferences</code> methods).
 * Every <code>RangeDifference</code> represents a single kind of difference
 * and the corresponding ranges of the underlying comparable entities in the
 * left, right, and optionally ancestor sides.
 * <p>
 * Alternatively, the <code>findRanges</code> methods not only return objects for
 * the differing ranges but for non-differing ranges too.
 * <p>
 * The algorithm used is an objectified version of one described in:
 * <it>A File Comparison Program,</it> by Webb Miller and Eugene W. Myers, 
 * Software Practice and Experience, Vol. 15, Nov. 1985.
 *
 * @see IRangeComparator
 * @see RangeDifference
 */
public final class RangeDifferencer {
	
	private static final RangeDifference[] EMPTY_RESULT= new RangeDifference[0];
	
	/* (non Javadoc)
	 * Non instantiatiable!
	 */
	private RangeDifferencer() { 
	}
	
	/**
	 * Finds the differences between two <code>IRangeComparator</code>s.
	 * The differences are returned as a list of <code>RangeDifference</code>s.
	 * If no differences are detected an empty list is returned.
	 * 
	 * @param left the left range comparator
	 * @param right the right range comparator
	 * @return an array of range differences, or an empty array if no differences were found
	 */
	public static RangeDifference[] findDifferences(IRangeComparator left, IRangeComparator right) {

		// assert that both IRangeComparators are of the same class
		Assert.isTrue(right.getClass().equals(left.getClass()));

		int rightSize= right.getRangeCount();
		int leftSize= left.getRangeCount();
		//
		// Differences matrix:
		// only the last d of each diagonal is stored, i.e., lastDiagonal[k] = row of d    
		//
		int diagLen= 2 * Math.max(rightSize, leftSize); // bound on the size of edit script
		int maxDiagonal= diagLen;
		int lastDiagonal[]= new int[diagLen + 1]; // the row containing the last d
		// on diagonal k (lastDiagonal[k] = row)
		int origin= diagLen / 2; // origin of diagonal 0

		// script corresponding to d[k] 
		LinkedRangeDifference script[]= new LinkedRangeDifference[diagLen + 1];
		int row, col;

		// find common prefix
		for (row= 0; row < rightSize && row < leftSize && rangesEqual(right, row, left, row) == true; ++row);

		lastDiagonal[origin]= row;
		script[origin]= null;
		int lower= (row == rightSize) ? origin + 1 : origin - 1;
		int upper= (row == leftSize) ? origin - 1 : origin + 1;

		if (lower > upper)
			return EMPTY_RESULT;

		// for each value of the edit distance
		for (int d= 1; d <= maxDiagonal; ++d) { // d is the current edit distance

			if (right.skipRangeComparison(d, maxDiagonal, left))
				return EMPTY_RESULT; // should be something we already found

			// for each relevant diagonal (-d, -d+2 ..., d-2, d)
			for (int k= lower; k <= upper; k += 2) { // k is the current diagonal
				LinkedRangeDifference edit;

				if (k == origin - d || k != origin + d && lastDiagonal[k + 1] >= lastDiagonal[k - 1]) {
					//
					// move down
					//
					row= lastDiagonal[k + 1] + 1;
					edit= new LinkedRangeDifference(script[k + 1], LinkedRangeDifference.DELETE);
				} else {
					//
					// move right
					//
					row= lastDiagonal[k - 1];
					edit= new LinkedRangeDifference(script[k - 1], LinkedRangeDifference.INSERT);
				}
				col= row + k - origin;
				edit.fRightStart= row;
				edit.fLeftStart= col;
				Assert.isTrue(k >= 0 && k <= maxDiagonal, "Indices out of range");
				script[k]= edit;

				// slide down the diagonal as far as possible 
				while (row < rightSize && col < leftSize && rangesEqual(right, row, left, col) == true) {
					++row;
					++col;
				}

				Assert.isTrue(k >= 0 && k <= maxDiagonal, "unreasonable value for diagonal index");
				lastDiagonal[k]= row;

				if (row == rightSize && col == leftSize) {
					//showScript(script[k], right, left);
					return createDifferencesRanges(script[k]);
				}
				if (row == rightSize)
					lower= k + 2;
				if (col == leftSize)
					upper= k - 2;
			}
			--lower;
			++upper;
		}
		// too many differences
		Assert.isTrue(false);
		return null;
	}

	/**
	 * Finds the differences among three <code>IRangeComparator</code>s.
	 * The differences are returned as a list of <code>RangeDifference</code>s.
	 * If no differences are detected an empty list is returned.
	 * If the ancestor range comparator is <code>null</code>, a two-way
	 * comparison is performed.
	 * 
	 * @param ancestor the ancestor range comparator or <code>null</code>
	 * @param left the left range comparator
	 * @param right the right range comparator
	 * @return an array of range differences, or an empty array if no differences were found
	 */
	public static RangeDifference[] findDifferences(IRangeComparator ancestor, IRangeComparator left, IRangeComparator right) {

		if (ancestor == null)
			return findDifferences(left, right);

		RangeDifference[] leftAncestorScript= null;
		RangeDifference[] rightAncestorScript= findDifferences(ancestor, right);
		if (rightAncestorScript != null)
			leftAncestorScript= findDifferences(ancestor, left);
		if (rightAncestorScript == null || leftAncestorScript == null)
			return null;

		DifferencesIterator myIter= new DifferencesIterator(rightAncestorScript);
		DifferencesIterator yourIter= new DifferencesIterator(leftAncestorScript);

		List diff3= new ArrayList();
		diff3.add(new RangeDifference(RangeDifference.ERROR)); // add a sentinel

		int changeRangeStart= 0;
		int changeRangeEnd= 0;
		//
		// Combine the two two-way edit scripts into one
		//
		while (myIter.fDifference != null || yourIter.fDifference != null) {

			DifferencesIterator startThread;
			myIter.removeAll();
			yourIter.removeAll();
			//
			// take the next diff that is closer to the start
			//
			if (myIter.fDifference == null)
				startThread= yourIter;
			else if (yourIter.fDifference == null)
				startThread= myIter;
			else { // not at end of both scripts take the lowest range
				if (myIter.fDifference.fLeftStart <= yourIter.fDifference.fLeftStart) // 2 -> common (Ancestor) change range
					startThread= myIter;
				else
					startThread= yourIter;
			}
			changeRangeStart= startThread.fDifference.fLeftStart;
			changeRangeEnd= startThread.fDifference.leftEnd();

			startThread.next();
			//
			// check for overlapping changes with other thread
			// merge overlapping changes with this range
			//
			DifferencesIterator other= startThread.other(myIter, yourIter);
			while (other.fDifference != null && other.fDifference.fLeftStart <= changeRangeEnd) {
				int newMax= other.fDifference.leftEnd();
				other.next();
				if (newMax >= changeRangeEnd) {
					changeRangeEnd= newMax;
					other= other.other(myIter, yourIter);
				}
			}
			diff3.add(createRangeDifference3(myIter, yourIter, diff3, right, left, changeRangeStart, changeRangeEnd));
		}

		// remove sentinel
		diff3.remove(0);
		return (RangeDifference[]) diff3.toArray(EMPTY_RESULT);
	}

	/**
	 * Finds the differences among two <code>IRangeComparator</code>s.
	 * In contrast to <code>findDifferences</code>, the result
	 * contains <code>RangeDifference</code> elements for non-differing ranges too.
	 * 
	 * @param left the left range comparator
	 * @param right the right range comparator
	 * @return an array of range differences
	 */
	public static RangeDifference[] findRanges(IRangeComparator left, IRangeComparator right) {
		RangeDifference[] in= findDifferences(left, right);
		List out= new ArrayList();

		RangeDifference rd;

		int mstart= 0;
		int ystart= 0;

		for (int i= 0; i < in.length; i++) {
			RangeDifference es= in[i];

			rd= new RangeDifference(RangeDifference.NOCHANGE, mstart, es.rightStart() - mstart, ystart, es.leftStart() - ystart);
			if (rd.maxLength() != 0)
				out.add(rd);

			out.add(es);

			mstart= es.rightEnd();
			ystart= es.leftEnd();
		}
		rd= new RangeDifference(RangeDifference.NOCHANGE, mstart, right.getRangeCount() - mstart, ystart, left.getRangeCount() - ystart);
		if (rd.maxLength() > 0)
			out.add(rd);

		return (RangeDifference[]) out.toArray(EMPTY_RESULT);
	}

	/**
	 * Finds the differences among three <code>IRangeComparator</code>s.
	 * In contrast to <code>findDifferences</code>, the result
	 * contains <code>RangeDifference</code> elements for non-differing ranges too.
	 * If the ancestor range comparator is <code>null</code>, a two-way
	 * comparison is performed.
	 * 
	 * @param ancestor the ancestor range comparator or <code>null</code>
	 * @param left the left range comparator
	 * @param right the right range comparator
	 * @return an array of range differences
	 */
	public static RangeDifference[] findRanges(IRangeComparator ancestor, IRangeComparator left, IRangeComparator right) {

		if (ancestor == null)
			return findRanges(left, right);

		RangeDifference[] in= findDifferences(ancestor, left, right);
		List out= new ArrayList();

		RangeDifference rd;

		int mstart= 0;
		int ystart= 0;
		int astart= 0;

		for (int i= 0; i < in.length; i++) {
			RangeDifference es= (RangeDifference) in[i];

			rd= new RangeDifference(RangeDifference.NOCHANGE, mstart, es.rightStart() - mstart, ystart, es.leftStart() - ystart, astart, es.ancestorStart() - astart);
			if (rd.maxLength() > 0)
				out.add(rd);

			out.add(es);

			mstart= es.rightEnd();
			ystart= es.leftEnd();
			astart= es.ancestorEnd();
		}
		rd= new RangeDifference(RangeDifference.NOCHANGE, mstart, right.getRangeCount() - mstart, ystart, left.getRangeCount() - ystart, astart, ancestor.getRangeCount() - astart);
		if (rd.maxLength() > 0)
			out.add(rd);

		return (RangeDifference[]) out.toArray(EMPTY_RESULT);
	}

	//---- private methods

	/**
	 * Creates a Vector of DifferencesRanges out of the LinkedRangeDifference.
	 * It coalesces adjacent changes.
	 * In addition, indices are changed such that the ranges are 1) open, i.e,
	 * the end of the range is not included, and 2) are zero based.
	 */
	private static RangeDifference[] createDifferencesRanges(LinkedRangeDifference start) {

		LinkedRangeDifference ep= reverseDifferences(start);
		ArrayList result= new ArrayList();
		RangeDifference es= null;

		while (ep != null) {
			es= new RangeDifference(RangeDifference.CHANGE);

			if (ep.isInsert()) {
				es.fRightStart= ep.fRightStart + 1;
				es.fLeftStart= ep.fLeftStart;
				RangeDifference b= ep;
				do {
					ep= ep.getNext();
					es.fLeftLength++;
				} while (ep != null && ep.isInsert() && ep.fRightStart == b.fRightStart);
			} else {
				es.fRightStart= ep.fRightStart;
				es.fLeftStart= ep.fLeftStart;

				RangeDifference a= ep;
				//
				// deleted lines
				//
				do {
					a= ep;
					ep= ep.getNext();
					es.fRightLength++;
				} while (ep != null && ep.isDelete() && ep.fRightStart == a.fRightStart + 1);

				boolean change= (ep != null && ep.isInsert() && ep.fRightStart == a.fRightStart);

				if (change) {
					RangeDifference b= ep;
					//
					// replacement lines
					//
					do {
						ep= ep.getNext();
						es.fLeftLength++;
					} while (ep != null && ep.isInsert() && ep.fRightStart == b.fRightStart);
				} else {
					es.fLeftLength= 0;
				}
				es.fLeftStart++; // meaning of range changes from "insert after", to "replace with"

			}
			//
			// the script commands are 1 based, subtract one to make them zero based
			//
			es.fRightStart--;
			es.fLeftStart--;
			result.add(es);
		}
		return (RangeDifference[]) result.toArray(EMPTY_RESULT);
	}

	/**
	 * Creates a <code>RangeDifference3</code> given the
	 * state of two DifferenceIterators.
	 */
	private static RangeDifference createRangeDifference3(
		DifferencesIterator myIter, 
		DifferencesIterator yourIter, 
		List diff3, 
		IRangeComparator right, 
		IRangeComparator left, 
		int changeRangeStart, 
		int changeRangeEnd) {

		int rightStart, rightEnd;
		int leftStart, leftEnd;
		int kind= RangeDifference.ERROR;
		RangeDifference last= (RangeDifference) diff3.get(diff3.size() - 1);

		Assert.isTrue((myIter.getCount() != 0 || yourIter.getCount() != 0), "at least one range array must be non-empty");
		//
		// find corresponding lines to fChangeRangeStart/End in right and left
		//
		if (myIter.getCount() == 0) { // only left changed
			rightStart= changeRangeStart - last.ancestorEnd() + last.rightEnd();
			rightEnd= changeRangeEnd - last.ancestorEnd() + last.rightEnd();
			kind= RangeDifference.LEFT;
		} else {
			RangeDifference f= (RangeDifference) myIter.fRange.get(0);
			RangeDifference l= (RangeDifference) myIter.fRange.get(myIter.fRange.size() - 1);
			rightStart= changeRangeStart - f.fLeftStart + f.fRightStart;
			rightEnd= changeRangeEnd - l.leftEnd() + l.rightEnd();
		}

		if (yourIter.getCount() == 0) { // only right changed
			leftStart= changeRangeStart - last.ancestorEnd() + last.leftEnd();
			leftEnd= changeRangeEnd - last.ancestorEnd() + last.leftEnd();
			kind= RangeDifference.RIGHT;
		} else {
			RangeDifference f= (RangeDifference) yourIter.fRange.get(0);
			RangeDifference l= (RangeDifference) yourIter.fRange.get(yourIter.fRange.size() - 1);
			leftStart= changeRangeStart - f.fLeftStart + f.fRightStart;
			leftEnd= changeRangeEnd - l.leftEnd() + l.rightEnd();
		}

		if (kind == RangeDifference.ERROR) { // overlapping change (conflict) -> compare the changed ranges
			if (rangeSpansEqual(right, rightStart, rightEnd - rightStart, left, leftStart, leftEnd - leftStart))
				kind= RangeDifference.ANCESTOR;
			else
				kind= RangeDifference.CONFLICT;
		}
		return new RangeDifference(kind, rightStart, rightEnd - rightStart, leftStart, leftEnd - leftStart, changeRangeStart, changeRangeEnd - changeRangeStart);
	}

	/**
	 * Tests if two ranges are equal
	 */
	private static boolean rangesEqual(IRangeComparator a, int ai, IRangeComparator b, int bi) {
		return a.rangesEqual(ai, b, bi);
	}

	/**
	 * Tests whether <code>right</code> and <code>left</left> changed in the same way
	 */
	private static boolean rangeSpansEqual(IRangeComparator right, int rightStart, int rightLen, IRangeComparator left, int leftStart, int leftLen) {
		if (rightLen == leftLen) {
			int i= 0;
			for (i= 0; i < rightLen; i++) {
				if (!rangesEqual(right, rightStart + i, left, leftStart + i))
					break;
			}
			if (i == rightLen)
				return true;
		}
		return false;
	}

	/**
	 * Reverses the range differences
	 */
	private static LinkedRangeDifference reverseDifferences(LinkedRangeDifference start) {
		LinkedRangeDifference ep, behind, ahead;

		ahead= start;
		ep= null;
		while (ahead != null) {
			behind= ep;
			ep= ahead;
			ahead= ahead.getNext();
			ep.setNext(behind);
		}
		return ep;
	}
}