/*******************************************************************************
* Copyright (c) 2000, 2006 IBM Corporation and others.
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License v1.0
* which accompanies this distribution, and is available at
* http://www.eclipse.org/legal/epl-v10.html
*
* Contributors:
* IBM Corporation - initial API and implementation
*******************************************************************************/
package org.eclipse.compare.internal;
/* Used to determine the change set responsible for each line */
public abstract class LCS {
private int max_differences; // the maximum number of differences
// from
// each end to consider
private int length;
/**
* Myers' algorithm for longest common subsequence. O((M + N)D) worst case
* time, O(M + N + D^2) expected time, O(M + N) space
* (http://citeseer.ist.psu.edu/myers86ond.html)
*
* Note: Beyond implementing the algorithm as described in the paper I have
* added diagonal range compression which helps when finding the LCS of a
* very long and a very short sequence, also bound the running time to (N +
* M)^1.5 when both sequences are very long.
*
* After this method is called, the longest common subsequence is available
* by calling getResult() where result[0] is composed of entries from l1 and
* result[1] is composed of entries from l2
*
* @param subMonitor
*/
public void longestCommonSubsequence(LCSSettings settings) {
int length1 = getLength1();
int length2 = getLength2();
if (length1 == 0 || length2 == 0) {
length = 0;
return;
}
max_differences = (length1 + length2 + 1) / 2; // ceil((N+M)/2)
if ((double) length1 * (double) length2 > settings.getTooLong()) {
// limit complexity to D^POW_LIMIT for long sequences
max_differences = (int) Math.pow(max_differences, settings
.getPowLimit() - 1.0);
}
initializeLcs(length1);
/*
* The common prefixes and suffixes are always part of some LCS, include
* them now to reduce our search space
*/
int forwardBound;
int max = Math.min(length1, length2);
for (forwardBound = 0; forwardBound < max
&& isRangeEqual(forwardBound, forwardBound); forwardBound++) {
setLcs(forwardBound, forwardBound);
}
int backBoundL1 = length1 - 1;
int backBoundL2 = length2 - 1;
while (backBoundL1 >= forwardBound && backBoundL2 >= forwardBound
&& isRangeEqual(backBoundL1, backBoundL2)) {
setLcs(backBoundL1, backBoundL2);
backBoundL1--;
backBoundL2--;
}
length = forwardBound
+ length1
- backBoundL1
- 1
+ lcs_rec(forwardBound, backBoundL1, forwardBound, backBoundL2,
new int[2][length1 + length2 + 1], new int[3]);
}
/**
* The recursive helper function for Myers' LCS. Computes the LCS of
* l1[bottoml1 .. topl1] and l2[bottoml2 .. topl2] fills in the appropriate
* location in lcs and returns the length
*
* @param l1
* The 1st sequence
* @param bottoml1
* Index in the 1st sequence to start from (inclusive)
* @param topl1
* Index in the 1st sequence to end on (inclusive)
* @param l2
* The 2nd sequence
* @param bottoml2
* Index in the 2nd sequence to start from (inclusive)
* @param topl2
* Index in the 2nd sequence to end on (inclusive)
* @param V
* should be allocated as int[2][l1.length + l2.length + 1],
* used to store furthest reaching D-paths
* @param snake
* should be allocated as int[3], used to store the beginning
* x, y coordinates and the length of the latest snake
* traversed
* @param subMonitor
* @param lcs
* should be allocated as TextLine[2][l1.length], used to
* store the common points found to be part of the LCS where
* lcs[0] references lines of l1 and lcs[1] references lines
* of l2.
*
* @return the length of the LCS
*/
private int lcs_rec(int bottoml1, int topl1, int bottoml2, int topl2,
int[][] V, int[] snake) {
// check that both sequences are non-empty
if (bottoml1 > topl1 || bottoml2 > topl2) {
return 0;
}
int d = find_middle_snake(bottoml1, topl1, bottoml2, topl2, V, snake);
// System.out.println(snake[0] + " " + snake[1] + " " + snake[2]);
// need to store these so we don't lose them when they're overwritten by
// the recursion
int len = snake[2];
int startx = snake[0];
int starty = snake[1];
// the middle snake is part of the LCS, store it
for (int i = 0; i < len; i++) {
setLcs(startx + i, starty + i);
}
if (d > 1) {
return len
+ lcs_rec(bottoml1, startx - 1, bottoml2, starty - 1, V,
snake)
+ lcs_rec(startx + len, topl1, starty + len, topl2, V,
snake);
} else if (d == 1) {
/*
* In this case the sequences differ by exactly 1 line. We have
* already saved all the lines after the difference in the for loop
* above, now we need to save all the lines before the difference.
*/
int max = Math.min(startx - bottoml1, starty - bottoml2);
for (int i = 0; i < max; i++) {
setLcs(bottoml1 + i, bottoml2 + i);
}
return max + len;
}
return len;
}
/**
* Helper function for Myers' LCS algorithm to find the middle snake for
* l1[bottoml1..topl1] and l2[bottoml2..topl2] The x, y coodrdinates of the
* start of the middle snake are saved in snake[0], snake[1] respectively
* and the length of the snake is saved in s[2].
*
* @param l1
* The 1st sequence
* @param bottoml1
* Index in the 1st sequence to start from (inclusive)
* @param topl1
* Index in the 1st sequence to end on (inclusive)
* @param l2
* The 2nd sequence
* @param bottoml2
* Index in the 2nd sequence to start from (inclusive)
* @param topl2
* Index in the 2nd sequence to end on (inclusive)
* @param V
* should be allocated as int[2][l1.length + l2.length + 1],
* used to store furthest reaching D-paths
* @param snake
* should be allocated as int[3], used to store the beginning
* x, y coordinates and the length of the middle snake
*
* @return The number of differences (SES) between l1[bottoml1..topl1] and
* l2[bottoml2..topl2]
*/
private int find_middle_snake(int bottoml1, int topl1, int bottoml2,
int topl2, int[][] V, int[] snake) {
int N = topl1 - bottoml1 + 1;
int M = topl2 - bottoml2 + 1;
// System.out.println("N: " + N + " M: " + M + " bottom: " + bottoml1 +
// ", " +
// bottoml2 + " top: " + topl1 + ", " + topl2);
int delta = N - M;
boolean isEven;
if ((delta & 1) == 1) {
isEven = false;
} else {
isEven = true;
}
int limit = Math.min(max_differences, (N + M + 1) / 2); // ceil((N+M)/2)
int value_to_add_forward; // a 0 or 1 that we add to the start
// offset
// to make it odd/even
if ((M & 1) == 1) {
value_to_add_forward = 1;
} else {
value_to_add_forward = 0;
}
int value_to_add_backward;
if ((N & 1) == 1) {
value_to_add_backward = 1;
} else {
value_to_add_backward = 0;
}
int start_forward = -M;
int end_forward = N;
int start_backward = -N;
int end_backward = M;
V[0][limit + 1] = 0;
V[1][limit - 1] = N;
for (int d = 0; d <= limit; d++) {
int start_diag = Math.max(value_to_add_forward + start_forward, -d);
int end_diag = Math.min(end_forward, d);
value_to_add_forward = 1 - value_to_add_forward;
// compute forward furthest reaching paths
for (int k = start_diag; k <= end_diag; k += 2) {
int x;
if (k == -d
|| (k < d && V[0][limit + k - 1] < V[0][limit + k + 1])) {
x = V[0][limit + k + 1];
} else {
x = V[0][limit + k - 1] + 1;
}
int y = x - k;
snake[0] = x + bottoml1;
snake[1] = y + bottoml2;
snake[2] = 0;
// System.out.println("1 x: " + x + " y: " + y + " k: " + k + "
// d: " + d );
while (x < N && y < M
&& isRangeEqual(x + bottoml1, y + bottoml2)) {
x++;
y++;
snake[2]++;
}
V[0][limit + k] = x;
// System.out.println(x + " " + V[1][limit+k -delta] + " " + k +
// " " + delta);
if (!isEven && k >= delta - d + 1 && k <= delta + d - 1
&& x >= V[1][limit + k - delta]) {
// System.out.println("Returning: " + (2*d-1));
return 2 * d - 1;
}
// check to see if we can cut down the diagonal range
if (x >= N && end_forward > k - 1) {
end_forward = k - 1;
} else if (y >= M) {
start_forward = k + 1;
value_to_add_forward = 0;
}
}
start_diag = Math.max(value_to_add_backward + start_backward, -d);
end_diag = Math.min(end_backward, d);
value_to_add_backward = 1 - value_to_add_backward;
// compute backward furthest reaching paths
for (int k = start_diag; k <= end_diag; k += 2) {
int x;
if (k == d
|| (k != -d && V[1][limit + k - 1] < V[1][limit + k + 1])) {
x = V[1][limit + k - 1];
} else {
x = V[1][limit + k + 1] - 1;
}
int y = x - k - delta;
snake[2] = 0;
// System.out.println("2 x: " + x + " y: " + y + " k: " + k + "
// d: " + d);
while (x > 0 && y > 0
&& isRangeEqual(x - 1 + bottoml1, y - 1 + bottoml2)) {
x--;
y--;
snake[2]++;
}
V[1][limit + k] = x;
if (isEven && k >= -delta - d && k <= d - delta
&& x <= V[0][limit + k + delta]) {
// System.out.println("Returning: " + 2*d);
snake[0] = bottoml1 + x;
snake[1] = bottoml2 + y;
return 2 * d;
}
// check to see if we can cut down our diagonal range
if (x <= 0) {
start_backward = k + 1;
value_to_add_backward = 0;
} else if (y <= 0 && end_backward > k - 1) {
end_backward = k - 1;
}
}
}
/*
* computing the true LCS is too expensive, instead find the diagonal
* with the most progress and pretend a midle snake of length 0 occurs
* there.
*/
int[] most_progress = findMostProgress(M, N, limit, V);
snake[0] = bottoml1 + most_progress[0];
snake[1] = bottoml2 + most_progress[1];
snake[2] = 0;
return 5; /*
* HACK: since we didn't really finish the LCS computation
* we don't really know the length of the SES. We don't do
* anything with the result anyway, unless it's <=1. We know
* for a fact SES > 1 so 5 is as good a number as any to
* return here
*/
}
/**
* Takes the array with furthest reaching D-paths from an LCS computation
* and returns the x,y coordinates and progress made in the middle diagonal
* among those with maximum progress, both from the front and from the back.
*
* @param M
* the length of the 1st sequence for which LCS is being
* computed
* @param N
* the length of the 2nd sequence for which LCS is being
* computed
* @param limit
* the number of steps made in an attempt to find the LCS
* from the front and back
* @param V
* the array storing the furthest reaching D-paths for the
* LCS computation
* @return The result as an array of 3 integers where result[0] is the x
* coordinate of the current location in the diagonal with the most
* progress, result[1] is the y coordinate of the current location
* in the diagonal with the most progress and result[2] is the
* amount of progress made in that diagonal
*/
private static int[] findMostProgress(int M, int N, int limit, int[][] V) {
int delta = N - M;
int forward_start_diag;
if ((M & 1) == (limit & 1)) {
forward_start_diag = Math.max(-M, -limit);
} else {
forward_start_diag = Math.max(1 - M, -limit);
}
int forward_end_diag = Math.min(N, limit);
int backward_start_diag;
if ((N & 1) == (limit & 1)) {
backward_start_diag = Math.max(-N, -limit);
} else {
backward_start_diag = Math.max(1 - N, -limit);
}
int backward_end_diag = Math.min(M, limit);
int[][] max_progress = new int[Math.max(forward_end_diag
- forward_start_diag, backward_end_diag - backward_start_diag) / 2 + 1][3];
int num_progress = 0; // the 1st entry is current, it is initialized
// with 0s
// first search the forward diagonals
for (int k = forward_start_diag; k <= forward_end_diag; k += 2) {
int x = V[0][limit + k];
int y = x - k;
if (x > N || y > M) {
continue;
}
int progress = x + y;
if (progress > max_progress[0][2]) {
num_progress = 0;
max_progress[0][0] = x;
max_progress[0][1] = y;
max_progress[0][2] = progress;
} else if (progress == max_progress[0][2]) {
num_progress++;
max_progress[num_progress][0] = x;
max_progress[num_progress][1] = y;
max_progress[num_progress][2] = progress;
}
}
boolean max_progress_forward = true; // initially the maximum
// progress is in the forward
// direction
// now search the backward diagonals
for (int k = backward_start_diag; k <= backward_end_diag; k += 2) {
int x = V[1][limit + k];
int y = x - k - delta;
if (x < 0 || y < 0) {
continue;
}
int progress = N - x + M - y;
if (progress > max_progress[0][2]) {
num_progress = 0;
max_progress_forward = false;
max_progress[0][0] = x;
max_progress[0][1] = y;
max_progress[0][2] = progress;
} else if (progress == max_progress[0][2] && !max_progress_forward) {
num_progress++;
max_progress[num_progress][0] = x;
max_progress[num_progress][1] = y;
max_progress[num_progress][2] = progress;
}
}
// return the middle diagonal with maximal progress.
return max_progress[num_progress / 2];
}
protected abstract int getLength2();
protected abstract int getLength1();
protected abstract boolean isRangeEqual(int i1, int i2);
protected abstract void setLcs(int sl1, int sl2);
protected abstract void initializeLcs(int lcsLength);
public int getLength() {
return length;
}
}