/*
    * Copyright (C) 2008-2009, Johannes E. Schindelin <johannes.schindelin@gmx.de>
    * Copyright (C) 2009, Johannes Schindelin <johannes.schindelin@gmx.de> and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
   */
  
  package org.eclipse.jgit.diff;
  
  import java.text.MessageFormat;
  
  import org.eclipse.jgit.errors.DiffInterruptedException;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.util.IntList;
  import org.eclipse.jgit.util.LongList;
  
  /**
   * Diff algorithm, based on "An O(ND) Difference Algorithm and its Variations",
   * by Eugene Myers.
   * <p>
   * The basic idea is to put the line numbers of text A as columns ("x") and the
   * lines of text B as rows ("y"). Now you try to find the shortest "edit path"
   * from the upper left corner to the lower right corner, where you can always go
   * horizontally or vertically, but diagonally from (x,y) to (x+1,y+1) only if
   * line x in text A is identical to line y in text B.
   * <p>
   * Myers' fundamental concept is the "furthest reaching D-path on diagonal k": a
   * D-path is an edit path starting at the upper left corner and containing
   * exactly D non-diagonal elements ("differences"). The furthest reaching D-path
   * on diagonal k is the one that contains the most (diagonal) elements which
   * ends on diagonal k (where k = y - x).
   * <p>
   * Example:
   *
   * <pre>
   *    H E L L O   W O R L D
   *    ____
   *  L     \___
   *  O         \___
   *  W             \________
   * </pre>
   * <p>
   * Since every D-path has exactly D horizontal or vertical elements, it can only
   * end on the diagonals -D, -D+2, ..., D-2, D.
   * <p>
   * Since every furthest reaching D-path contains at least one furthest reaching
   * (D-1)-path (except for D=0), we can construct them recursively.
   * <p>
   * Since we are really interested in the shortest edit path, we can start
   * looking for a 0-path, then a 1-path, and so on, until we find a path that
   * ends in the lower right corner.
   * <p>
   * To save space, we do not need to store all paths (which has quadratic space
   * requirements), but generate the D-paths simultaneously from both sides. When
   * the ends meet, we will have found "the middle" of the path. From the end
   * points of that diagonal part, we can generate the rest recursively.
   * <p>
   * This only requires linear space.
   * <p>
   * The overall (runtime) complexity is:
   *
   * <pre>
   *     O(N * D^2 + 2 * N/2 * (D/2)^2 + 4 * N/4 * (D/4)^2 + ...)
   *     = O(N * D^2 * 5 / 4) = O(N * D^2),
   * </pre>
   * <p>
   * (With each step, we have to find the middle parts of twice as many regions as
   * before, but the regions (as well as the D) are halved.)
   * <p>
   * So the overall runtime complexity stays the same with linear space, albeit
   * with a larger constant factor.
   *
   * @param <S>
   *            type of sequence.
   */
  @SuppressWarnings("hiding")
  public class MyersDiff<S extends Sequence> {
  	/** Singleton instance of MyersDiff. */
  	public static final DiffAlgorithm INSTANCE = new LowLevelDiffAlgorithm() {
  		@SuppressWarnings("unused")
  		@Override
  		public <S extends Sequence> void diffNonCommon(EditList edits,
  				HashedSequenceComparator<S> cmp, HashedSequence<S> a,
  				HashedSequence<S> b, Edit region) {
  			new MyersDiff<>(edits, cmp, a, b, region);
  		}
  	};
  
  	/**
  	 * The list of edits found during the last call to
  	 * {@link #calculateEdits(Edit)}
  	 */
  	protected EditList edits;
  
  	/** Comparison function for sequences. */
 	protected HashedSequenceComparator<S> cmp;
 
 	/**
 	 * The first text to be compared. Referred to as "Text A" in the comments
 	 */
 	protected HashedSequence<S> a;
 
 	/**
 	 * The second text to be compared. Referred to as "Text B" in the comments
 	 */
 	protected HashedSequence<S> b;
 
 	private MyersDiff(EditList edits, HashedSequenceComparator<S> cmp,
 			HashedSequence<S> a, HashedSequence<S> b, Edit region) {
 		this.edits = edits;
 		this.cmp = cmp;
 		this.a = a;
 		this.b = b;
 		calculateEdits(region);
 	}
 
 	// TODO: use ThreadLocal for future multi-threaded operations
 	MiddleEdit middle = new MiddleEdit();
 
 	/**
 	 * Entrypoint into the algorithm this class is all about. This method triggers that the
 	 * differences between A and B are calculated in form of a list of edits.
 	 * @param r portion of the sequences to examine.
 	 */
 	private void calculateEdits(Edit r) {
 		middle.initialize(r.beginA, r.endA, r.beginB, r.endB);
 		if (middle.beginA >= middle.endA &&
 				middle.beginB >= middle.endB)
 			return;
 
 		calculateEdits(middle.beginA, middle.endA,
 				middle.beginB, middle.endB);
 	}
 
 	/**
 	 * Calculates the differences between a given part of A against another
 	 * given part of B
 	 *
 	 * @param beginA
 	 *            start of the part of A which should be compared
 	 *            (0&lt;=beginA&lt;sizeof(A))
 	 * @param endA
 	 *            end of the part of A which should be compared
 	 *            (beginA&lt;=endA&lt;sizeof(A))
 	 * @param beginB
 	 *            start of the part of B which should be compared
 	 *            (0&lt;=beginB&lt;sizeof(B))
 	 * @param endB
 	 *            end of the part of B which should be compared
 	 *            (beginB&lt;=endB&lt;sizeof(B))
 	 */
 	protected void calculateEdits(int beginA, int endA,
 			int beginB, int endB) {
 		Edit edit = middle.calculate(beginA, endA, beginB, endB);
 
 		if (beginA < edit.beginA || beginB < edit.beginB) {
 			int k = edit.beginB - edit.beginA;
 			int x = middle.backward.snake(k, edit.beginA);
 			calculateEdits(beginA, x, beginB, k + x);
 		}
 
 		if (edit.getType() != Edit.Type.EMPTY)
 			edits.add(edits.size(), edit);
 
 		// after middle
 		if (endA > edit.endA || endB > edit.endB) {
 			int k = edit.endB - edit.endA;
 			int x = middle.forward.snake(k, edit.endA);
 			calculateEdits(x, endA, k + x, endB);
 		}
 	}
 
 	/**
 	 * A class to help bisecting the sequences a and b to find minimal
 	 * edit paths.
 	 *
 	 * As the arrays are reused for space efficiency, you will need one
 	 * instance per thread.
 	 *
 	 * The entry function is the calculate() method.
 	 */
 	class MiddleEdit {
 		void initialize(int beginA, int endA, int beginB, int endB) {
 			this.beginA = beginA; this.endA = endA;
 			this.beginB = beginB; this.endB = endB;
 
 			// strip common parts on either end
 			int k = beginB - beginA;
 			this.beginA = forward.snake(k, beginA);
 			this.beginB = k + this.beginA;
 
 			k = endB - endA;
 			this.endA = backward.snake(k, endA);
 			this.endB = k + this.endA;
 		}
 
 		/*
 		 * This function calculates the "middle" Edit of the shortest
 		 * edit path between the given subsequences of a and b.
 		 *
 		 * Once a forward path and a backward path meet, we found the
 		 * middle part.  From the last snake end point on both of them,
 		 * we construct the Edit.
 		 *
 		 * It is assumed that there is at least one edit in the range.
 		 */
 		// TODO: measure speed impact when this is synchronized
 		Edit calculate(int beginA, int endA, int beginB, int endB) {
 			if (beginA == endA || beginB == endB)
 				return new Edit(beginA, endA, beginB, endB);
 			this.beginA = beginA; this.endA = endA;
 			this.beginB = beginB; this.endB = endB;
 
 			/*
 			 * Following the conventions in Myers' paper, "k" is
 			 * the difference between the index into "b" and the
 			 * index into "a".
 			 */
 			int minK = beginB - endA;
 			int maxK = endB - beginA;
 
 			forward.initialize(beginB - beginA, beginA, minK, maxK);
 			backward.initialize(endB - endA, endA, minK, maxK);
 
 			for (int d = 1; ; d++)
 				if (forward.calculate(d) ||
 						backward.calculate(d))
 					return edit;
 		}
 
 		/*
 		 * For each d, we need to hold the d-paths for the diagonals
 		 * k = -d, -d + 2, ..., d - 2, d.  These are stored in the
 		 * forward (and backward) array.
 		 *
 		 * As we allow subsequences, too, this needs some refinement:
 		 * the forward paths start on the diagonal forwardK =
 		 * beginB - beginA, and backward paths start on the diagonal
 		 * backwardK = endB - endA.
 		 *
 		 * So, we need to hold the forward d-paths for the diagonals
 		 * k = forwardK - d, forwardK - d + 2, ..., forwardK + d and
 		 * the analogue for the backward d-paths.  This means that
 		 * we can turn (k, d) into the forward array index using this
 		 * formula:
 		 *
 		 *	i = (d + k - forwardK) / 2
 		 *
 		 * There is a further complication: the edit paths should not
 		 * leave the specified subsequences, so k is bounded by
 		 * minK = beginB - endA and maxK = endB - beginA.  However,
 		 * (k - forwardK) _must_ be odd whenever d is odd, and it
 		 * _must_ be even when d is even.
 		 *
 		 * The values in the "forward" and "backward" arrays are
 		 * positions ("x") in the sequence a, to get the corresponding
 		 * positions ("y") in the sequence b, you have to calculate
 		 * the appropriate k and then y:
 		 *
 		 *	k = forwardK - d + i * 2
 		 *	y = k + x
 		 *
 		 * (substitute backwardK for forwardK if you want to get the
 		 * y position for an entry in the "backward" array.
 		 */
 		EditPaths forward = new ForwardEditPaths();
 		EditPaths backward = new BackwardEditPaths();
 
 		/* Some variables which are shared between methods */
 		protected int beginA, endA, beginB, endB;
 		protected Edit edit;
 
 		abstract class EditPaths {
 			private IntListtList.html#IntList">IntList x = new IntList();
 			private LongListt.html#LongList">LongList snake = new LongList();
 			int beginK, endK, middleK;
 			int prevBeginK, prevEndK;
 			/* if we hit one end early, no need to look further */
 			int minK, maxK; // TODO: better explanation
 
 			final int getIndex(int d, int k) {
 // TODO: remove
 if (((d + k - middleK) % 2) != 0)
 	throw new RuntimeException(MessageFormat.format(JGitText.get().unexpectedOddResult, Integer.valueOf(d), Integer.valueOf(k), Integer.valueOf(middleK)));
 				return (d + k - middleK) / 2;
 			}
 
 			final int getX(int d, int k) {
 // TODO: remove
 if (k < beginK || k > endK)
 	throw new RuntimeException(MessageFormat.format(JGitText.get().kNotInRange, Integer.valueOf(k), Integer.valueOf(beginK), Integer.valueOf(endK)));
 				return x.get(getIndex(d, k));
 			}
 
 			final long getSnake(int d, int k) {
 // TODO: remove
 if (k < beginK || k > endK)
 	throw new RuntimeException(MessageFormat.format(JGitText.get().kNotInRange, Integer.valueOf(k), Integer.valueOf(beginK), Integer.valueOf(endK)));
 				return snake.get(getIndex(d, k));
 			}
 
 			private int forceKIntoRange(int k) {
 				/* if k is odd, so must be the result */
 				if (k < minK)
 					return minK + ((k ^ minK) & 1);
 				else if (k > maxK)
 					return maxK - ((k ^ maxK) & 1);
 				return k;
 			}
 
 			void initialize(int k, int x, int minK, int maxK) {
 				this.minK = minK;
 				this.maxK = maxK;
 				beginK = endK = middleK = k;
 				this.x.clear();
 				this.x.add(x);
 				snake.clear();
 				snake.add(newSnake(k, x));
 			}
 
 			abstract int snake(int k, int x);
 			abstract int getLeft(int x);
 			abstract int getRight(int x);
 			abstract boolean isBetter(int left, int right);
 			abstract void adjustMinMaxK(int k, int x);
 			abstract boolean meets(int d, int k, int x, long snake);
 
 			final long newSnake(int k, int x) {
 				long y = k + x;
 				long ret = ((long) x) << 32;
 				return ret | y;
 			}
 
 			final int snake2x(long snake) {
 				return (int) (snake >>> 32);
 			}
 
 			final int snake2y(long snake) {
 				return (int) snake;
 			}
 
 			final boolean makeEdit(long snake1, long snake2) {
 				int x1 = snake2x(snake1), x2 = snake2x(snake2);
 				int y1 = snake2y(snake1), y2 = snake2y(snake2);
 				/*
 				 * Check for incompatible partial edit paths:
 				 * when there are ambiguities, we might have
 				 * hit incompatible (i.e. non-overlapping)
 				 * forward/backward paths.
 				 *
 				 * In that case, just pretend that we have
 				 * an empty edit at the end of one snake; this
 				 * will force a decision which path to take
 				 * in the next recursion step.
 				 */
 				if (x1 > x2 || y1 > y2) {
 					x1 = x2;
 					y1 = y2;
 				}
 				edit = new Edit(x1, x2, y1, y2);
 				return true;
 			}
 
 			boolean calculate(int d) {
 				prevBeginK = beginK;
 				prevEndK = endK;
 				beginK = forceKIntoRange(middleK - d);
 				endK = forceKIntoRange(middleK + d);
 				// TODO: handle i more efficiently
 				// TODO: walk snake(k, getX(d, k)) only once per (d, k)
 				// TODO: move end points out of the loop to avoid conditionals inside the loop
 				// go backwards so that we can avoid temp vars
 				for (int k = endK; k >= beginK; k -= 2) {
 					if (Thread.interrupted()) {
 						throw new DiffInterruptedException();
 					}
 					int left = -1, right = -1;
 					long leftSnake = -1L, rightSnake = -1L;
 					// TODO: refactor into its own function
 					if (k > prevBeginK) {
 						int i = getIndex(d - 1, k - 1);
 						left = x.get(i);
 						int end = snake(k - 1, left);
 						leftSnake = left != end ?
 							newSnake(k - 1, end) :
 							snake.get(i);
 						if (meets(d, k - 1, end, leftSnake))
 							return true;
 						left = getLeft(end);
 					}
 					if (k < prevEndK) {
 						int i = getIndex(d - 1, k + 1);
 						right = x.get(i);
 						int end = snake(k + 1, right);
 						rightSnake = right != end ?
 							newSnake(k + 1, end) :
 							snake.get(i);
 						if (meets(d, k + 1, end, rightSnake))
 							return true;
 						right = getRight(end);
 					}
 					int newX;
 					long newSnake;
 					if (k >= prevEndK ||
 							(k > prevBeginK &&
 							 isBetter(left, right))) {
 						newX = left;
 						newSnake = leftSnake;
 					}
 					else {
 						newX = right;
 						newSnake = rightSnake;
 					}
 					if (meets(d, k, newX, newSnake))
 						return true;
 					adjustMinMaxK(k, newX);
 					int i = getIndex(d, k);
 					x.set(i, newX);
 					snake.set(i, newSnake);
 				}
 				return false;
 			}
 		}
 
 		class ForwardEditPaths extends EditPaths {
 			@Override
 			final int snake(int k, int x) {
 				for (; x < endA && k + x < endB; x++)
 					if (!cmp.equals(a, x, b, k + x))
 						break;
 				return x;
 			}
 
 			@Override
 			final int getLeft(int x) {
 				return x;
 			}
 
 			@Override
 			final int getRight(int x) {
 				return x + 1;
 			}
 
 			@Override
 			final boolean isBetter(int left, int right) {
 				return left > right;
 			}
 
 			@Override
 			final void adjustMinMaxK(int k, int x) {
 				if (x >= endA || k + x >= endB) {
 					if (k > backward.middleK)
 						maxK = k;
 					else
 						minK = k;
 				}
 			}
 
 			@Override
 			final boolean meets(int d, int k, int x, long snake) {
 				if (k < backward.beginK || k > backward.endK)
 					return false;
 				// TODO: move out of loop
 				if (((d - 1 + k - backward.middleK) % 2) != 0)
 					return false;
 				if (x < backward.getX(d - 1, k))
 					return false;
 				makeEdit(snake, backward.getSnake(d - 1, k));
 				return true;
 			}
 		}
 
 		class BackwardEditPaths extends EditPaths {
 			@Override
 			final int snake(int k, int x) {
 				for (; x > beginA && k + x > beginB; x--)
 					if (!cmp.equals(a, x - 1, b, k + x - 1))
 						break;
 				return x;
 			}
 
 			@Override
 			final int getLeft(int x) {
 				return x - 1;
 			}
 
 			@Override
 			final int getRight(int x) {
 				return x;
 			}
 
 			@Override
 			final boolean isBetter(int left, int right) {
 				return left < right;
 			}
 
 			@Override
 			final void adjustMinMaxK(int k, int x) {
 				if (x <= beginA || k + x <= beginB) {
 					if (k > forward.middleK)
 						maxK = k;
 					else
 						minK = k;
 				}
 			}
 
 			@Override
 			final boolean meets(int d, int k, int x, long snake) {
 				if (k < forward.beginK || k > forward.endK)
 					return false;
 				// TODO: move out of loop
 				if (((d + k - forward.middleK) % 2) != 0)
 					return false;
 				if (x > forward.getX(d, k))
 					return false;
 				makeEdit(forward.getSnake(d, k), snake);
 				return true;
 			}
 		}
 	}
 
 	/**
 	 * Main method
 	 *
 	 * @param args
 	 *            two filenames specifying the contents to be diffed
 	 */
 	public static void main(String[] args) {
 		if (args.length != 2) {
 			System.err.println(JGitText.get().need2Arguments);
 			System.exit(1);
 		}
 		try {
 			RawText a = new RawText(new java.io.File(args[0]));
 			RawText b = new RawText(new java.io.File(args[1]));
 			EditList r = INSTANCE.diff(RawTextComparator.DEFAULT, a, b);
 			System.out.println(r.toString());
 		} catch (Exception e) {
 			e.printStackTrace();
 		}
 	}
 }