/*******************************************************************************
 * Copyright (c) 2000, 2005 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.jface.text;


/**
 * Implements a gap managing text store. The gap text store
 * relies on the assumption that subsequent changes of a document are co-located.
 * The start of the gap is always moved to the location of the last change. The
 * size of the gap varies between the low water mark and the high water mark.
 * <p>
 * This class is not intended to be subclassed.
 * </p>
 */
public class GapTextStore implements ITextStore {

	/** The store's content */
	private char[] fContent= new char[0];
	/** Starting index of the gap */
	private int fGapStart= -1;
	/** End index of the gap */
	private int fGapEnd= -1;

	/** The high water mark. If the gap is larger than this, it will be shrunken */
	private int fHighWatermark;
	/** The low water mark, If this gap is smaller than this, it will be extended */
	private int fLowWatermark;

	/**
	 * Creates a new empty text store using the specified low and high watermarks.
	 *
	 * @param lowWatermark if this gap is ever smaller than this, it will automatically be extended
	 * @param highWatermark if the gap is ever larger than this, it will automatically be shrunken
	 */
	public GapTextStore(int lowWatermark, int highWatermark) {
		Assert.isTrue(lowWatermark < highWatermark);
		fLowWatermark= lowWatermark;
		fHighWatermark= highWatermark;
	}

	/**
	 * Adjusts the gap so that is at the right offset and capable of handling
	 * the addition of a specified number of characters without having to be shifted.
	 * The <code>sizeHint</code> represents the range that will be filled afterwards.
	 * If the gap is already at the right offset, it must only be
	 * resized if it will be no longer between the low and high watermark. However,
	 * on delete (sizeHint &lt; 0) at the edges of the gap, the gap is only enlarged.
	 *
	 * @param offset the offset at which the change happens
	 * @param sizeHint the number of character which will be inserted
	 */
	private void adjustGap(int offset, int sizeHint) {

		if (offset == fGapStart) {
			int size= (fGapEnd - fGapStart) - sizeHint;
			if (fLowWatermark <= size && size <= fHighWatermark)
				return;
		}

		moveAndResizeGap(offset, sizeHint);
	}

	/**
	 * Moves the gap to the specified offset and adjust its size to the
	 * anticipated change size. The given size represents the expected
	 * range of the gap that will be filled after the gap has been moved.
	 * Thus the gap is resized to actual size + the specified size and
	 * moved to the given offset.
	 *
	 * @param offset the offset where the gap is moved to
	 * @param size the anticipated size of the change
	 */
	private void moveAndResizeGap(int offset, int size) {

		char[] content= null;
		int oldSize= fGapEnd - fGapStart;
		int newSize= fHighWatermark + size;


		if (newSize < 0) {

			if (oldSize > 0) {
				content= new char[fContent.length - oldSize];
				System.arraycopy(fContent, 0, content, 0, fGapStart);
				System.arraycopy(fContent, fGapEnd, content, fGapStart, content.length - fGapStart);
				fContent= content;
			}
			fGapStart= fGapEnd= offset;
			return;
		}


		content= new char[fContent.length + (newSize - oldSize)];

		int newGapStart= offset;
		int newGapEnd= newGapStart + newSize;

		if (oldSize == 0) {

			System.arraycopy(fContent, 0, content, 0, newGapStart);
			System.arraycopy(fContent, newGapStart, content, newGapEnd, content.length - newGapEnd);

		} else if (newGapStart < fGapStart) {

			int delta= fGapStart - newGapStart;
			System.arraycopy(fContent, 0, content, 0, newGapStart);
			System.arraycopy(fContent, newGapStart, content, newGapEnd, delta);
			System.arraycopy(fContent, fGapEnd, content, newGapEnd + delta, fContent.length - fGapEnd);

		} else {

			int delta= newGapStart - fGapStart;
			System.arraycopy(fContent, 0, content, 0, fGapStart);
			System.arraycopy(fContent, fGapEnd, content, fGapStart, delta);
			System.arraycopy(fContent, fGapEnd + delta, content, newGapEnd, content.length - newGapEnd);
		}


		fContent= content;
		fGapStart= newGapStart;
		fGapEnd= newGapEnd;
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#get(int)
	 */
	public char get(int offset) {

		if (offset < fGapStart)
			return fContent[offset];

		int gapLength= fGapEnd - fGapStart;
		return fContent[offset + gapLength];
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#get(int, int)
	 */
	public String get(int offset, int length) {

		int end= offset + length;

		if (fContent == null)
			return ""; //$NON-NLS-1$

		if (end <= fGapStart)
			return new String(fContent, offset, length);

		if (fGapStart < offset) {
			int gapLength= fGapEnd - fGapStart;
			return new String(fContent, offset + gapLength , length);
		}

		StringBuffer buf= new StringBuffer(length);
		buf.append(fContent, offset, fGapStart - offset);
		buf.append(fContent, fGapEnd, end - fGapStart);
		return buf.toString();
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#getLength()
	 */
	public int getLength() {
		int length= fGapEnd - fGapStart;
		return (fContent.length - length);
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#replace(int, int, java.lang.String)
	 */
	public void replace(int offset, int length, String text) {

		int textLength= (text == null ? 0 : text.length());

		// handle delete at the edges of the gap
		if (textLength == 0) {
			if (offset <= fGapStart && offset + length >= fGapStart && fGapStart > -1 && fGapEnd > -1) {
				length -= fGapStart - offset;
				fGapStart= offset;
				fGapEnd += length;
				return;
			}
		}

		// move gap
		adjustGap(offset + length, textLength - length);

		// overwrite
		int min= Math.min(textLength, length);
		for (int i= offset, j= 0; i < offset + min; i++, j++)
			fContent[i]= text.charAt(j);

		if (length > textLength) {
			// enlarge the gap
			fGapStart -= (length - textLength);
		} else if (textLength > length) {
			// shrink gap
			fGapStart += (textLength - length);
			for (int i= length; i < textLength; i++)
				fContent[offset + i]= text.charAt(i);
		}
	}

	/**
	 * Sets the content to <code>text</code> and removes the gap
	 * since there are no sensible predictions about
	 * where the next change will occur.
	 *
	 * @see ITextStore#set(String)
	 */
	public void set(String text) {

		if (text == null)
			text= ""; //$NON-NLS-1$

		fContent= text.toCharArray();

		fGapStart= -1;
		fGapEnd=   -1;
	}

	/**
	 * Returns a copy of the content of this text store.
	 * For internal use only.
	 *
	 * @return a copy of the content of this text store
	 */
	protected String getContentAsString() {
		return new String(fContent);
	}

	/**
	 * Returns the start index of the gap managed by this text store.
	 * For internal use only.
	 *
	 * @return the start index of the gap managed by this text store
	 */
	protected int getGapStartIndex() {
		return fGapStart;
	}

	/**
	 * Returns the end index of the gap managed by this text store.
	 * For internal use only.
	 *
	 * @return the end index of the gap managed by this text store
	 */
	protected int getGapEndIndex() {
		return fGapEnd;
	}
}