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/*******************************************************************************
* Copyright (c) 2000, 2008 IBM Corporation and others.
*
* This program and the accompanying materials
* are made available under the terms of the Eclipse Public License 2.0
* which accompanies this distribution, and is available at
* https://www.eclipse.org/legal/epl-2.0/
*
* SPDX-License-Identifier: EPL-2.0
*
* Contributors:
* IBM Corporation - initial API and implementation
*******************************************************************************/
package org.eclipse.jface.text;
import org.eclipse.core.runtime.Assert;
/**
* Implements a gap managing text store. The gap text store relies on the assumption that
* consecutive changes to a document are co-located. The start of the gap is always moved to the
* location of the last change.
*
* Performance: Typing-style changes perform in constant time unless re-allocation
* becomes necessary. Generally, a change that does not cause re-allocation will cause at most one
* {@linkplain System#arraycopy(Object, int, Object, int, int) arraycopy} operation of a length of
* about d, where d is the distance from the previous change. Let a(x)
* be the algorithmic performance of an arraycopy operation of the length x,
* then such a change then performs in O(a(x)),
* {@linkplain #get(int, int) get(int, length)} performs in O(a(length)),
* {@link #get(int)} in O(1).
*
* How frequently the array needs re-allocation is controlled by the constructor parameters.
*
*
* This class is not intended to be subclassed.
*
*
* @see CopyOnWriteTextStore for a copy-on-write text store wrapper
* @noextend This class is not intended to be subclassed by clients.
*/
public class GapTextStore implements ITextStore {
/**
* The minimum gap size allocated when re-allocation occurs.
* @since 3.3
*/
private final int fMinGapSize;
/**
* The maximum gap size allocated when re-allocation occurs.
* @since 3.3
*/
private final int fMaxGapSize;
/**
* The multiplier to compute the array size from the content length
* (1 <= fSizeMultiplier <= 2).
*
* @since 3.3
*/
private final float fSizeMultiplier;
/** The store's content */
private char[] fContent= new char[0];
/** Starting index of the gap */
private int fGapStart= 0;
/** End index of the gap */
private int fGapEnd= 0;
/**
* The current high water mark. If a change would cause the gap to grow larger than this, the
* array is re-allocated.
* @since 3.3
*/
private int fThreshold= 0;
/**
* Creates a new empty text store using the specified low and high watermarks.
*
* @param lowWatermark unused - at the lower bound, the array is only resized when the content
* does not fit
* @param highWatermark if the gap is ever larger than this, it will automatically be shrunken
* (>= 0)
* @deprecated use {@link GapTextStore#GapTextStore(int, int, float)} instead
*/
@Deprecated
public GapTextStore(int lowWatermark, int highWatermark) {
/*
* Legacy constructor. The API contract states that highWatermark is the upper bound for the
* gap size. Albeit this contract was not previously adhered to, it is now: The allocated
* gap size is fixed at half the highWatermark. Since the threshold is always twice the
* allocated gap size, the gap will never grow larger than highWatermark. Previously, the
* gap size was initialized to highWatermark, causing re-allocation if the content length
* shrunk right after allocation. The fixed gap size is now only half of the previous value,
* circumventing that problem (there was no API contract specifying the initial gap size).
*
* The previous implementation did not allow the gap size to become smaller than
* lowWatermark, which doesn't make any sense: that area of the gap was simply never ever
* used.
*/
this(highWatermark / 2, highWatermark / 2, 0f);
}
/**
* Equivalent to
* {@linkplain GapTextStore#GapTextStore(int, int, float) new GapTextStore(256, 4096, 0.1f)}.
*
* @since 3.3
*/
public GapTextStore() {
this(256, 4096, 0.1f);
}
/**
* Creates an empty text store that uses re-allocation thresholds relative to the content
* length. Re-allocation is controlled by the gap factor, which is the quotient of
* the gap size and the array size. Re-allocation occurs if a change causes the gap factor to go
* outside [0, maxGapFactor]. When re-allocation occurs, the array is sized
* such that the gap factor is 0.5 * maxGapFactor. The gap size computed in this
* manner is bounded by the minSize and maxSize parameters.
*
* A maxGapFactor of 0 creates a text store that never has a gap
* at all (if minSize is 0); a maxGapFactor of 1
* creates a text store that doubles its size with every re-allocation and that never shrinks.
*
*
* The minSize and maxSize parameters are absolute bounds to the
* allocated gap size. Use minSize to avoid frequent re-allocation for small
* documents. Use maxSize to avoid a huge gap being allocated for large
* documents.
*
*
* @param minSize the minimum gap size to allocate (>= 0; use 0 for no minimum)
* @param maxSize the maximum gap size to allocate (>= minSize; use
* {@link Integer#MAX_VALUE} for no maximum)
* @param maxGapFactor is the maximum fraction of the array that is occupied by the gap (0 <= maxGapFactor <= 1)
* @since 3.3
*/
public GapTextStore(int minSize, int maxSize, float maxGapFactor) {
Assert.isLegal(0f <= maxGapFactor && maxGapFactor <= 1f);
Assert.isLegal(0 <= minSize && minSize <= maxSize);
fMinGapSize= minSize;
fMaxGapSize= maxSize;
fSizeMultiplier= 1 / (1 - maxGapFactor / 2);
}
@Override
public final char get(int offset) {
if (offset < fGapStart)
return fContent[offset];
return fContent[offset + gapSize()];
}
@Override
public final String get(int offset, int length) {
if (fGapStart <= offset)
return new String(fContent, offset + gapSize() , length);
final int end= offset + length;
if (end <= fGapStart)
return new String(fContent, offset, length);
StringBuilder buf= new StringBuilder(length); // see Bug 113871
buf.append(fContent, offset, fGapStart - offset);
buf.append(fContent, fGapEnd, end - fGapStart);
return buf.toString();
}
@Override
public final int getLength() {
return fContent.length - gapSize();
}
@Override
public final void set(String text) {
/*
* Moves the gap to the end of the content. There is no sensible prediction of where the
* next change will occur, but at least the next change will not trigger re-allocation. This
* is especially important when using the GapTextStore within a CopyOnWriteTextStore, where
* the GTS is only initialized right before a modification.
*/
replace(0, getLength(), text);
}
@Override
public final void replace(int offset, int length, String text) {
if (text == null) {
adjustGap(offset, length, 0);
} else {
int textLength= text.length();
adjustGap(offset, length, textLength);
if (textLength != 0)
text.getChars(0, textLength, fContent, offset);
}
}
/**
* Moves the gap to offset + add, moving any content after
* offset + remove behind the gap. The gap size is kept between 0 and
* {@link #fThreshold}, leading to re-allocation if needed. The content between
* offset and offset + add is undefined after this operation.
*
* @param offset the offset at which a change happens
* @param remove the number of character which are removed or overwritten at offset
* @param add the number of character which are inserted or overwriting at offset
*/
private void adjustGap(int offset, int remove, int add) {
final int oldGapSize= gapSize();
final int newGapSize= oldGapSize - add + remove;
final boolean reuseArray= 0 <= newGapSize && newGapSize <= fThreshold;
final int newGapStart= offset + add;
final int newGapEnd;
if (reuseArray)
newGapEnd= moveGap(offset, remove, oldGapSize, newGapSize, newGapStart);
else
newGapEnd= reallocate(offset, remove, oldGapSize, newGapSize, newGapStart);
fGapStart= newGapStart;
fGapEnd= newGapEnd;
}
/**
* Moves the gap to newGapStart.
*
* @param offset the change offset
* @param remove the number of removed / overwritten characters
* @param oldGapSize the old gap size
* @param newGapSize the gap size after the change
* @param newGapStart the offset in the array to move the gap to
* @return the new gap end
* @since 3.3
*/
private int moveGap(int offset, int remove, int oldGapSize, int newGapSize, int newGapStart) {
/*
* No re-allocation necessary. The area between the change offset and gap can be copied
* in at most one operation. Don't copy parts that will be overwritten anyway.
*/
final int newGapEnd= newGapStart + newGapSize;
if (offset < fGapStart) {
int afterRemove= offset + remove;
if (afterRemove < fGapStart) {
final int betweenSize= fGapStart - afterRemove;
arrayCopy(afterRemove, fContent, newGapEnd, betweenSize);
}
// otherwise, only the gap gets enlarged
} else {
final int offsetShifted= offset + oldGapSize;
final int betweenSize= offsetShifted - fGapEnd; // in the typing case, betweenSize is 0
arrayCopy(fGapEnd, fContent, fGapStart, betweenSize);
}
return newGapEnd;
}
/**
* Reallocates a new array and copies the data from the previous one.
*
* @param offset the change offset
* @param remove the number of removed / overwritten characters
* @param oldGapSize the old gap size
* @param newGapSize the gap size after the change if no re-allocation would occur (can be negative)
* @param newGapStart the offset in the array to move the gap to
* @return the new gap end
* @since 3.3
*/
private int reallocate(int offset, int remove, final int oldGapSize, int newGapSize, final int newGapStart) {
// the new content length (without any gap)
final int newLength= fContent.length - newGapSize;
// the new array size based on the gap factor
int newArraySize= (int) (newLength * fSizeMultiplier);
newGapSize= newArraySize - newLength;
// bound the gap size within min/max
if (newGapSize < fMinGapSize) {
newGapSize= fMinGapSize;
newArraySize= newLength + newGapSize;
} else if (newGapSize > fMaxGapSize) {
newGapSize= fMaxGapSize;
newArraySize= newLength + newGapSize;
}
// the upper threshold is always twice the gapsize
fThreshold= newGapSize * 2;
final char[] newContent= allocate(newArraySize);
final int newGapEnd= newGapStart + newGapSize;
/*
* Re-allocation: The old content can be copied in at most 3 operations to the newly allocated
* array. Either one of change offset and the gap may come first.
* - unchanged area before the change offset / gap
* - area between the change offset and the gap (either one may be first)
* - rest area after the change offset / after the gap
*/
if (offset < fGapStart) {
// change comes before gap
arrayCopy(0, newContent, 0, offset);
int afterRemove= offset + remove;
if (afterRemove < fGapStart) {
// removal is completely before the gap
final int betweenSize= fGapStart - afterRemove;
arrayCopy(afterRemove, newContent, newGapEnd, betweenSize);
final int restSize= fContent.length - fGapEnd;
arrayCopy(fGapEnd, newContent, newGapEnd + betweenSize, restSize);
} else {
// removal encompasses the gap
afterRemove += oldGapSize;
final int restSize= fContent.length - afterRemove;
arrayCopy(afterRemove, newContent, newGapEnd, restSize);
}
} else {
// gap comes before change
arrayCopy(0, newContent, 0, fGapStart);
final int offsetShifted= offset + oldGapSize;
final int betweenSize= offsetShifted - fGapEnd;
arrayCopy(fGapEnd, newContent, fGapStart, betweenSize);
final int afterRemove= offsetShifted + remove;
final int restSize= fContent.length - afterRemove;
arrayCopy(afterRemove, newContent, newGapEnd, restSize);
}
fContent= newContent;
return newGapEnd;
}
/**
* Allocates a new char[size].
*
* @param size the length of the new array.
* @return a newly allocated char array
* @since 3.3
*/
private char[] allocate(int size) {
return new char[size];
}
/*
* Executes System.arraycopy if length != 0. A length < 0 cannot happen -> don't hide coding
* errors by checking for negative lengths.
* @since 3.3
*/
private void arrayCopy(int srcPos, char[] dest, int destPos, int length) {
if (length != 0)
System.arraycopy(fContent, srcPos, dest, destPos, length);
}
/**
* Returns the gap size.
*
* @return the gap size
* @since 3.3
*/
private int gapSize() {
return fGapEnd - fGapStart;
}
/**
* 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;
}
}
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