package aQute.bnd.properties;

/**
 * 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.
 * <p>
 * <strong>Performance:</strong> 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 <var>d</var>, where <var>d</var> is the
 * distance from the previous change. Let <var>a(x)</var> be the algorithmic
 * performance of an <code>arraycopy</code> operation of the length <var>x</var>
 * , then such a change then performs in <i>O(a(x))</i>,
 * {@linkplain #get(int, int) get(int, <var>length</var>)} performs in
 * <i>O(a(length))</i>, {@link #get(int)} in <i>O(1)</i>.
 * <p>
 * How frequently the array needs re-allocation is controlled by the constructor
 * parameters.
 * </p>
 * <p>
 * This class is not intended to be subclassed.
 * </p>
 *
 * @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&nbsp;&lt;=&nbsp;fSizeMultiplier&nbsp;&lt;=&nbsp;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 (&gt;=&nbsp;0) @deprecated use
	 *            {@link GapTextStore#GapTextStore(int, int, float)} instead
	 */
	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 <em>gap
	 * factor</em>, 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
	 * <code>[0,&nbsp;maxGapFactor]</code>. When re-allocation occurs, the array
	 * is sized such that the gap factor is <code>0.5 * maxGapFactor</code>. The
	 * gap size computed in this manner is bounded by the <code>minSize</code>
	 * and <code>maxSize</code> parameters.
	 * <p>
	 * A <code>maxGapFactor</code> of <code>0</code> creates a text store that
	 * never has a gap at all (if <code>minSize</code> is 0); a
	 * <code>maxGapFactor</code> of <code>1</code> creates a text store that
	 * doubles its size with every re-allocation and that never shrinks.
	 * </p>
	 * <p>
	 * The <code>minSize</code> and <code>maxSize</code> parameters are absolute
	 * bounds to the allocated gap size. Use <code>minSize</code> to avoid
	 * frequent re-allocation for small documents. Use <code>maxSize</code> to
	 * avoid a huge gap being allocated for large documents.
	 * </p>
	 *
	 * @param minSize the minimum gap size to allocate (&gt;=&nbsp;0; use 0 for
	 *            no minimum)
	 * @param maxSize the maximum gap size to allocate (&gt;=&nbsp;minSize; use
	 *            {@link Integer#MAX_VALUE} for no maximum)
	 * @param maxGapFactor is the maximum fraction of the array that is occupied
	 *            by the gap (
	 *            <code>0&nbsp;&lt;=&nbsp;maxGapFactor&nbsp;&lt;=&nbsp;1</code>
	 *            ) @since 3.3
	 */
	public GapTextStore(int minSize, int maxSize, float maxGapFactor) {
		fMinGapSize = minSize;
		fMaxGapSize = maxSize;
		fSizeMultiplier = 1 / (1 - maxGapFactor / 2);
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#get(int)
	 */
	@Override
	public final char get(int offset) {
		if (offset < fGapStart)
			return fContent[offset];

		return fContent[offset + gapSize()];
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#get(int, int)
	 */
	@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);
		buf.append(fContent, offset, fGapStart - offset);
		buf.append(fContent, fGapEnd, end - fGapStart);
		return buf.toString();
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#getLength()
	 */
	@Override
	public final int getLength() {
		return fContent.length - gapSize();
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#set(java.lang.String)
	 */
	@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);
	}

	/*
	 * @see org.eclipse.jface.text.ITextStore#replace(int, int,
	 * java.lang.String)
	 */
	@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 <code>offset + add</code>, moving any content after
	 * <code>offset + remove</code> behind the gap. The gap size is kept between
	 * 0 and {@link #fThreshold}, leading to re-allocation if needed. The
	 * content between <code>offset</code> and <code>offset + add</code> 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
	 *            <code>offset</code>
	 * @param add the number of character which are inserted or overwriting at
	 *            <code>offset</code>
	 */
	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 <code>newGapStart</code>.
	 *
	 * @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 <code>char[size]</code>.
	 *
	 * @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;
	}
}
