runtime/arrays/ArrayData.java

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package jdk.nashorn.internal.runtime.arrays;

import static jdk.nashorn.internal.codegen.CompilerConstants.staticCall;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.reflect.Array;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import jdk.internal.dynalink.CallSiteDescriptor;
import jdk.internal.dynalink.linker.GuardedInvocation;
import jdk.internal.dynalink.linker.LinkRequest;
import jdk.nashorn.internal.codegen.CompilerConstants;
import jdk.nashorn.internal.codegen.types.Type;
import jdk.nashorn.internal.objects.Global;
import jdk.nashorn.internal.runtime.JSType;
import jdk.nashorn.internal.runtime.PropertyDescriptor;
import jdk.nashorn.internal.runtime.ScriptRuntime;
import jdk.nashorn.internal.runtime.UnwarrantedOptimismException;

/**
 * ArrayData - abstraction for wrapping array elements
 */
public abstract class ArrayData {
    /** Minimum chunk size for underlying arrays */
    protected static final int CHUNK_SIZE = 32;

    /** Mask for getting a chunk */
    protected static final int CHUNK_MASK = CHUNK_SIZE - 1;

    /** Untouched data - still link callsites as IntArrayData, but expands to
     *  a proper ArrayData when we try to write to it */
    public static final ArrayData EMPTY_ARRAY = new UntouchedArrayData();

    /**
     * Length of the array data. Not necessarily length of the wrapped array.
     * This is private to ensure that no one in a subclass is able to touch the length
     * without going through {@link setLength}. This is used to implement
     * {@link LengthNotWritableFilter}s, ensuring that there are no ways past
     * a {@link setLength} function replaced by a nop
     */
    private long length;

    /**
     * Method handle to throw an {@link UnwarrantedOptimismException} when getting an element
     * of the wrong type
     */
    protected static final CompilerConstants.Call THROW_UNWARRANTED = staticCall(MethodHandles.lookup(), ArrayData.class, "throwUnwarranted", void.class, ArrayData.class, int.class, int.class);

    /**
     * Immutable empty array to get ScriptObjects started.
     * Use the same array and convert it to mutable as soon as it is modified
     */
    private static class UntouchedArrayData extends ContinuousArrayData {
        private UntouchedArrayData() {
            this(0);
        }

        private UntouchedArrayData(final int length) {
            super(length);
        }

        private ArrayData toRealArrayData() {
            return toRealArrayData(0);
        }

        private ArrayData toRealArrayData(final int index) {
            final IntArrayData newData = new IntArrayData(index + 1);
            if (index == 0) {
                return newData;
            }
            return new DeletedRangeArrayFilter(newData, 0, index);
        }

        @Override
        public ContinuousArrayData copy() {
            return new UntouchedArrayData((int)length());
        }

        @Override
        public Object asArrayOfType(final Class<?> componentType) {
            return Array.newInstance(componentType, 0);
        }

        @Override
        public Object[] asObjectArray() {
            return ScriptRuntime.EMPTY_ARRAY;
        }

        @Override
        public ArrayData ensure(final long safeIndex) {
            if (safeIndex > 0L) {
                if (safeIndex >= SparseArrayData.MAX_DENSE_LENGTH) {
                    return new SparseArrayData(this, safeIndex + 1);
                }
                //known to fit in int
                return toRealArrayData((int)safeIndex).ensure(safeIndex);
           }
           return this;
        }

        @Override
        public ArrayData convert(final Class<?> type) {
            return toRealArrayData(0).convert(type);
        }

        @Override
        public ArrayData delete(final int index) {
            return new DeletedRangeArrayFilter(this, index, index);
        }

        @Override
        public ArrayData delete(final long fromIndex, final long toIndex) {
            return new DeletedRangeArrayFilter(this, fromIndex, toIndex);
        }

        @Override
        public void shiftLeft(final int by) {
            //nop, always empty or we wouldn't be of this class
        }

        @Override
        public ArrayData shiftRight(final int by) {
            return this; //always empty or we wouldn't be of this class
        }

        @Override
        public ArrayData shrink(final long newLength) {
            return this;
        }

        @Override
        public ArrayData set(final int index, final Object value, final boolean strict) {
            return toRealArrayData(index).set(index, value, strict);
        }

        @Override
        public ArrayData set(final int index, final int value, final boolean strict) {
            return toRealArrayData(index).set(index, value, strict);
        }

        @Override
        public ArrayData set(final int index, final long value, final boolean strict) {
            return toRealArrayData(index).set(index, value, strict);
        }

        @Override
        public ArrayData set(final int index, final double value, final boolean strict) {
            return toRealArrayData(index).set(index, value, strict);
        }

        @Override
        public int getInt(final int index) {
            throw new ArrayIndexOutOfBoundsException(index); //empty
        }

        @Override
        public long getLong(final int index) {
            throw new ArrayIndexOutOfBoundsException(index); //empty
        }

        @Override
        public double getDouble(final int index) {
            throw new ArrayIndexOutOfBoundsException(index); //empty
        }

        @Override
        public Object getObject(final int index) {
            throw new ArrayIndexOutOfBoundsException(index); //empty
        }

        @Override
        public boolean has(final int index) {
            return false; //empty
        }

        @Override
        public Object pop() {
            return ScriptRuntime.UNDEFINED;
        }

        @Override
        public ArrayData push(final boolean strict, final Object item) {
            return toRealArrayData().push(strict, item);
        }

        @Override
        public ArrayData slice(final long from, final long to) {
            return this; //empty
        }

        @Override
        public ContinuousArrayData fastConcat(final ContinuousArrayData otherData) {
            return otherData.copy();
        }

        //no need to override fastPopInt, as the default behavior is to throw classcast exception so we
        //can relink and return an undefined, this is the IntArrayData default behavior
        @Override
        public String toString() {
            return getClass().getSimpleName();
        }

        @Override
        public MethodHandle getElementGetter(final Class<?> returnType, final int programPoint) {
            return null;
        }

        @Override
        public MethodHandle getElementSetter(final Class<?> elementType) {
            return null;
        }

        @Override
        public Class<?> getElementType() {
            return int.class;
        }

        @Override
        public Class<?> getBoxedElementType() {
            return Integer.class;
        }
    };

    /**
     * Constructor
     * @param length Virtual length of the array.
     */
    protected ArrayData(final long length) {
        this.length = length;
    }

    /**
     * Factory method for unspecified array - start as int
     * @return ArrayData
     */
    public final static ArrayData initialArray() {
        return new IntArrayData();
    }

    /**
     * Unwarranted thrower
     *
     * @param data         array data
     * @param programPoint program point
     * @param index        array index
     */
    protected static void throwUnwarranted(final ArrayData data, final int programPoint, final int index) {
        throw new UnwarrantedOptimismException(data.getObject(index), programPoint);
    }

    /**
     * Align an array size up to the nearest array chunk size
     * @param size size required
     * @return size given, always >= size
     */
    protected final static int alignUp(final int size) {
        return size + CHUNK_SIZE - 1 & ~(CHUNK_SIZE - 1);
    }

    /**
     * Factory method for unspecified array with given length - start as int array data
     *
     * @param length the initial length
     * @return ArrayData
     */
    public static final ArrayData allocate(final int length) {
        if (length == 0) {
            return new IntArrayData();
        } else if (length >= SparseArrayData.MAX_DENSE_LENGTH) {
            return new SparseArrayData(EMPTY_ARRAY, length);
        } else {
            return new DeletedRangeArrayFilter(new IntArrayData(length), 0, length - 1);
        }
    }

    /**
     * Factory method for unspecified given an array object
     *
     * @param  array the array
     * @return ArrayData wrapping this array
     */
    public static final ArrayData allocate(final Object array) {
        final Class<?> clazz = array.getClass();

        if (clazz == int[].class) {
            return new IntArrayData((int[])array, ((int[])array).length);
        } else if (clazz == long[].class) {
            return new LongArrayData((long[])array, ((long[])array).length);
        } else if (clazz == double[].class) {
            return new NumberArrayData((double[])array, ((double[])array).length);
        } else {
            return new ObjectArrayData((Object[])array, ((Object[])array).length);
        }
    }

    /**
     * Allocate an ArrayData wrapping a given array
     *
     * @param array the array to use for initial elements
     * @return the ArrayData
     */
    public static final ArrayData allocate(final int[] array) {
         return new IntArrayData(array, array.length);
    }

    /**
     * Allocate an ArrayData wrapping a given array
     *
     * @param array the array to use for initial elements
     * @return the ArrayData
     */
    public static final ArrayData allocate(final long[] array) {
        return new LongArrayData(array, array.length);
    }

    /**
     * Allocate an ArrayData wrapping a given array
     *
     * @param array the array to use for initial elements
     * @return the ArrayData
     */
    public static final ArrayData allocate(final double[] array) {
        return new NumberArrayData(array, array.length);
    }

    /**
     * Allocate an ArrayData wrapping a given array
     *
     * @param array the array to use for initial elements
     * @return the ArrayData
     */
    public static final ArrayData allocate(final Object[] array) {
        return new ObjectArrayData(array, array.length);
    }

    /**
     * Allocate an ArrayData wrapping a given nio ByteBuffer
     *
     * @param buf the nio ByteBuffer to wrap
     * @return the ArrayData
     */
    public static final ArrayData allocate(final ByteBuffer buf) {
        return new ByteBufferArrayData(buf);
    }

    /**
     * Apply a freeze filter to an ArrayData.
     *
     * @param underlying  the underlying ArrayData to wrap in the freeze filter
     * @return the frozen ArrayData
     */
    public static final ArrayData freeze(final ArrayData underlying) {
        return new FrozenArrayFilter(underlying);
    }

    /**
     * Apply a seal filter to an ArrayData.
     *
     * @param underlying  the underlying ArrayData to wrap in the seal filter
     * @return the sealed ArrayData
     */
    public static final ArrayData seal(final ArrayData underlying) {
        return new SealedArrayFilter(underlying);
    }

    /**
     * Prevent this array from being extended
     *
     * @param  underlying the underlying ArrayData to wrap in the non extensible filter
     * @return new array data, filtered
     */
    public static final ArrayData preventExtension(final ArrayData underlying) {
        return new NonExtensibleArrayFilter(underlying);
    }

    /**
     * Prevent this array from having its length reset
     *
     * @param underlying the underlying ArrayDAta to wrap in the non extensible filter
     * @return new array data, filtered
     */
    public static final ArrayData setIsLengthNotWritable(final ArrayData underlying) {
        return new LengthNotWritableFilter(underlying);
    }

    /**
     * Return the length of the array data. This may differ from the actual
     * length of the array this wraps as length may be set or gotten as any
     * other JavaScript Property
     *
     * Even though a JavaScript array length may be a long, we only store
     * int parts for the optimized array access. For long lengths there
     * are special cases anyway.
     *
     * TODO: represent arrays with "long" lengths as a special ArrayData
     * that basically maps to the ScriptObject directly for better abstraction
     *
     * @return the length of the data
     */
    public final long length() {
        return length;
    }

    /**
     * Return a copy of the array that can be modified without affecting this instance.
     * It is safe to return themselves for immutable subclasses.
     *
     * @return a new array
     */
    public abstract ArrayData copy();

    /**
     * Return a copy of the array data as an Object array.
     *
     * @return an Object array
     */
    public abstract Object[] asObjectArray();

    /**
     * Return a copy of the array data as an array of the specified type.
     *
     * @param componentType  the type of elements in the array
     * @return and array of the given type
     */
    public Object asArrayOfType(final Class<?> componentType) {
        return JSType.convertArray(asObjectArray(), componentType);
    }

    /**
     * Set the length of the data array
     *
     * @param length the new length for the data array
     */
    public void setLength(final long length) {
        this.length = length;
    }

    /**
     * Increase length by 1
     * @return the new length, not the old one (i.e. pre-increment)
     */
    protected final long increaseLength() {
        return ++this.length;
    }

    /**
     * Decrease length by 1.
     * @return the new length, not the old one (i.e. pre-decrement)
     */
    protected final long decreaseLength() {
        return --this.length;
    }

    /**
     * Shift the array data left
     *
     * TODO: explore start at an index and not at zero, to make these operations
     * even faster. Offset everything from the index. Costs memory but is probably
     * worth it
     *
     * @param by offset to shift
     */
    public abstract void shiftLeft(final int by);

    /**
     * Shift the array right
     *
     * @param by offset to shift

     * @return New arraydata (or same)
     */
    public abstract ArrayData shiftRight(final int by);

    /**
     * Ensure that the given index exists and won't fail subsequent
     *
     * @param safeIndex the index to ensure wont go out of bounds
     * @return new array data (or same)
     */
    public abstract ArrayData ensure(final long safeIndex);

    /**
     * Shrink the array to a new length, may or may not retain the
     * inner array
     *
     * @param newLength new max length
     *
     * @return new array data (or same)
     */
    public abstract ArrayData shrink(final long newLength);

    /**
     * Set an object value at a given index
     *
     * @param index the index
     * @param value the value
     * @param strict are we in strict mode
     * @return new array data (or same)
     */
    public abstract ArrayData set(final int index, final Object value, final boolean strict);

    /**
     * Set an int value at a given index
     *
     * @param index the index
     * @param value the value
     * @param strict are we in strict mode
     * @return new array data (or same)
     */
    public abstract ArrayData set(final int index, final int value, final boolean strict);

    /**
     * Set a long value at a given index
     *
     * @param index the index
     * @param value the value
     * @param strict are we in strict mode
     * @return new array data (or same)
     */
    public abstract ArrayData set(final int index, final long value, final boolean strict);

    /**
     * Set an double value at a given index
     *
     * @param index the index
     * @param value the value
     * @param strict are we in strict mode
     * @return new array data (or same)
     */
    public abstract ArrayData set(final int index, final double value, final boolean strict);

    /**
     * Set an empty value at a given index. Should only affect Object array.
     *
     * @param index the index
     * @return new array data (or same)
     */
    public ArrayData setEmpty(final int index) {
        // Do nothing.
        return this;
    }

    /**
     * Set an empty value for a given range. Should only affect Object array.
     *
     * @param lo range low end
     * @param hi range high end
     * @return new array data (or same)
     */
    public ArrayData setEmpty(final long lo, final long hi) {
        // Do nothing.
        return this;
    }

    /**
     * Get an int value from a given index
     *
     * @param index the index
     * @return the value
     */
    public abstract int getInt(final int index);

    /**
     * Returns the optimistic type of this array data. Basically, when an array data object needs to throw an
     * {@link UnwarrantedOptimismException}, this type is used as the actual type of the return value.
     * @return the optimistic type of this array data.
     */
    public Type getOptimisticType() {
        return Type.OBJECT;
    }

    /**
     * Get optimistic int - default is that it's impossible. Overridden
     * by arrays that actually represents ints
     *
     * @param index        the index
     * @param programPoint program point
     * @return the value
     */
    public int getIntOptimistic(final int index, final int programPoint) {
        throw new UnwarrantedOptimismException(getObject(index), programPoint, getOptimisticType());
    }

    /**
     * Get a long value from a given index
     *
     * @param index the index
     * @return the value
     */
    public abstract long getLong(final int index);

    /**
     * Get optimistic long - default is that it's impossible. Overridden
     * by arrays that actually represents longs or narrower
     *
     * @param index        the index
     * @param programPoint program point
     * @return the value
     */
    public long getLongOptimistic(final int index, final int programPoint) {
        throw new UnwarrantedOptimismException(getObject(index), programPoint, getOptimisticType());
    }

    /**
     * Get a double value from a given index
     *
     * @param index the index
     * @return the value
     */
    public abstract double getDouble(final int index);

    /**
     * Get optimistic double - default is that it's impossible. Overridden
     * by arrays that actually represents doubles or narrower
     *
     * @param index        the index
     * @param programPoint program point
     * @return the value
     */
    public double getDoubleOptimistic(final int index, final int programPoint) {
        throw new UnwarrantedOptimismException(getObject(index), programPoint, getOptimisticType());
    }

    /**
     * Get an Object value from a given index
     *
     * @param index the index
     * @return the value
     */
    public abstract Object getObject(final int index);

    /**
     * Tests to see if an entry exists (avoids boxing.)
     * @param index the index
     * @return true if entry exists
     */
    public abstract boolean has(final int index);

    /**
     * Returns if element at specific index can be deleted or not.
     *
     * @param index the index of the element
     * @param strict are we in strict mode
     *
     * @return true if element can be deleted
     */
    public boolean canDelete(final int index, final boolean strict) {
        return true;
    }

    /**
     * Returns if element at specific index range can be deleted or not.
     *
     * @param fromIndex  the start index
     * @param toIndex    the end index
     * @param strict     are we in strict mode
     *
     * @return true if range can be deleted
     */
    public boolean canDelete(final long fromIndex, final long toIndex, final boolean strict) {
        return true;
    }

    /**
     * Returns property descriptor for element at a given index
     *
     * @param global the global object
     * @param index  the index
     *
     * @return property descriptor for element
     */
    public PropertyDescriptor getDescriptor(final Global global, final int index) {
        return global.newDataDescriptor(getObject(index), true, true, true);
    }

    /**
     * Delete an array value at the given index, substituting
     * for an undefined
     *
     * @param index the index
     * @return new array data (or same)
     */
    public abstract ArrayData delete(final int index);

    /**
     * Delete a given range from this array;
     *
     * @param fromIndex  from index (inclusive)
     * @param toIndex    to index (inclusive)
     *
     * @return new ArrayData after deletion
     */
    public abstract ArrayData delete(final long fromIndex, final long toIndex);

    /**
     * Convert the ArrayData to one with a different element type
     * Currently Arrays are not collapsed to narrower types, just to
     * wider ones. Attempting to narrow an array will assert
     *
     * @param type new element type
     * @return new array data
     */
    public abstract ArrayData convert(final Class<?> type);

    /**
     * Push an array of items to the end of the array
     *
     * @param strict are we in strict mode
     * @param items  the items
     * @return new array data (or same)
     */
    public ArrayData push(final boolean strict, final Object... items) {
        if (items.length == 0) {
            return this;
        }

        final Class<?>  widest  = widestType(items);

        ArrayData newData = convert(widest);
        long      pos     = newData.length;
        for (final Object item : items) {
            newData = newData.ensure(pos); //avoid sparse array
            newData.set((int)pos++, item, strict);
        }
        return newData;
    }

    /**
     * Push an array of items to the end of the array
     *
     * @param strict are we in strict mode
     * @param item   the item
     * @return new array data (or same)
     */
    public ArrayData push(final boolean strict, final Object item) {
        return push(strict, new Object[] { item });
    }

    /**
     * Push an array of items to the end of the array
     *
     * @param strict are we in strict mode
     * @param item   the item
     * @return new array data (or same)
     */
    public ArrayData push(final boolean strict, final double item) {
        return push(strict, item);
    }

    /**
     * Push an array of items to the end of the array
     *
     * @param strict are we in strict mode
     * @param item   the item
     * @return new array data (or same)
     */
    public ArrayData push(final boolean strict, final long item) {
        return push(strict, item);
    }

    /**
     * Push an array of items to the end of the array
     *
     * @param strict are we in strict mode
     * @param item   the item
     * @return new array data (or same)
     */
    public ArrayData push(final boolean strict, final int item) {
        return push(strict, item);
    }

    /**
     * Pop an element from the end of the array
     *
     * @return the popped element
     */
    public abstract Object pop();

    /**
     * Slice out a section of the array and return that
     * subsection as a new array data: [from, to)
     *
     * @param from start index
     * @param to   end index + 1
     * @return new array data
     */
    public abstract ArrayData slice(final long from, final long to);

    /**
     * Fast splice operation. This just modifies the array according to the number of
     * elements added and deleted but does not insert the added elements. Throws
     * {@code UnsupportedOperationException} if fast splice operation is not supported
     * for this class or arguments.
     *
     * @param start start index of splice operation
     * @param removed number of removed elements
     * @param added number of added elements
     * @throws UnsupportedOperationException if fast splice is not supported for the class or arguments.
     * @return new arraydata, but this never happens because we always throw an exception
     */
    public ArrayData fastSplice(final int start, final int removed, final int added) throws UnsupportedOperationException {
        throw new UnsupportedOperationException();
    }

    static Class<?> widestType(final Object... items) {
        assert items.length > 0;

        Class<?> widest = Integer.class;

        for (final Object item : items) {
            if (item == null) {
                return Object.class;
            }
            final Class<?> itemClass = item.getClass();
            if (itemClass == Long.class) {
                if (widest == Integer.class) {
                    widest = Long.class;
                }
            } else if (itemClass == Double.class || itemClass == Float.class) {
                if (widest == Integer.class || widest == Long.class) {
                    widest = Double.class;
                }
            } else if (itemClass != Integer.class && itemClass != Short.class && itemClass != Byte.class) {
                return Object.class;
            }
        }

        return widest;
    }

    /**
     * Return a list of keys in the array for the iterators
     * @return iterator key list
     */
    protected List<Long> computeIteratorKeys() {
        final List<Long> keys = new ArrayList<>();

        final long len = length();
        for (long i = 0L; i < len; i = nextIndex(i)) {
            if (has((int)i)) {
                keys.add(i);
            }
        }

        return keys;
    }

    /**
     * Return an iterator that goes through all indexes of elements
     * in this array. This includes those after array.length if
     * they exist
     *
     * @return iterator
     */
    public Iterator<Long> indexIterator() {
        return computeIteratorKeys().iterator();
    }

    /**
     * Exponential growth function for array size when in
     * need of resizing.
     *
     * @param size current size
     * @return next size to allocate for internal array
     */
    public static int nextSize(final int size) {
        return alignUp(size + 1) * 2;
    }

    /**
     * Return the next valid index from a given one. Subclassed for various
     * array representation
     *
     * @param index the current index
     *
     * @return the next index
     */
    long nextIndex(final long index) {
        return index + 1;
    }

    static Object invoke(final MethodHandle mh, final Object arg) {
        try {
            return mh.invoke(arg);
        } catch (final RuntimeException | Error e) {
            throw e;
        } catch (final Throwable t) {
            throw new RuntimeException(t);
        }
    }

   /**
     * Find a fast call if one exists
     *
     * @param clazz    array data class
     * @param desc     callsite descriptor
     * @param request  link request
     * @return fast property getter if one is found
     */
    public GuardedInvocation findFastCallMethod(final Class<? extends ArrayData> clazz, final CallSiteDescriptor desc, final LinkRequest request) {
        return null;
    }

    /**
     * Find a fast property getter if one exists
     *
     * @param clazz    array data class
     * @param desc     callsite descriptor
     * @param request  link request
     * @param operator operator
     * @return fast property getter if one is found
     */
    public GuardedInvocation findFastGetMethod(final Class<? extends ArrayData> clazz, final CallSiteDescriptor desc, final LinkRequest request, final String operator) {
        return null;
    }

    /**
     * Find a fast element getter if one exists
     *
     * @param clazz   array data class
     * @param desc    callsite descriptor
     * @param request link request
     * @return fast index getter if one is found
     */
    public GuardedInvocation findFastGetIndexMethod(final Class<? extends ArrayData> clazz, final CallSiteDescriptor desc, final LinkRequest request) { // array, index, value
        return null;
    }

    /**
     * Find a fast element setter if one exists
     *
     * @param clazz   array data class
     * @param desc    callsite descriptor
     * @param request link request
     * @return fast index getter if one is found
     */
    public GuardedInvocation findFastSetIndexMethod(final Class<? extends ArrayData> clazz, final CallSiteDescriptor desc, final LinkRequest request) { // array, index, value
        return null;
    }
}