Un semplice elenco collegato di array in Java

Introduzione

Questa semplice struttura di dati combina ArrayList con LinkedList. In altre parole, è un elenco collegato di array:

Codice

package net.coderodde.util.experimental; /** * This class implements an experimental linked list data structure that * combines linked list with array-based list. * * @author Rodion "rodde" Efremov * @version 1.6 (Aug 22, 2018) */ public final class LinkedBlockList<T> { private static final int DEFAULT_BLOCK_CAPACITY = 64; private static final int MINIMUM_BLOCK_CAPACITY = 4; /** * This static inner class implements the actual blocks storing the * elements. * * @param <T> the element type. */ private static final class Block<T> { /** * The length of {@code array}. */ final int capacity; /** * The mask used for modulo computation. */ final int indexMask; /** * The number of elements in this block. */ int size; /** * The index of the very first element in this block. */ int headIndex; /** * The array holding all the elements belonging to this block. */ T[] array; /** * The previous block. */ Block<T> previousBlock; /** * The next block. */ Block<T> nextBlock; Block(int capacity) { this.capacity = capacity; this.indexMask = capacity - 1; this.array = (T[]) new Object[capacity]; } T get(int logicalIndex) { return array[(headIndex + logicalIndex) & indexMask]; } void setNull(int logicalIndex) { array[(headIndex + logicalIndex) & indexMask] = null; } } /** * The number of elements in this list. */ private int size; /** * The number of blocks contained by this list. */ private int blocks; /** * The first block in the chain. */ private Block<T> headBlock; /** * The last block in the chain. */ private Block<T> tailBlock; /** * The block capacity. */ private int blockCapacity; /** * The mask used for index computation. */ private int indexMask; public LinkedBlockList(int blockCapacity) { blockCapacity = Math.max(blockCapacity, MINIMUM_BLOCK_CAPACITY); blockCapacity = ceilToPowerOfTwo(blockCapacity); this.blockCapacity = blockCapacity; this.indexMask = blockCapacity - 1; } public LinkedBlockList() { this(DEFAULT_BLOCK_CAPACITY); } public void add(int index, T element) { checkAddIndex(index); if (size == 0) { headBlock = new Block<>(blockCapacity); tailBlock = headBlock; headBlock.array[0] = element; headBlock.size = 1; size = 1; return; } Block<T> block = headBlock; while (index > block.size) { index -= block.size; block = block.nextBlock; } if (block.size == block.capacity) { // Create a new block and move to it as little elements as possible: int elementsOnLeft = index; int elementsOnRight = block.size - index; Block<T> newBlock = new Block<>(blockCapacity); if (elementsOnLeft < elementsOnRight) { // Add newBlock before block and move to it the prefix of the // current block and append the new element: for (int newBlockIndex = 0; newBlockIndex < elementsOnLeft; newBlockIndex++) { newBlock.array[newBlockIndex] = block.get(newBlockIndex); block.setNull(newBlockIndex); } newBlock.array[elementsOnLeft] = element; newBlock.size = elementsOnLeft + 1; newBlock.nextBlock = block; newBlock.previousBlock = block.previousBlock; block.previousBlock = newBlock; block.size -= elementsOnLeft; block.headIndex = (block.headIndex + elementsOnLeft) & indexMask; if (newBlock.previousBlock == null) { headBlock = newBlock; } else { newBlock.previousBlock.nextBlock = newBlock; } } else { block.size -= elementsOnRight; newBlock.array[0] = element; int targetIndex = 1; for (int newBlockIndex = index; newBlockIndex < elementsOnRight; newBlockIndex++) { newBlock.array[targetIndex] = block.get(newBlockIndex); block.setNull(newBlockIndex); targetIndex++; } newBlock.size = elementsOnRight + 1; newBlock.previousBlock = block; newBlock.nextBlock = block.nextBlock; block.nextBlock = newBlock; if (newBlock.nextBlock == null) { tailBlock = newBlock; } else { newBlock.nextBlock.previousBlock = newBlock; } } } else { // The current block is not full so insert into it: int elementsOnLeft = index; int elementsOnRight = block.size - index; if (elementsOnLeft < elementsOnRight) { // Shift the leftmost elements one position to the left: for (int elementIndex = 0; elementIndex < elementsOnLeft; elementIndex++) { int sourceIndex = (block.headIndex + elementIndex) & indexMask; int targetIndex = (block.headIndex + elementIndex - 1) & indexMask; block.array[targetIndex] = block.array[sourceIndex]; } block.array[(block.headIndex + index - 1) & indexMask] = element; block.headIndex = (block.headIndex - 1) & indexMask; block.size++; } else { // Shift the rightmost elements one position to the right: for (int elementIndex = 0; elementIndex < elementsOnRight; elementIndex++) { int sourceIndex = (block.headIndex + block.size - elementIndex - 1) & indexMask; int targetIndex = (block.headIndex + block.size - elementIndex) & indexMask; block.array[targetIndex] = block.array[sourceIndex]; } block.array[(block.headIndex + index) & indexMask] = element; block.size++; } } size++; } public T get(int index) { checkAccessIndex(index); Block<T> block = headBlock; while (index >= block.size) { index -= block.size; block = block.nextBlock; } return block.get(index); } public void remove(int index) { checkAccessIndex(index); Block<T> targetBlock = headBlock; while (index >= targetBlock.size) { index -= targetBlock.size; targetBlock = targetBlock.nextBlock; } if (targetBlock.size == 1) { // The target block contains only one element. Unlink it from the // chain of blocks: if (targetBlock == headBlock) { headBlock = headBlock.nextBlock; if (headBlock != null) { headBlock.previousBlock = null; } } else { targetBlock.previousBlock.nextBlock = targetBlock.nextBlock; } if (targetBlock == tailBlock) { tailBlock = tailBlock.previousBlock; if (tailBlock != null) { tailBlock.nextBlock = null; } } else { targetBlock.nextBlock.previousBlock = targetBlock.previousBlock; } } else { int elementsOnLeft = index; int elementsOnRight = targetBlock.size - index - 1; if (elementsOnLeft < elementsOnRight) { // Shift the leftmost elements in the target block one position // to the right: for (int i = index - 1; i >= 0; i--) { int sourceIndex = (targetBlock.headIndex + i) & indexMask; int targetIndex = (targetBlock.headIndex + i + 1) & indexMask; targetBlock.array[targetIndex] = targetBlock.array[sourceIndex]; } targetBlock.setNull(0); targetBlock.headIndex = (targetBlock.headIndex + 1) & indexMask; targetBlock.size--; } else { // Shift the rightmost elements in the target block one position // to the left: for (int i = index + 1; i < targetBlock.size; i++) { int sourceIndex = (targetBlock.headIndex + i) & indexMask; int targetIndex = (targetBlock.headIndex + i - 1) & indexMask; targetBlock.array[targetIndex] = targetBlock.array[sourceIndex]; } targetBlock.size--; targetBlock.setNull(targetBlock.size); } } size--; } public int size() { return size; } /** * Returns a number between zero and one indicating how densely the blocks * are. * * @return density factor. */ public float getDensityFactor() { return ((float) size) / blocks * blockCapacity; } private void checkAccessIndex(int index) { if (index < 0) { throw new IndexOutOfBoundsException("index(" + index + ") < 0"); } if (index >= size) { throw new IndexOutOfBoundsException( "index(" + index + ") >= (" + size + ")"); } } private void checkAddIndex(int index) { if (index < 0) { throw new IndexOutOfBoundsException("index(" + index + ") < 0"); } if (index > size) { throw new IndexOutOfBoundsException( "index(" + index + ") > (" + size + ")"); } } private static int ceilToPowerOfTwo(int number) { int ret = 1; while (ret < number) { ret <<= 1; } return ret; } } 

Dati sul rendimento

  LinkedBlockList.add in 106 ms. LinkedBlockList.get in 214 ms. LinkedBlockList.remove in 223 ms. LinkedBlockList total time: 543 ms. LinkedList.add in 4810 ms. LinkedList.get in 13839 ms. LinkedList.remove in 7292 ms. LinkedList total time: 25941 ms.  

Consulta GitHub per pseudo benchmark e unit test.

Qualsiasi critica è molto apprezzata.