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package com.fasterxml.jackson.databind.util;

import java.lang.reflect.Array;
import java.util.List;

/**
 * Helper class to use for constructing Object arrays by appending entries
 * to create arrays of various lengths (length that is not known a priori). 
 */
public final class ObjectBuffer
{
    // // // Config constants

    /**
     * Let's start with small chunks; typical usage is for small arrays anyway.
     */
    final static int INITIAL_CHUNK_SIZE = 12;

    /**
     * Also: let's expand by doubling up until 64k chunks (which is 16k entries for
     * 32-bit machines)
     */
    final static int SMALL_CHUNK_SIZE = (1 << 14);

    /**
     * Let's limit maximum size of chunks we use; helps avoid excessive allocation
     * overhead for huge data sets.
     * For now, let's limit to quarter million entries, 1 meg chunks for 32-bit
     * machines.
     */
    final static int MAX_CHUNK_SIZE = (1 << 18);

    // // // Data storage

    private Node _bufferHead;

    private Node _bufferTail;

    /**
     * Number of total buffered entries in this buffer, counting all instances
     * within linked list formed by following {@link #_bufferHead}.
     */
    private int _bufferedEntryCount;

    // // // Simple reuse

    /**
     * Reusable Object array, stored here after buffer has been released having
     * been used previously.
     */
    private Object[] _freeBuffer;

    /*
    /**********************************************************
    /* Construction
    /**********************************************************
     */

    public ObjectBuffer() { }

    /*
    /**********************************************************
    /* Public API
    /**********************************************************
     */

    /**
     * Method called to start buffering process. Will ensure that the buffer
     * is empty, and then return an object array to start chunking content on
     */
    public Object[] resetAndStart()
    {
        _reset();
        if (_freeBuffer == null) {
            return new Object[INITIAL_CHUNK_SIZE];
        }
        return _freeBuffer;
    }

    /**
     * Method called to add a full Object array as a chunk buffered within
     * this buffer, and to obtain a new array to fill. Caller is not to use
     * the array it gives; but to use the returned array for continued
     * buffering.
     *
     * @param fullChunk Completed chunk that the caller is requesting
     *   to append to this buffer. It is generally chunk that was
     *   returned by an earlier call to {@link #resetAndStart} or
     *   {@link #appendCompletedChunk} (although this is not required or
     *   enforced)
     *
     * @return New chunk buffer for caller to fill
     */
    public Object[] appendCompletedChunk(Object[] fullChunk)
    {
        Node next = new Node(fullChunk);
        if (_bufferHead == null) { // first chunk
            _bufferHead = _bufferTail = next;
        } else { // have something already
            _bufferTail.linkNext(next);
            _bufferTail = next;
        }
        int len = fullChunk.length;
        _bufferedEntryCount += len;
        // double the size for small chunks
        if (len < SMALL_CHUNK_SIZE) {
            len += len;
        } else { // but by +25% for larger (to limit overhead)
            len += (len >> 2);
        }
        return new Object[len];
    }

    /**
     * Method called to indicate that the buffering process is now
     * complete; and to construct a combined exactly-sized result
     * array. Additionally the buffer itself will be reset to
     * reduce memory retention.
     *

* Resulting array will be of generic Object[] type: * if a typed array is needed, use the method with additional * type argument. */ public Object[] completeAndClearBuffer(Object[] lastChunk, int lastChunkEntries) { int totalSize = lastChunkEntries + _bufferedEntryCount; Object[] result = new Object[totalSize]; _copyTo(result, totalSize, lastChunk, lastChunkEntries); return result; } /** * Type-safe alternative to * {@link #completeAndClearBuffer(Object[], int)}, to allow * for constructing explicitly typed result array. * * @param componentType Type of elements included in the buffer. Will be * used for constructing the result array. */ public T[] completeAndClearBuffer(Object[] lastChunk, int lastChunkEntries, Class componentType) { int totalSize = lastChunkEntries + _bufferedEntryCount; @SuppressWarnings("unchecked") T[] result = (T[]) Array.newInstance(componentType, totalSize); _copyTo(result, totalSize, lastChunk, lastChunkEntries); _reset(); return result; } public void completeAndClearBuffer(Object[] lastChunk, int lastChunkEntries, List resultList) { for (Node n = _bufferHead; n != null; n = n.next()) { Object[] curr = n.getData(); for (int i = 0, len = curr.length; i < len; ++i) { resultList.add(curr[i]); } } // and then the last one for (int i = 0; i < lastChunkEntries; ++i) { resultList.add(lastChunk[i]); } } /** * Helper method that can be used to check how much free capacity * will this instance start with. Can be used to choose the best * instance to reuse, based on size of reusable object chunk * buffer holds reference to. */ public int initialCapacity() { return (_freeBuffer == null) ? 0 : _freeBuffer.length; } /** * Method that can be used to check how many Objects have been buffered * within this buffer. */ public int bufferedSize() { return _bufferedEntryCount; } /* /********************************************************** /* Internal methods /********************************************************** */ protected void _reset() { // can we reuse the last (and thereby biggest) array for next time? if (_bufferTail != null) { _freeBuffer = _bufferTail.getData(); } // either way, must discard current contents _bufferHead = _bufferTail = null; _bufferedEntryCount = 0; } protected final void _copyTo(Object resultArray, int totalSize, Object[] lastChunk, int lastChunkEntries) { int ptr = 0; for (Node n = _bufferHead; n != null; n = n.next()) { Object[] curr = n.getData(); int len = curr.length; System.arraycopy(curr, 0, resultArray, ptr, len); ptr += len; } System.arraycopy(lastChunk, 0, resultArray, ptr, lastChunkEntries); ptr += lastChunkEntries; // sanity check (could have failed earlier due to out-of-bounds, too) if (ptr != totalSize) { throw new IllegalStateException("Should have gotten "+totalSize+" entries, got "+ptr); } } /* /********************************************************** /* Helper classes /********************************************************** */ /** * Helper class used to store actual data, in a linked list. */ final static class Node { /** * Data stored in this node. Array is considered to be full. */ final Object[] _data; Node _next; public Node(Object[] data) { _data = data; } public Object[] getData() { return _data; } public Node next() { return _next; } public void linkNext(Node next) { if (_next != null) { // sanity check throw new IllegalStateException(); } _next = next; } } }