threadblockiterator.java

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package com.redmoon.forum;


public class ThreadBlockIterator {
    String groupKey;

    private long[] infoBlock;

    private long blockID;
    private long blockStart;

    private String query;

    private long startIndex;
    private long currentIndex;
    private long endIndex;

    private Object previousElement = null;
    private Object nextElement = null;

    /**
     * Constructs a new ForumThreadBlockIterator.
     *
     * @param threadBlock the starting threadBlock of elements to iterate
     *      through.
     * @param query the SQL query corresponding to this iteration.
     * @param startIndex the starting index of the iteration.
     * @param endIndex the ending index of the iteration.
     */
    protected ThreadBlockIterator(long[] threadBlock, String query,
                                  String groupKey,
                                  long startIndex, long endIndex) {
        this.infoBlock = threadBlock;
        this.blockID = startIndex / MsgCache.MSG_BLOCK_SIZE;
        this.blockStart = blockID * MsgCache.MSG_BLOCK_SIZE;
        this.query = query;
        this.currentIndex = startIndex - 1;
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.groupKey = groupKey;
    }

    public boolean hasNext() {
        // If we are at the end of the list there are no more elements.
        if (currentIndex == endIndex) {
            return false;
        }
        // Otherwise, see if nextElement is null. If so, try to load the next
        // element to make sure it exists. If nextElement isn't null, that
        // means we've already checked to make sure it exists.
        if (nextElement == null) {
            nextElement = getNextElement();
            // If getting the next element failed, return false.
            if (nextElement == null) {
                return false;
            }
        }
        return true;
    }

    public boolean hasPrevious() {
        // If we are at the start of the list there are no previous elements.
        if (currentIndex == startIndex) {
            return false;
        }
        // Otherwise, see if previousElement is null. If so, try to load the
        // previous element to make sure it exists.
        if (previousElement == null) {
            previousElement = getPreviousElement();
            // If getting the previous element failed, return false.
            if (previousElement == null) {
                return false;
            }
        }
        return true;
    }

    public Object next() throws java.util.NoSuchElementException {
        Object element = null;
        if (nextElement != null) {
            element = nextElement;
            this.nextElement = null;
        } else {
            element = getNextElement();
            if (element == null) {
                throw new java.util.NoSuchElementException();
            }
        }
        return element;
    }

    public Object previous() {
        Object element = null;
        if (previousElement != null) {
            element = previousElement;
            previousElement = null;
        } else {
            element = getPreviousElement();
            if (element == null) {
                throw new java.util.NoSuchElementException();
            }
        }
        return element;
    }

    /**
     * Returns the next element, or null if there are no more
     * elements to return.
     *
     * @return the next element.
     */
    private Object getNextElement() {
        previousElement = null;
        Object element = null;
        if (endIndex != Integer.MAX_VALUE) { //如果为max_value还用下句则有可能会导致CPU 100%
            while (currentIndex + 1 < endIndex && element == null) {
                currentIndex++;
                element = getElement(currentIndex);
            }
        } else { //如果next没有，则不再继续找
            if (currentIndex + 1 < endIndex) {
                currentIndex++;
                element = getElement(currentIndex);
            }
        }
        return element;
    }

    /**
     * Returns the previous element, or null if there are no more elements
     * to return.
     *
     * @return the previous element.
     */
    private Object getPreviousElement() {
        nextElement = null;

        Object element = null;
        while (currentIndex >= startIndex && element == null) {
            currentIndex--;
            element = getElement(currentIndex);
        }
        return element;
    }

    /**
     *
     * @param index int 对应第多少条记录
     * @return Object
     */
    private Object getElement(long index) {
        if (index < 0) {
            return null;
        }
        // See if element isn't in the current info block
        // 因为endIndex可能会大于本block的最末尾的序号
        if (index < blockStart || index >= blockStart + MsgCache.THREAD_BLOCK_SIZE) {
            // Then load up the appropriate block
            MsgCache mc = new MsgCache();
            this.infoBlock = mc.getThreadsBlock(query, groupKey, index);
            this.blockID = index / MsgCache.THREAD_BLOCK_SIZE;
            this.blockStart = blockID * MsgCache.THREAD_BLOCK_SIZE;
        }
        Object element = null;
        // Compute the relative index of the element, which is the index in the
        // current thread block.
        int relativeIndex = (int)(index % MsgCache.THREAD_BLOCK_SIZE);
        // Make sure index isn't too large
        if (relativeIndex < infoBlock.length) {
            MsgMgr msgmgr = new MsgMgr();
            element = msgmgr.getMsgDb(infoBlock[relativeIndex]);
        }
        return element;
    }

    public void setIndex(MsgDb msgDb) {
        // Set nextElement and previousElement to null since we may be moving
        // the index.
        nextElement = null;
        previousElement = null;

        // Scan through all blocks looking for thread.
        long threadID = msgDb.getId();
        long[] currentBlock;
        for (long i = startIndex; i < endIndex; i++) {
            currentBlock = msgDb.getThreadsBlock(query, msgDb.getboardcode(), i);
            if (currentBlock.length == 0) {
                //throw new ErrMsgException("Thread with id " +
                //    threadID + " is not a valid index in the iteration.");
            }
            long blockID = i / MsgCache.THREAD_BLOCK_SIZE;
            long blockEnd = blockID * MsgCache.THREAD_BLOCK_SIZE +
                           MsgCache.THREAD_BLOCK_SIZE;
            // If in first block of threads
            if (startIndex < blockEnd) {
                // If we are in the first block, j should start at the
                // start index instead of the beginning of the block.
                for (long j = startIndex % MsgCache.THREAD_BLOCK_SIZE;
                             j < currentBlock.length; j++, i++) {
                    if (currentBlock[(int)j] == threadID) {
                        this.currentIndex = i;
                        return;
                    }
                }
            }
            // Otherwise, not in first block so start looking at beginning
            else {
                for (int j = 0; j < currentBlock.length; j++, i++) {
                    if (currentBlock[j] == threadID) {
                        this.currentIndex = i;
                        return;
                    }
                }
            }
        }

    }
}