msgblockiterator.java

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package com.redmoon.forum;

public class MsgBlockIterator {
    // 如果是在搜索时，则groupKey取all，如果是dir_code类别的消息列表，则取dir_code
    // 当del create消息时，需要对groupKey组的缓存进行更新
    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.
     * @param forumID the forumID the threads are a part of.
     * @param factory a ForumFactory to load data from.
     */
    protected MsgBlockIterator(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 != Long.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) {
                // System.out.println("MsgBlockIterator.java:currentIndex:" + currentIndex);
                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.MSG_BLOCK_SIZE) {
            // Then load up the appropriate block
            MsgCache mc = new MsgCache();
            this.infoBlock = mc.getMsgBlock(query, groupKey, index);
            this.blockID = index / MsgCache.MSG_BLOCK_SIZE;
            this.blockStart = blockID * MsgCache.MSG_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.MSG_BLOCK_SIZE);
        // Make sure index isn't too large
        if (relativeIndex < infoBlock.length) {
            MsgMgr msgmgr = new MsgMgr();
            element = msgmgr.getMsgDb(infoBlock[relativeIndex]);
        }
        return element;
    }
}