一个随机播放的算法II

// 指定列表有n首歌，初始比重为1.
mRandomPicker = new RandomPicker(n, 1); 
// 进入切歌模式。
mRandomPicker.enterCutMode();
// 随机获取下一首
int nextPos = mRandomPicker.next();
...
// 退出切歌模式
mRandomPicker.exitCutMode();

// 更默认的比重计算器
mRandomPicker.setCalculator(new Calculator() {
@Override
public int calculateNextWeight(int currentWeight, int originWeight) {
    return (currentWeight + 1) * originWeight;
    }
 });
// 改变某个item的初始比重
mRandomPicker.changeOriginWeight(0, 3);
// 指定下次随机到的数
mRandomPicker.setNextPick(3);
//添加一个item至尾部，并为其赋值初始比重
mRandomPicker.add(2);

    /*执行随机算法*/
    private int randomPick() {
        // 若列表长度小于2，则下一次位置必为0.
        if (mCurrentWeightList.size() < 2) {
            return 0;
        }

        int nextPos = 0;
        // 算出下一次选中的位置
        if (mNextPickPosition != null) {
            nextPos = mNextPickPosition;
            mNextPickPosition = null;
        } else {
            int allWeight = 0;
            for (int i = 0; i < mCurrentWeightList.size(); i++) {
                allWeight += mCurrentWeightList.get(i);
            }

            if (allWeight <= 0) {
                //TODO avoid this situation.
                allWeight = Integer.MAX_VALUE;
                //Log.e(TAG, "...");
            }

            int nextPosInWeight = mRandom.nextInt(allWeight);
            int currentWeight = 0;
            for (int i = 0; i < mCurrentWeightList.size(); i++) {
                currentWeight += mCurrentWeightList.get(i);
                if (currentWeight > nextPosInWeight) {
                    nextPos = i;
                    break;
                }
            }
        }

        // 预先算好下一次的比重
        for (int i = 0; i < mCurrentWeightList.size(); i++) {
            if (isCutMode()) {
                if (mCutOutSet.contains(i)) {
                    continue;
                }
            }
            int weight = calculateWeight(mCurrentWeightList.get(i), mOriginWeightList.get(i));
            mCurrentWeightList.set(i, weight);
        }
        if (isRepeatable)
            mCurrentWeightList.set(nextPos, calculateWeight(0, mOriginWeightList.get(nextPos)));
        else
            mCurrentWeightList.set(nextPos, 0);

        if (isCutMode()) {
            mCurrentWeightList.set(nextPos, 0);
            mCutOutSet.add(nextPos);
            if (mCutOutSet.size() >= getSize())
                mCutOutSet.clear();
        }
        return nextPos;
    }