一個關于Android音樂隨機播放的算法

想法

偽隨機。

你的音樂列表里有一些歌，每首歌的初始隨機因數為1。

每次你點擊下一首時，每首歌的隨機因數都會加1，然后隨機到的那首歌隨機因數變為0。

隨機因數越大，被隨機到的幾率就越高。

比如有4首歌，那么下表是一種可能出現的情況：

...

可以看到，Refrain 這首歌連續6次沒有出現，它的隨機因數累加到了6，那么第十次它被隨機到的概率是6/(1+2+6)，即三分之二。

上面使用的是隨機因數累加，其實我們還可以讓隨機因數累乘等等...

Demo及實現

RandomPicker

Demo中的的大圖截圖自網易云音樂。

前往GitHub Star/Fork/Compile

如何使用

快速開始:

RandomPicker randomPicker = new RandomPicker(12);
int nextPos = randomPicker.next();

更多方法:

randomPicker.setMultiplyNumber(3);
randomPicker.setAddNumber(2);
randomPicker.setNextPick(5);
randomPicker.add();
randomPicker.changeOriginWeight(0,3);
randomPicker.getHistoryList();

更多更多:

請下載項目查看源碼

RandomPicker源碼

package top.wefor.randompicker;

import java.util.ArrayList;
import java.util.Random;
/**

Created on 16/8/26.
<p/>
適用于音樂隨機播放等
GitHub: https://github.com/XunMengWinter
<p/>
latest edited date: 2016-08-26
*
@author ice
*/
public class RandomPicker {
private ArrayList<Integer> mOriginWeightList = new ArrayList<>();
 private ArrayList<Integer> mCurrentWeightList = new ArrayList<>();
 private ArrayList<Integer> mHistoryList = new ArrayList<>();
private int mMultiplyNumber = 1;
 private int mAddNumber = 1;
 private int mPickedPosition;
 private boolean isRepeatable;
 private Integer mNextPickPosition;
 Random mRandom = new Random();
public RandomPicker() {

 //默認一個，避免報錯。
 new RandomPicker(1);

}
public RandomPicker(int size) {

 initSize(size);

}
/設置累乘積數/
 public void setMultiplyNumber(int multiplyNumber) {

 mMultiplyNumber = multiplyNumber;

}
/設置累加積數/
 public void setAddNumber(int addNumber) {

 mAddNumber = addNumber;

}
/指定下一次選中的位置/
 public void setNextPick(int pickedPosition) {

 mNextPickPosition = pickedPosition;

}
/是否允許連續兩次出現同一個位置/
 public void setRepeatable(boolean repeatable) {

 isRepeatable = repeatable;

}
/初始化列表長度/
 public void initSize(int size) {

 mOriginWeightList.clear();
 mCurrentWeightList.clear();
 mHistoryList.clear();
 for (int i = 0; i < size; i++)
     add();

}
/獲得當前條目數/
 public int getSize() {

 return mOriginWeightList.size();

}
/獲取歷史條目的位置列表/
 public ArrayList<Integer> getHistoryList() {

 return mHistoryList;

}



         /*上為配置參數*/
         /*下為邏輯實現*/


/*獲得下一個隨機條目的位置*/
public int next() {
    random();
    mHistoryList.add(mPickedPosition);
    return mPickedPosition;
}

public void add() {
    // 默認每個條目的比重為1.
    add(getSize(), 1);
}

/*添加一個條目*/
public void add(int index, int weight) {
    mOriginWeightList.add(index, weight);
    mCurrentWeightList.add(index, calculateWeight(0, weight));
}

/*修改一個條目的比重*/
public void changeOriginWeight(int index, int weight) {
    mOriginWeightList.set(index, weight);
    int currentWeight = mCurrentWeightList.get(index);
    mCurrentWeightList.set(index, currentWeight / mOriginWeightList.get(index) * weight);
}

/*移除一個條目*/
public void remove(int index) {
    mOriginWeightList.remove(index);
    mCurrentWeightList.remove(index);
}

/*執行隨機算法*/
private void random() {
    // 算出下一次選中的位置
    if (mNextPickPosition != null) {
        mPickedPosition = mNextPickPosition;
        mNextPickPosition = null;
    } else {
        long allCount = 0;
        for (int i = 0; i < mCurrentWeightList.size(); i++) {
            allCount += mCurrentWeightList.get(i);
        }

        long randomLong = (long) (mRandom.nextDouble() * allCount);
        long currentLong = 0;
        for (int i = 0; i < mCurrentWeightList.size(); i++) {
            currentLong += mCurrentWeightList.get(i);
            if (currentLong > randomLong) {
                mPickedPosition = i;
                break;
            }
        }
    }

    // 若列表長度小于2，則下一次位置必為0.
    if (mCurrentWeightList.size() < 2) {
        mPickedPosition = 0;
        return;
    }

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

/*計算下一次的比重*/
private int calculateWeight(int currentWeight, int originWeight) {
    return (currentWeight + mAddNumber) * mMultiplyNumber * originWeight;
}


}</code></pre> 
  
每次調用next()的時候，都要做兩次for循環遍歷列表，列表里12首歌倒是毫無壓力，列表要是有500首歌的話肯定會延遲了，此處待改進。
 
  
 
 
  
來自：http://www.jianshu.com/p/472fed76690a