java 自下而上合并排序与n auxilary空间完全相同。包括一些测试代码。有趣的是，我的机器排序时间比线路增长慢

Posted 2021-05-08

public class MergeSort{
    
    
    public static void sort(int[] arr){
        int granularity = 1;
        int[] copy = new int[arr.length];
        boolean sourceOriginal=true;
        while(granularity <= arr.length){
//            System.out.println("    Used Memory in granularity : " + granularity + "   is "
//                        + (instance.totalMemory() - instance.freeMemory()) / mb);
            for(int i = 0 ; i < arr.length ; i+= granularity*2){
                merge( i ,
                        Math.min(i + granularity, arr.length) ,
                        Math.min(i + granularity, arr.length) ,
                        Math.min(i + 2 * granularity, arr.length) ,
                        i,
                        sourceOriginal ? arr : copy,
                        sourceOriginal ? copy : arr);
            }
            granularity <<= 1;
            sourceOriginal = !sourceOriginal;
        }
        if(!sourceOriginal)
            merge(0,arr.length, 0 , 0 ,0, copy , arr);
        
    }
    
    
    
    private static void merge(int startFirst , int endFirst , int startSecond , int endSecond , int targetIndex, int[] source , int[] target){
        while(startFirst < endFirst || startSecond < endSecond){
            if(startFirst < endFirst && (startSecond == endSecond || source[startFirst]  < source[startSecond]  ) ){
                target[targetIndex] = source[startFirst];
                startFirst++;
            }
            else{
                target[targetIndex] = source[startSecond];
                startSecond++;
            }
            targetIndex++;
        }
        
    }
    static Runtime instance = Runtime.getRuntime();
    static int mb = 1024 * 1024;
    public static void main(String... args){
        Random r = new Random();
          
         
        
     
        for(int j = 1 ; j < 100000002 ; j*=10) {
            System.out.println();
            
            System.gc();
            System.out.println("Used Memory after gc: "
                    + (instance.totalMemory() - instance.freeMemory()) / mb);
            int[] arr = new int[j];
            for (int i = 0; i < arr.length; i++) {
                arr[i] = r.nextInt();
            }
            System.out.println("Used Memory after array creation: "
                    + (instance.totalMemory() - instance.freeMemory()) / mb);
            long startTime = System.nanoTime();
            MergeSort.sort(arr);
            long endTime = System.nanoTime();
            System.out.println("Used Memory after sort: "
                    + (instance.totalMemory() - instance.freeMemory()) / mb);
            System.out.println("time passed : " +(endTime - startTime)/1000+" μs");
            for (int i = 1; i < arr.length; i++) {
                if(arr[i] < arr[i-1] )
                    throw new IllegalStateException("sort failed");
            }
        }
    }
    
}