Is there any way to optimize this Mergesort implementation? - sorting

Question

I apologize in advance for any unconventional writing patterns, or any other apparent mistakes. Any advice or tips would be appreciated!
public class MergeSort {
private static final int INSERTION_THRESHOLD = 8;
public static void mergeSort(int[] x) {
mergeSort(x, 0, x.length);
}
private static void mergeSort(int[] x, int start, int end) {
int length=end-start;
if(length<INSERTION_THRESHOLD) {
for (int i=start; i<end; i++)
for (int j=i; j>start && x[j-1]>x[j]; j--)
swap(x, j, j-1);
return;
}
int mid=(start+end)>>>1;
mergeSort(x, 0, mid);
mergeSort(x, mid, end);
int[] space=Arrays.copyOfRange(x, 0, mid);
int i=0, j=mid, k=0;
while(i<mid&&j<end)
x[k++]=(x[j]<space[i])?x[j++]:space[i++];
while(i<mid)
x[k++]=space[i++];
}
private static void swap(int[] x, int n, int m) {
int temp = x[n];
x[n] = x[m];
x[m] = temp;
}
Here are a few test cases to show that it works.
public static void main(String[] args) {
int[] test1={ 4, 3, 2, 1, 6, 7, 4, 2 }; // 1 2 2 3 4 4 6 7
int[] test2={ 3, 2, 3, 5, 4, 2, 1, 7 }; // 1 2 2 3 3 4 5 7
int[] test3={ 1, 5, 4, 6, 7, 3, 2, 8 }; // 1 2 3 4 5 6 7 8
int[] test4={ 9, 8, 7, 6, 5, 4, 3, 2 }; // 2 3 4 5 6 7 8 9
int[] test5={-8, 7,-3, 4, 5, 1, 0,-3 }; //-8 -3 -3 0 1 4 5 7
mergeSort(test1);
mergeSort(test2);
mergeSort(test3);
mergeSort(test4);
mergeSort(test5);
System.out.println(Arrays.toString(test1));
System.out.println(Arrays.toString(test2));
System.out.println(Arrays.toString(test3));
System.out.println(Arrays.toString(test4));
System.out.println(Arrays.toString(test5));
}
}
What I am looking for here, is really any advice or ideas that might make this a better implementation. Am I getting it right, or is there some horribly hilarious mess-up here? Are there any spots that might be unnecessary in the code? Is there something that could be added to make this run more quickly/smoothly/more along the lines of a good Merge Sort?

Answer 1

Here you can take a look at different merge sort implementation in different languages, and find out the best solution for you, maybe improve your existing code. Here is the example of implementation in PHP from that website:
function mergesort($arr){
if(count($arr) == 1 ) return $arr;
$mid = count($arr) / 2;
$left = array_slice($arr, 0, $mid);
$right = array_slice($arr, $mid);
$left = mergesort($left);
$right = mergesort($right);
return merge($left, $right);
}
function merge($left, $right){
$res = array();
while (count($left) > 0 && count($right) > 0){
if($left[0] > $right[0]){
$res[] = $right[0];
$right = array_slice($right , 1);
}else{
$res[] = $left[0];
$left = array_slice($left, 1);
}
}
while (count($left) > 0){
$res[] = $left[0];
$left = array_slice($left, 1);
}
while (count($right) > 0){
$res[] = $right[0];
$right = array_slice($right, 1);
}
return $res;
}
$arr = array( 1, 5, 2, 7, 3, 9, 4, 6, 8);
$arr = mergesort($arr);
echo implode(',',$arr);
Hope it will help. Here is the website: http://rosettacode.org/wiki/Sorting_algorithms/Merge_sort