# Question

I wrote my own Quicksort method for educational purposes. In order to improve it, I took a look at .NET source code to see how to LINQ `OrderBy()`

method is implemented.

I found the following Quicksort method :

```
void QuickSort(int[] map, int left, int right) {
do {
int i = left;
int j = right;
int x = map[i + ((j - i) >> 1)];
do {
while (i < map.Length && CompareKeys(x, map[i]) > 0) i++;
while (j >= 0 && CompareKeys(x, map[j]) < 0) j--;
if (i > j) break;
if (i < j) {
int temp = map[i];
map[i] = map[j];
map[j] = temp;
}
i++;
j--;
} while (i <= j);
if (j - left <= right - i) {
if (left < j) QuickSort(map, left, j);
left = i;
}
else {
if (i < right) QuickSort(map, i, right);
right = j;
}
} while (left < right);
}
```

I am trying to understand the inner workings.

AFAIK it looks very similar to Hoare partition scheme but with some slight optimisations.

What is unclear to me is :

- Why do we recurse only one side of the pivot after partitionning ? (depending the result of the
`if (j - left <= right - i)`

)
- Why do we have a
`do { ... } while (left < right)`

over the whole thing ? Is it because we recurse only one side of the pivot as suggested above ?
- Why is there a
`if (i < j)`

conditional test before the swap ? Isn’t the `break;`

statement before enough?

In comparison, here is how my actual implementation Quicksort looks (straight implementation of Hoare partition scheme)

```
void QuickSort(int[] map, int left, int right)
{
if (left < right)
{
int i = left - 1;
int j = right + 1;
int x = map[left + ((right - left) >> 1)];
while (true)
{
do { i++; } while (Compare(map[i], x) < 0);
do { j--; } while (Compare(map[j], x) > 0);
if (i >= j) break;
int temp = map[i];
map[i] = map[j];
map[j] = temp;
}
QuickSort(map, left, j);
QuickSort(map, j + 1, right);
}
}
```