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qsort: clean up the implementation (forward walk rather than backward)

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This commit is contained in:
Aryadev Chavali
2025-10-27 23:09:16 +00:00
parent 964e001dd5
commit 946006096b
1 changed files with 20 additions and 38 deletions
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58
impls/qsort.cpp
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@@ -16,8 +16,6 @@
(A) ^= (B); \
} while (0)
#define TEST_SIZE 10
void print_arr(int *arr, int size)
{
printf("{\n");
@@ -28,44 +26,28 @@ void print_arr(int *arr, int size)
void quicksort(int *arr, int arr_size)
{
#ifdef TRACE
printf("before, arr=");
print_arr(arr, arr_size);
printf("\n");
#endif
// TODO Optimisation: for small arr_size, a different sort like the bubble
// sort may be better, instead of initialising new stack frames.
if (arr_size < 2)
return;
// TODO Optimisation: Use some heuristics to figure out a better pivot than
// some constant choice of element.
int pivot = 0;
int pivot_value = arr[0];
for (int i = 1; i < arr_size; ++i)
// TODO: Use a different sort for small enough sizes?
if (arr_size > 1)
{
if (arr[i] <= pivot_value)
{
if (i != pivot)
SWAP(arr[i], arr[pivot]);
++pivot;
}
}
// TODO: Choose a better pivot maybe?
int pivot = arr[arr_size - 1]; // Choose the last element as pivot
int pivot_index = -1;
#ifdef TRACE
printf("after, arr=");
print_arr(arr, arr_size);
printf("\n");
#endif
// TODO Optimisation: Use a stack to do this instead of recursion.
//
// This algorithm is similar to a pre order traversal/dfs, in this case on an
// array where left and right partition are children nodes to the original
// array node. Also with recursion there's a real risk for very large
// ARR_SIZE of running out of space in the call stack.
if (pivot != 0)
quicksort(arr, pivot);
if (pivot < arr_size - 1)
quicksort(arr + pivot + 1, arr_size - pivot - 1);
for (int j = 0; j < arr_size; ++j)
{
if (arr[j] < pivot)
{
++pivot_index;
std::swap(arr[pivot_index], arr[j]);
}
}
std::swap(arr[pivot_index + 1], arr[arr_size - 1]);
// TODO: Use a stack here instead?
quicksort(arr, pivot_index + 1);
quicksort(arr + pivot_index + 1, arr_size - pivot_index - 1);
}
}
int main(void)