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/* qsort.cpp
* Date: 2024-06-22
* Author: Aryadev Chavali
* Commentary: The inplace O(nlog(n)) sortitng algorithm. Originally designed
* in C, hence the lack of iostream.
*/
#include <cstdio>
#include <cstdlib>
#define ARR_SIZE(XS) (sizeof(XS) / sizeof((XS)[0]))
#define SWAP(A, B) \
do \
{ \
(A) ^= (B); \
(B) ^= (A); \
(A) ^= (B); \
} while (0)
#define TEST_SIZE 10
void print_arr(int *arr, int size)
{
printf("{\n");
for (int i = 0; i < size; ++i)
printf(" %d,\n", arr[i]);
printf("}\n");
}
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)
{
if (arr[i] <= pivot_value)
{
if (i != pivot)
SWAP(arr[i], arr[pivot]);
++pivot;
}
}
#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);
}
int main(void)
{
// randomly generate some numbers
int *arr = new int[TEST_SIZE];
for (size_t i = 0; i < TEST_SIZE; ++i)
{
arr[i] = rand() % (2L << 30);
}
printf("Before: ");
print_arr(arr, TEST_SIZE);
printf("\n");
quicksort(arr, TEST_SIZE);
printf("After: ");
print_arr(arr, TEST_SIZE);
printf("\n");
delete[] arr;
return 0;
}
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