Dual Pivot Quicksort

A version of quicksort originally deemed slower than traditional quicksort, but proven by Vladimir Yaroslavskiy to actually be faster than traditional quicksort on larger arrays, n > 286, so this implementation takes advantage of this by using Insertion Sort, Quick Sort, and Dual Pivot Quick Sort to produce a very fast numeric sorting interface. This is the current sorting method used on the Java Virtual Machine (JVM) for sorting large arrays of primitive values (int, float, double, etc).

How to use

#include <dpqs.h>

int main(void) {
  int foo[2000];
  
  for (int i = 0; i < 2000; i++) {
    foo[i] = i + 39;
  }
  
  dpqs_sort(foo, 0, 1999);
  
  // Foo is sorted :)
}

How The Algorithm Works

• Take in the array and bounds Lo and Hi
• For A with less than 27 elements, use Insertion sort
• For A with greater than 27 elements, and less than 286 elements, use Hoare's quicksort
• A[Lo] < A[Hi] or else swap them
• Choose 2 pivot elements
  • P = A[Lo]
  • Q = A[Hi]
• Assign three pointers:
  • L = location of P + 1
  • K = L
  • G = location of Q - 1
• While K is less than or equal to G
  • Swap A[K] with A[L] if A[K] < P, increment K, and L
  • Swap A[K] with G if A[K] > Q, decrement G
  • Otherwise, increment K
• Swap P with A[L + 1]
• Swap Q with A[G - 1]
• Increment L
• Decrement G
• Recursively sort the sub-array from Lo to L
• Recursively sort the sub-array from L + 1 to G if A[L] < A[G]
• Recursively sort the sub-array from G + 1 to Hi
• The array is sorted

Credits

• Vladimir Yaroslavskiy (Dual-Pivot Quicksort algorithm)

License

This implementation is available under the MIT License, more information is provided by the LICENSE file at the root of the project.