rollback quicksort to random pivot version

Author: xtaci

include/quick_sort.h

@@ -19,63 +19,42 @@
 #define __QUICKSORT_H__
 
 #include <generic.h> 
-#include <cassert>
 
 namespace alg {
-
-/**
- * Return median of begin, middle, and end.
- * Order these and hide the pivot.
- */
-template<typename T>
-static const T & __median3(T list[], int begin, int end) {
-assert(begin + 2 <= end);
-int middle = end - (end - begin) / 2;
-if (list[middle] < list[begin])
-swap(list[middle], list[begin]);
-if (list[end] < list[begin])
-swap(list[end], list[begin]);
-if (list[end] < list[middle])
-swap(list[end], list[middle]);
-
-//Place pivot at position [end - 1]
-swap(list[middle], list[end - 1]);
-return list[end - 1];
-}
-
 /**
  * the quick-sort partition routine
  */
 template<typename T>
 static int __partition(T list[],int begin, int end) {
-T pivot = __median3<T>(list, begin, end);
-
-int i = begin;
-int j = end - 1;
-
-while(i < j) {
-while(list[++i] < pivot) {}
-while(pivot < list[--j]) {}
+int pivot_idx = RANDOM(begin,end);
+T pivot = list[pivot_idx];
+swap(list[begin], list[pivot_idx]);
+
+int i = begin + 1;
+int j = end;
+
+while(i <= j) {
+while((i <= end) && (list[i] <= pivot))
+i++;
+while((j >= begin) && (list[j] > pivot))
+j--;
 if(i < j)
 swap(list[i],list[j]);
 }
 
-swap(list[i],list[end - 1]);
-return i; // final pivot position
+swap(list[begin],list[j]);
+return j; // final pivot position
 }
 
 /**
  * quick sort an array of range [begin, end]
  */
 template<typename T>
 static void quicksort(T list[],int begin,int end) {
-if( begin + 1 < end) {
+if( begin < end) {
 int pivot_idx = __partition<T>(list, begin, end);
 quicksort(list, begin, pivot_idx-1);
 quicksort(list, pivot_idx+1, end);
-} else if ( begin + 1 == end) {
-if (list[begin] > list[end])
-swap(list[begin], list[end]);
 }
 }
 }