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252 lines
8.5 KiB
C
252 lines
8.5 KiB
C
/*******************************************************************************
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*
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* Author: Remi Dufour - remi.dufour@gmail.com
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* Date: July 23rd, 2012
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*
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* Name: Quicksort
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*
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* Description: This is a well-known sorting algorithm developed by C. A. R.
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* Hoare. It is a comparison sort and in this implementation,
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* is not a stable sort.
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*
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* Note: This is public-domain C implementation written from
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* scratch. Use it at your own risk.
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*
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*******************************************************************************/
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#include <limits.h>
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#include <stddef.h>
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/* Insertion sort threshold shift
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*
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* This macro defines the threshold shift (power of 2) at which the insertion
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* sort algorithm replaces the Quicksort. A zero threshold shift disables the
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* insertion sort completely.
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*
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* The value is optimized for Linux and MacOS on the Intel x86 platform.
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*/
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#ifndef INSERTION_SORT_THRESHOLD_SHIFT
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# ifdef __APPLE__ & __MACH__
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# define INSERTION_SORT_THRESHOLD_SHIFT 0
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# else
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# define INSERTION_SORT_THRESHOLD_SHIFT 2
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# endif
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#endif
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/* Macro SWAP
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*
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* Swaps the elements of two arrays.
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*
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* The length of the swap is determined by the value of "SIZE". While both
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* arrays can't overlap, the case in which both pointers are the same works.
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*/
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#define SWAP(A,B,SIZE) \
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{ \
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register char *a_byte = A; \
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register char *b_byte = B; \
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register const char *a_end = a_byte + SIZE; \
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\
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while (a_byte < a_end) \
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{ \
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register const char swap_byte = *b_byte; \
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*b_byte++ = *a_byte; \
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*a_byte++ = swap_byte; \
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} \
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}
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/* Macro SWAP_NEXT
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*
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* Swaps the elements of an array with its next value.
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*
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* The length of the swap is determined by the value of "SIZE". This macro
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* must be used at the beginning of a scope and "A" shouldn't be an expression.
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*/
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#define SWAP_NEXT(A,SIZE) \
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register char *a_byte = A; \
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register const char *a_end = A + SIZE; \
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\
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while (a_byte < a_end) \
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{ \
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register const char swap_byte = *(a_byte + SIZE); \
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*(a_byte + SIZE) = *a_byte; \
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*a_byte++ = swap_byte; \
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}
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/* Function Quicksort
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*
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* This function performs a basic Quicksort. This implementation is the
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* in-place version of the algorithm and is done in he following way:
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*
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* 1. In the middle of the array, we determine a pivot that we temporarily swap
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* to the end.
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* 2. From the beginning to the end of the array, we swap any elements smaller
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* than this pivot to the start, adjacent to other elements that were
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* already moved.
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* 3. We swap the pivot next to these smaller elements.
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* 4. For both sub-arrays on sides of the pivot, we repeat this process
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* recursively.
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* 5. For a sub-array smaller than a certain threshold, the insertion sort
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* algorithm takes over.
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*
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* As an optimization, rather than performing a real recursion, we keep a
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* global stack to track boundaries for each recursion level.
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*
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* To ensure that at most O(log2 N) space is used, we recurse into the smaller
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* partition first. The log2 of the highest unsigned value of an integer type
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* is the number of bits needed to store that integer.
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*/
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void qsort(void *array,
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size_t length,
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size_t size,
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int(*compare)(const void *, const void *))
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{
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/* Recursive stacks for array boundaries (both inclusive) */
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struct stackframe
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{
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void *left;
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void *right;
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} stack[CHAR_BIT * sizeof(void *)];
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/* Recursion level */
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struct stackframe *recursion = stack;
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#if INSERTION_SORT_THRESHOLD_SHIFT != 0
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/* Insertion sort threshold */
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const int threshold = size << INSERTION_SORT_THRESHOLD_SHIFT;
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#endif
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/* Assign the first recursion level of the sorting */
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recursion->left = array;
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recursion->right = (char *)array + size * (length - 1);
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do
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{
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/* Partition the array */
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register char *index = recursion->left;
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register char *right = recursion->right;
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char *left = index;
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/* Assigning store to the left */
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register char *store = index;
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/* Pop the stack */
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--recursion;
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/* Determine a pivot (in the middle) and move it to the end */
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const size_t middle = (right - left) >> 1;
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SWAP(left + middle - middle % size,right,size)
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/* From left to right */
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while (index < right)
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{
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/* If item is smaller than pivot */
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if (compare(right, index) > 0)
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{
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/* Swap item and store */
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SWAP(index,store,size)
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/* We increment store */
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store += size;
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}
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index += size;
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}
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/* Move the pivot to its final place */
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SWAP(right,store,size)
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/* Performs a recursion to the left */
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#define RECURSE_LEFT \
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if (left < store - size) \
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{ \
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(++recursion)->left = left; \
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recursion->right = store - size; \
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}
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/* Performs a recursion to the right */
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#define RECURSE_RIGHT \
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if (store + size < right) \
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{ \
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(++recursion)->left = store + size; \
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recursion->right = right; \
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}
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/* Insertion sort inner-loop */
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#define INSERTION_SORT_LOOP(LEFT) \
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{ \
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register char *trail = index - size; \
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while (trail >= LEFT && compare(trail, trail + size) > 0) \
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{ \
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SWAP_NEXT(trail,size) \
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trail -= size; \
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} \
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}
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/* Performs insertion sort left of the pivot */
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#define INSERTION_SORT_LEFT \
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for (index = left + size; index < store; index +=size) \
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INSERTION_SORT_LOOP(left)
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/* Performs insertion sort right of the pivot */
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#define INSERTION_SORT_RIGHT \
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for (index = store + (size << 1); index <= right; index +=size) \
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INSERTION_SORT_LOOP(store + size)
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/* Sorts to the left */
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#if INSERTION_SORT_THRESHOLD_SHIFT == 0
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# define SORT_LEFT RECURSE_LEFT
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#else
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# define SORT_LEFT \
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if (store - left <= threshold) \
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{ \
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INSERTION_SORT_LEFT \
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} \
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else \
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{ \
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RECURSE_LEFT \
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}
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#endif
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/* Sorts to the right */
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#if INSERTION_SORT_THRESHOLD_SHIFT == 0
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# define SORT_RIGHT RECURSE_RIGHT
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#else
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# define SORT_RIGHT \
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if (right - store <= threshold) \
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{ \
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INSERTION_SORT_RIGHT \
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} \
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else \
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{ \
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RECURSE_RIGHT \
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}
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#endif
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/* Recurse into the smaller partition first */
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if (store - left < right - store)
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{
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/* Left side is smaller */
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SORT_RIGHT
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SORT_LEFT
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continue;
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}
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/* Right side is smaller */
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SORT_LEFT
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SORT_RIGHT
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#undef RECURSE_LEFT
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#undef RECURSE_RIGHT
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#undef INSERTION_SORT_LOOP
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#undef INSERTION_SORT_LEFT
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#undef INSERTION_SORT_RIGHT
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#undef SORT_LEFT
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#undef SORT_RIGHT
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}
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while (recursion >= stack);
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}
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#undef INSERTION_SORT_THRESHOLD_SHIFT
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#undef SWAP
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#undef SWAP_NEXT |