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fixes to partitioning with pivot values - also simplified algorithm probably even faster actually because no avg calculation now

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This commit is contained in:
Richard Thier 2025-04-07 04:58:18 +02:00
parent 22bb267000
commit 69adbe4ae9
1 changed files with 26 additions and 22 deletions
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42
qsort.h
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@ -15,13 +15,6 @@
* - VALPART: Partitioning by value - not by random or given pivot index!
*/
/** Hand-coded abs difference - I wanted to minimize dependencies by not needing stdlib.h */
static inline int64_t absdiff(uint32_t a, uint32_t b) {
/* Should be branchless and fast */
int64_t n = (a - b);
return (n > 0) ? n : -n;
}
/* BASICS */
/** Swap operation */
@ -228,11 +221,15 @@ static inline void quicksort_rand(uint32_t array[], int low, int high, rpivotsta
* - Elements smaller than pivot are on left of pivot
* - Elements greater than pivot are on right of pivot
*
* (***): The left min-max outputs can return not just the left partition, but left partition ++ pivotpoint region min-max!
* This does not only happen if partition is empty, but also when pivot was already highest and last earlier.
* Would be hard to handle this edge-case and min-max out is generally used as output hints only so I prefer speed..
*
* @param array The array to partition
* @param low From when. (inclusive)
* @param high Until when. (inclusive too!)
* @param minout_left OUT: Will be filled with the minimum key of left partition or pivot value when partition empty
* @param maxout_left OUT: Will be filled with the maximum key of left partition or pivot value when partition empty
* @param minout_left OUT: Will be filled with the minimum key of left partition or pivot value when partition empty. Also (***)
* @param maxout_left OUT: Will be filled with the maximum key of left partition or pivot value when partition empty. Also (***)
* @param minout_right OUT: Will be filled with the minimum key of right partition or pivot value when partition empty
* @param maxout_right OUT: Will be filled with the maximum key of right partition or pivot value when partition empty
* @returns The partition point.
@ -320,7 +317,7 @@ static inline int partition_and_pminmax_with_pivot(
/* swap pivot with rightmost */
swapit(&array[high], &array[pi]);
/* delegate to previous sol. */
return partition_and_pminmax(array, low, high, minout_left, maxout_left, maxout_left, maxout_right);
return partition_and_pminmax(array, low, high, minout_left, maxout_left, minout_right, maxout_right);
}
/* VALPART */
@ -328,14 +325,15 @@ static inline int partition_and_pminmax_with_pivot(
/**
* Partition the array using pivot value - and find pivot closest to that value (and place them at proper pivot index)
*
* - Elements smaller than pivot are on left of pivot
* - Elements greater than pivot are on right of pivot
* - The "pivot" element we have found is the closest to the "pivotval" given.
* - Elements smaller-eq than pivotval are on left of pivot
* - Elements greater than pivotval are on right of pivot
* - The "pivot" element we find is the biggest among the ones smaller-eq to the pivot value
* or if there is no such (all is greater) we return first greater-than value index (right[0])
*
* @param array The array to partition
* @param low From when. (inclusive)
* @param high Until when. (inclusive too!)
* @param pivotval This value is used to partition the array - pivot will be the closest to this valued element!
* @param pivotval This value is used to partition the array.
* @returns The partition point.
*/
static inline int partition_with_pivotval(uint32_t array[], int low, int high, uint32_t pivotval) {
@ -343,7 +341,7 @@ static inline int partition_with_pivotval(uint32_t array[], int low, int high, u
/* Select the rightmost element as pivot just because */
/* Need some start-value for min(abs(pv - p)) search! */
int64_t mindiff = absdiff(array[low], pivotval);
int64_t leftmax = -1;
/* Index of currently found pivot value */
uint32_t pivoti = low;
@ -366,17 +364,23 @@ static inline int partition_with_pivotval(uint32_t array[], int low, int high, u
/* After this, array[i] can never change - so we can save it as a found pivot-index */
/* Min-search on elements by telling which is closest to pivotval by abs difference! */
int64_t diff = absdiff(array[i], pivotval);
pivoti = (diff < mindiff) ? i : pivoti;
if(array[i] > leftmax) {
pivoti = i;
leftmax = array[i];
}
}
}
/* Must check if above loop found elem at all - because its guessing */
if(i != (low - 1)) {
// TODO: Remove debug stuff
printf("array[%d]: %d <---> %d :array[%d] @ pivotval: %u\n", i, array[i], array[pivoti], pivoti, pivotval);
if(pivotval == 2147494598u) raise(SIGINT);
printf("array:i[%d:i]: %d <---> %d :array:pivoti[%d] @ pivotval: %u && [%u..%u]\n",
i, array[i], array[pivoti], pivoti, pivotval, low, high);
/* swap the pivot element into its place */
swapit(&array[i], &array[pivoti]);
} else printf("array:i[%d:i]: %d <-/-> %d :array:pivoti[%d] @ pivotval: %u && [%u..%u]\n",
i, array[i], array[pivoti], pivoti, pivotval, low, high);
/* return the partition point: index of pivot element */
return i;