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Improve qsort_r(3) run-time complexity using quicksort.

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This commit is contained in:
Jonas 'Sortie' Termansen 2014-03-09 16:09:35 +01:00
parent a9cf3cc684
commit f3eb87c3ee
1 changed files with 61 additions and 20 deletions
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81
libc/stdlib/qsort_r.cpp
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@ -22,13 +22,12 @@
*******************************************************************************/
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
static void memswap(uint8_t* a, uint8_t* b, size_t size)
static void memswap(unsigned char* a, unsigned char* b, size_t size)
{
uint8_t tmp;
unsigned char tmp;
for ( size_t i = 0; i < size; i++ )
{
tmp = a[i];
@ -37,8 +36,51 @@ static void memswap(uint8_t* a, uint8_t* b, size_t size)
}
}
// TODO: This is just a quick and dirty insertion sort. It'd be nice to have a
// good old quick sort here soon.
static
unsigned char* array_index(unsigned char* base,
size_t element_size,
size_t index)
{
return base + element_size * index;
}
static
size_t partition(unsigned char* base,
size_t element_size,
size_t num_elements,
size_t pivot_index,
int (*compare)(const void*, const void*, void*),
void* arg)
{
if ( pivot_index != num_elements - 1 )
{
unsigned char* pivot = array_index(base, element_size, pivot_index);
unsigned char* other = array_index(base, element_size, num_elements - 1);
memswap(pivot, other, element_size);
pivot_index = num_elements - 1;
}
size_t store_index = 0;
for ( size_t i = 0; i < num_elements - 1; i++ )
{
unsigned char* pivot = array_index(base, element_size, pivot_index);
unsigned char* value = array_index(base, element_size, i);
if ( compare(value, pivot, arg) <= 0 )
{
unsigned char* other = array_index(base, element_size, store_index);
if ( value != other )
memswap(value, other, element_size);
store_index++;
}
}
unsigned char* pivot = array_index(base, element_size, pivot_index);
unsigned char* value = array_index(base, element_size, store_index);
memswap(pivot, value, element_size);
return store_index;
}
extern "C"
void qsort_r(void* base_ptr,
size_t num_elements,
@ -46,19 +88,18 @@ void qsort_r(void* base_ptr,
int (*compare)(const void*, const void*, void*),
void* arg)
{
uint8_t* base = (uint8_t*) base_ptr;
for ( size_t i = 0; i < num_elements; i++ )
{
for ( size_t c = i; c; c-- )
{
size_t currentoff = c * element_size;
uint8_t* current = base + currentoff;
size_t p = c-1;
size_t prevoff = p * element_size;
uint8_t* prev = base + prevoff;
int cmp = compare(prev, current, arg);
if ( cmp <= 0 ) { break; }
memswap(prev, current, element_size);
}
}
unsigned char* base = (unsigned char*) base_ptr;
if ( !element_size || num_elements < 2 )
return;
size_t pivot_index = num_elements / 2;
pivot_index = partition(base, element_size, num_elements, pivot_index, compare, arg);
if ( 2 <= pivot_index )
qsort_r(base, pivot_index, element_size, compare, arg);
if ( 2 <= num_elements - (pivot_index + 1) )
qsort_r(array_index(base, element_size, pivot_index + 1),
num_elements - (pivot_index + 1), element_size, compare, arg);
}