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mf_qsort.c: 

  qsort implementation backported from 4.0 tree, the old one was buggy


mysys/mf_qsort.c:
  qsort implementation backported from 4.0 tree, the old one was buggy
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mysys/mf_qsort.c
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/* Copyright (C) 1991, 1992, 1996, 1997 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
/* Copyright (C) 2000 MySQL AB
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Library General Public License for more details.
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB. If not,
write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA. */
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
/*
Modifications by monty:
- Uses mysys include files
- Small fixes to make the it a bit faster
- Can be compiled with a cmp function that takes one extra argument.
qsort implementation optimized for comparison of pointers
Inspired by the qsort implementations by Douglas C. Schmidt,
and Bentley & McIlroy's "Engineering a Sort Function".
*/
#include "mysys_priv.h"
#ifndef SCO
#include <m_string.h>
#endif
/* Envoke the comparison function, returns either 0, < 0, or > 0. */
/* We need to use qsort with 2 different compare functions */
#ifdef QSORT_EXTRA_CMP_ARGUMENT
#define CMP(A,B) ((*cmp)(cmp_argument,(A),(B)))
#else
#define CMP(A,B) ((*cmp)((A),(B)))
#endif
/* Byte-wise swap two items of size SIZE. */
#define SWAP(a, b, size) \
do \
{ \
register size_t __size = (size); \
register char *__a = (a), *__b = (b); \
do \
{ \
char __tmp = *__a; \
*__a++ = *__b; \
*__b++ = __tmp; \
} while (--__size > 0); \
} while (0)
/* Discontinue quicksort algorithm when partition gets below this size.
This particular magic number was chosen to work best on a Sun 4/260. */
#define MAX_THRESH 8
/* Stack node declarations used to store unfulfilled partition obligations. */
typedef struct _qsort_stack_node
{
char *lo;
char *hi;
} stack_node;
/* The next 4 #defines implement a very fast in-line stack abstraction. */
#define STACK_SIZE (8 * sizeof(unsigned long int))
#define PUSH(LOW,HIGH) do {top->lo = LOW;top++->hi = HIGH;} while (0)
#define POP(LOW,HIGH) do {LOW = (--top)->lo;HIGH = top->hi;} while (0)
#define STACK_NOT_EMPTY (stack < top)
/* Order size using quicksort. This implementation incorporates
four optimizations discussed in Sedgewick:
1. Non-recursive, using an explicit stack of pointer that store the
next array partition to sort. To save time, this maximum amount
of space required to store an array of MAX_INT is allocated on the
stack. Assuming a 32-bit integer, this needs only 32 *
sizeof (stack_node) == 136 bits. Pretty cheap, actually.
#define SWAP(A, B, size,swap_ptrs) \
do { \
if (swap_ptrs) \
{ \
reg1 char **a = (char**) (A), **b = (char**) (B); \
char *tmp = *a; *a++ = *b; *b++ = tmp; \
} \
else \
{ \
reg1 char *a = (A), *b = (B); \
reg3 char *end= a+size; \
do \
{ \
char tmp = *a; *a++ = *b; *b++ = tmp; \
} while (a < end); \
} \
} while (0)
/* Put the median in the middle argument */
#define MEDIAN(low, mid, high) \
{ \
if (CMP(high,low) < 0) \
SWAP(high, low, size, ptr_cmp); \
if (CMP(mid, low) < 0) \
SWAP(mid, low, size, ptr_cmp); \
else if (CMP(high, mid) < 0) \
SWAP(mid, high, size, ptr_cmp); \
}
2. Chose the pivot element using a median-of-three decision tree.
This reduces the probability of selecting a bad pivot value and
eliminates certain extraneous comparisons.
/* The following node is used to store ranges to avoid recursive calls */
3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
insertion sort to order the MAX_THRESH items within each partition.
This is a big win, since insertion sort is faster for small, mostly
sorted array segments.
typedef struct st_stack
{
char *low,*high;
} stack_node;
4. The larger of the two sub-partitions is always pushed onto the
stack first, with the algorithm then concentrating on the
smaller partition. This *guarantees* no more than log (n)
stack size is needed (actually O(1) in this case)! */
#define PUSH(LOW,HIGH) {stack_ptr->low = LOW; stack_ptr++->high = HIGH;}
#define POP(LOW,HIGH) {LOW = (--stack_ptr)->low; HIGH = stack_ptr->high;}
/* The following stack size is enough for ulong ~0 elements */
#define STACK_SIZE (8 * sizeof(unsigned long int))
#define THRESHOLD_FOR_INSERT_SORT 10
#if defined(QSORT_TYPE_IS_VOID)
#define SORT_RETURN return
#else
#define SORT_RETURN return 0
#endif
/****************************************************************************
** 'standard' quicksort with the following extensions:
**
** Can be compiled with the qsort2_cmp compare function
** Store ranges on stack to avoid recursion
** Use insert sort on small ranges
** Optimize for sorting of pointers (used often by MySQL)
** Use median comparison to find partition element
*****************************************************************************/
#ifdef QSORT_EXTRA_CMP_ARGUMENT
qsort_t qsort2(void *base_ptr, size_t total_elems, size_t size, qsort2_cmp cmp,
qsort_t qsort2(void *base_ptr, size_t count, size_t size, qsort2_cmp cmp,
void *cmp_argument)
#else
qsort_t qsort(void *base_ptr, size_t total_elems, size_t size, qsort_cmp cmp)
qsort_t qsort(void *base_ptr, size_t count, size_t size, qsort_cmp cmp)
#endif
{
/* Allocating SIZE bytes for a pivot buffer facilitates a better
algorithm below since we can do comparisons directly on the pivot.
*/
size_t max_thresh = (size_t) (MAX_THRESH * size);
if (total_elems <= 1)
SORT_RETURN; /* Crashes on MSDOS if continues */
if (total_elems > MAX_THRESH)
{
char *lo = (char*) base_ptr;
char *hi = &lo[size * (total_elems - 1)];
stack_node stack[STACK_SIZE]; /* Largest size needed for 32-bit int!!! */
stack_node *top = stack + 1;
char *pivot = (char *) my_alloca ((int) size);
char *low, *high, *pivot;
stack_node stack[STACK_SIZE], *stack_ptr;
my_bool ptr_cmp;
/* Handle the simple case first */
/* This will also make the rest of the code simpler */
if (count <= 1)
SORT_RETURN;
low = (char*) base_ptr;
high = low+ size * (count - 1);
stack_ptr = stack + 1;
#ifdef HAVE_purify
stack[0].lo=stack[0].hi=0;
/* The first element in the stack will be accessed for the last POP */
stack[0].low=stack[0].high=0;
#endif
pivot = (char *) my_alloca((int) size);
ptr_cmp= size == sizeof(char*) && !((low - (char*) 0)& (sizeof(char*)-1));
do
/* The following loop sorts elements between high and low */
do
{
char *low_ptr, *high_ptr, *mid;
count=((size_t) (high - low) / size)+1;
/* If count is small, then an insert sort is faster than qsort */
if (count < THRESHOLD_FOR_INSERT_SORT)
{
char *left_ptr,*right_ptr;
/* Select median value from among LO, MID, and HI. Rearrange
LO and HI so the three values are sorted. This lowers the
probability of picking a pathological pivot value and
skips a comparison for both the LEFT_PTR and RIGHT_PTR. */
char *mid = lo + size * (((ulong) (hi - lo) / (ulong) size) >> 1);
if (CMP(hi,lo) < 0)
SWAP (hi, lo, size);
if (CMP (mid, lo) < 0)
SWAP (mid, lo, size);
else if (CMP (hi, mid) < 0)
SWAP (mid, hi, size);
memcpy (pivot, mid, size);
left_ptr = lo + size;
right_ptr = hi - size;
/* Here's the famous ``collapse the walls'' section of quicksort.
Gotta like those tight inner loops! They are the main reason
that this algorithm runs much faster than others. */
do
for (low_ptr = low + size; low_ptr <= high; low_ptr += size)
{
while (CMP (left_ptr, pivot) < 0)
left_ptr += size;
while (CMP (pivot, right_ptr) < 0)
right_ptr -= size;
if (left_ptr < right_ptr)
{
SWAP (left_ptr, right_ptr, size);
left_ptr += size;
right_ptr -= size;
}
else if (left_ptr == right_ptr)
{
left_ptr += size;
right_ptr -= size;
break;
}
else
break; /* left_ptr > right_ptr */
char *ptr;
for (ptr = low_ptr; ptr > low && CMP(ptr - size, ptr) > 0;
ptr -= size)
SWAP(ptr, ptr - size, size, ptr_cmp);
}
while (left_ptr <= right_ptr);
POP(low, high);
continue;
}
/* Try to find a good middle element */
mid= low + size * (count >> 1);
if (count > 40) /* Must be bigger than 24 */
{
size_t step = size* (count / 8);
MEDIAN(low, low + step, low+step*2);
MEDIAN(mid - step, mid, mid+step);
MEDIAN(high - 2 * step, high-step, high);
/* Put best median in 'mid' */
MEDIAN(low+step, mid, high-step);
low_ptr = low;
high_ptr = high;
}
else
{
MEDIAN(low, mid, high);
/* The low and high argument are already in sorted against 'pivot' */
low_ptr = low + size;
high_ptr = high - size;
}
memcpy(pivot, mid, size);
/* Set up pointers for next iteration. First determine whether
left and right partitions are below the threshold size. If so,
ignore one or both. Otherwise, push the larger partition's
bounds on the stack and continue sorting the smaller one. */
do
{
while (CMP(low_ptr, pivot) < 0)
low_ptr += size;
while (CMP(pivot, high_ptr) < 0)
high_ptr -= size;
if ((size_t) (right_ptr - lo) <= max_thresh)
if (low_ptr < high_ptr)
{
if ((size_t) (hi - left_ptr) <= max_thresh)
POP (lo, hi); /* Ignore both small partitions. */
else
lo = left_ptr; /* Ignore small left part. */
SWAP(low_ptr, high_ptr, size, ptr_cmp);
low_ptr += size;
high_ptr -= size;
}
else if ((size_t) (hi - left_ptr) <= max_thresh)
hi = right_ptr; /* Ignore small right partition. */
else if ((right_ptr - lo) > (hi - left_ptr))
{
PUSH (lo, right_ptr); /* Push larger left part */
lo = left_ptr;
}
else
else
{
PUSH (left_ptr, hi); /* Push larger right part */
hi = right_ptr;
if (low_ptr == high_ptr)
{
low_ptr += size;
high_ptr -= size;
}
break;
}
} while (STACK_NOT_EMPTY);
my_afree(pivot);
}
/* Once the BASE_PTR array is partially sorted by quicksort the rest
is completely sorted using insertion sort, since this is efficient
for partitions below MAX_THRESH size. BASE_PTR points to the beginning
of the array to sort, and END_PTR points at the very last element in
the array (*not* one beyond it!). */
{
char *end_ptr = (char*) base_ptr + size * (total_elems - 1);
char *tmp_ptr = (char*) base_ptr;
char *thresh = min (end_ptr, (char*) base_ptr + max_thresh);
register char *run_ptr;
/* Find smallest element in first threshold and place it at the
array's beginning. This is the smallest array element,
and the operation speeds up insertion sort's inner loop. */
for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
if (CMP (run_ptr, tmp_ptr) < 0)
tmp_ptr = run_ptr;
if (tmp_ptr != (char*) base_ptr)
SWAP (tmp_ptr, (char*) base_ptr, size);
}
while (low_ptr <= high_ptr);
/* Insertion sort, running from left-hand-side up to right-hand-side. */
/*
Prepare for next iteration.
Skip partitions of size 1 as these doesn't have to be sorted
Push the larger partition and sort the smaller one first.
This ensures that the stack is keept small.
*/
for (run_ptr = (char*) base_ptr + size;
(run_ptr += size) <= end_ptr; )
if ((int) (high_ptr - low) <= 0)
{
if (CMP (run_ptr, (tmp_ptr = run_ptr-size)) < 0)
if ((int) (high - low_ptr) <= 0)
{
char *trav;
while (CMP (run_ptr, tmp_ptr -= size) < 0) ;
tmp_ptr += size;
/* Shift down all smaller elements, put found element in 'run_ptr' */
for (trav = run_ptr + size; --trav >= run_ptr;)
{
char c = *trav;
char *hi, *lo;
for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
*hi = *lo;
*hi = c;
}
POP(low, high); /* Nothing more to sort */
}
else
low = low_ptr; /* Ignore small left part. */
}
else if ((int) (high - low_ptr) <= 0)
high = high_ptr; /* Ignore small right part. */
else if ((high_ptr - low) > (high - low_ptr))
{
PUSH(low, high_ptr); /* Push larger left part */
low = low_ptr;
}
else
{
PUSH(low_ptr, high); /* Push larger right part */
high = high_ptr;
}
}
} while (stack_ptr > stack);
my_afree(pivot);
SORT_RETURN;
}
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