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47 pj 1
/* Copyright (C) 1991, 1992, 1996, 1997, 1999 Free Software Foundation, Inc.
2
   This file is part of the GNU C Library.
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   Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
4
 
5
   The GNU C Library is free software; you can redistribute it and/or
6
   modify it under the terms of the GNU Lesser General Public
7
   License as published by the Free Software Foundation; either
8
   version 2.1 of the License, or (at your option) any later version.
9
 
10
   The GNU C Library is distributed in the hope that it will be useful,
11
   but WITHOUT ANY WARRANTY; without even the implied warranty of
12
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
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   Lesser General Public License for more details.
14
 
15
   You should have received a copy of the GNU Lesser General Public
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   License along with the GNU C Library; if not, write to the Free
17
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
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   02111-1307 USA.  */
19
 
20
/* If you consider tuning this algorithm, you should consult first:
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   Engineering a sort function; Jon Bentley and M. Douglas McIlroy;
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   Software - Practice and Experience; Vol. 23 (11), 1249-1265, 1993.  */
23
 
24
//#include <alloca.h> //SHARK
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#include <limits.h>
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#include <stdlib.h>
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#include <string.h>
28
 
29
/* Byte-wise swap two items of size SIZE. */
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#define SWAP(a, b, size)                                                      \
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  do                                                                          \
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    {                                                                         \
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      register size_t __size = (size);                                        \
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      register char *__a = (a), *__b = (b);                                   \
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      do                                                                      \
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        {                                                                     \
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          char __tmp = *__a;                                                  \
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          *__a++ = *__b;                                                      \
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          *__b++ = __tmp;                                                     \
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        } while (--__size > 0);                                               \
41
    } while (0)
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43
/* Discontinue quicksort algorithm when partition gets below this size.
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   This particular magic number was chosen to work best on a Sun 4/260. */
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#define MAX_THRESH 4
46
 
47
/* Stack node declarations used to store unfulfilled partition obligations. */
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typedef struct
49
  {
50
    char *lo;
51
    char *hi;
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  } stack_node;
53
 
54
/* The next 4 #defines implement a very fast in-line stack abstraction. */
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/* The stack needs log (total_elements) entries (we could even subtract
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   log(MAX_THRESH)).  Since total_elements has type size_t, we get as
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   upper bound for log (total_elements):
58
   bits per byte (CHAR_BIT) * sizeof(size_t).  */
379 giacomo 59
#define QSORT_STACK_SIZE        (CHAR_BIT * sizeof(size_t))
47 pj 60
#define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
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#define POP(low, high)  ((void) (--top, (low = top->lo), (high = top->hi)))
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#define STACK_NOT_EMPTY (stack < top)
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64
 
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/* Order size using quicksort.  This implementation incorporates
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   four optimizations discussed in Sedgewick:
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68
   1. Non-recursive, using an explicit stack of pointer that store the
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      next array partition to sort.  To save time, this maximum amount
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      of space required to store an array of SIZE_MAX is allocated on the
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      stack.  Assuming a 32-bit (64 bit) integer for size_t, this needs
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      only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes).
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      Pretty cheap, actually.
74
 
75
   2. Chose the pivot element using a median-of-three decision tree.
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      This reduces the probability of selecting a bad pivot value and
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      eliminates certain extraneous comparisons.
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79
   3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
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      insertion sort to order the MAX_THRESH items within each partition.
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      This is a big win, since insertion sort is faster for small, mostly
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      sorted array segments.
83
 
84
   4. The larger of the two sub-partitions is always pushed onto the
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      stack first, with the algorithm then concentrating on the
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      smaller partition.  This *guarantees* no more than log (total_elems)
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      stack size is needed (actually O(1) in this case)!  */
88
 
89
void
90
_quicksort (void *const pbase, size_t total_elems, size_t size,
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            __compar_fn_t cmp)
92
{
93
  register char *base_ptr = (char *) pbase;
94
 
95
  const size_t max_thresh = MAX_THRESH * size;
96
 
97
  if (total_elems == 0)
98
    /* Avoid lossage with unsigned arithmetic below.  */
99
    return;
100
 
101
  if (total_elems > MAX_THRESH)
102
    {
103
      char *lo = base_ptr;
104
      char *hi = &lo[size * (total_elems - 1)];
379 giacomo 105
      stack_node stack[QSORT_STACK_SIZE];
47 pj 106
      stack_node *top = stack + 1;
107
 
108
      while (STACK_NOT_EMPTY)
109
        {
110
          char *left_ptr;
111
          char *right_ptr;
112
 
113
          /* Select median value from among LO, MID, and HI. Rearrange
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             LO and HI so the three values are sorted. This lowers the
115
             probability of picking a pathological pivot value and
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             skips a comparison for both the LEFT_PTR and RIGHT_PTR in
117
             the while loops. */
118
 
119
          char *mid = lo + size * ((hi - lo) / size >> 1);
120
 
121
          if ((*cmp) ((void *) mid, (void *) lo) < 0)
122
            SWAP (mid, lo, size);
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          if ((*cmp) ((void *) hi, (void *) mid) < 0)
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            SWAP (mid, hi, size);
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          else
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            goto jump_over;
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          if ((*cmp) ((void *) mid, (void *) lo) < 0)
128
            SWAP (mid, lo, size);
129
        jump_over:;
130
 
131
          left_ptr  = lo + size;
132
          right_ptr = hi - size;
133
 
134
          /* Here's the famous ``collapse the walls'' section of quicksort.
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             Gotta like those tight inner loops!  They are the main reason
136
             that this algorithm runs much faster than others. */
137
          do
138
            {
139
              while ((*cmp) ((void *) left_ptr, (void *) mid) < 0)
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                left_ptr += size;
141
 
142
              while ((*cmp) ((void *) mid, (void *) right_ptr) < 0)
143
                right_ptr -= size;
144
 
145
              if (left_ptr < right_ptr)
146
                {
147
                  SWAP (left_ptr, right_ptr, size);
148
                  if (mid == left_ptr)
149
                    mid = right_ptr;
150
                  else if (mid == right_ptr)
151
                    mid = left_ptr;
152
                  left_ptr += size;
153
                  right_ptr -= size;
154
                }
155
              else if (left_ptr == right_ptr)
156
                {
157
                  left_ptr += size;
158
                  right_ptr -= size;
159
                  break;
160
                }
161
            }
162
          while (left_ptr <= right_ptr);
163
 
164
          /* Set up pointers for next iteration.  First determine whether
165
             left and right partitions are below the threshold size.  If so,
166
             ignore one or both.  Otherwise, push the larger partition's
167
             bounds on the stack and continue sorting the smaller one. */
168
 
169
          if ((size_t) (right_ptr - lo) <= max_thresh)
170
            {
171
              if ((size_t) (hi - left_ptr) <= max_thresh)
172
                /* Ignore both small partitions. */
173
                POP (lo, hi);
174
              else
175
                /* Ignore small left partition. */
176
                lo = left_ptr;
177
            }
178
          else if ((size_t) (hi - left_ptr) <= max_thresh)
179
            /* Ignore small right partition. */
180
            hi = right_ptr;
181
          else if ((right_ptr - lo) > (hi - left_ptr))
182
            {
183
              /* Push larger left partition indices. */
184
              PUSH (lo, right_ptr);
185
              lo = left_ptr;
186
            }
187
          else
188
            {
189
              /* Push larger right partition indices. */
190
              PUSH (left_ptr, hi);
191
              hi = right_ptr;
192
            }
193
        }
194
    }
195
 
196
  /* Once the BASE_PTR array is partially sorted by quicksort the rest
197
     is completely sorted using insertion sort, since this is efficient
198
     for partitions below MAX_THRESH size. BASE_PTR points to the beginning
199
     of the array to sort, and END_PTR points at the very last element in
200
     the array (*not* one beyond it!). */
201
 
202
 
203
  {
204
    char *const end_ptr = &base_ptr[size * (total_elems - 1)];
205
    char *tmp_ptr = base_ptr;
206
    char *thresh = min(end_ptr, base_ptr + max_thresh);
207
    register char *run_ptr;
208
 
209
    /* Find smallest element in first threshold and place it at the
210
       array's beginning.  This is the smallest array element,
211
       and the operation speeds up insertion sort's inner loop. */
212
 
213
    for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
214
      if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr) < 0)
215
        tmp_ptr = run_ptr;
216
 
217
    if (tmp_ptr != base_ptr)
218
      SWAP (tmp_ptr, base_ptr, size);
219
 
220
    /* Insertion sort, running from left-hand-side up to right-hand-side.  */
221
 
222
    run_ptr = base_ptr + size;
223
    while ((run_ptr += size) <= end_ptr)
224
      {
225
        tmp_ptr = run_ptr - size;
226
        while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr) < 0)
227
          tmp_ptr -= size;
228
 
229
        tmp_ptr += size;
230
        if (tmp_ptr != run_ptr)
231
          {
232
            char *trav;
233
 
234
            trav = run_ptr + size;
235
            while (--trav >= run_ptr)
236
              {
237
                char c = *trav;
238
                char *hi, *lo;
239
 
240
                for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
241
                  *hi = *lo;
242
                *hi = c;
243
              }
244
          }
245
      }
246
  }
247
}