libctf/ctf-qsort_r.c

   1 /* Copyright (C) 1991-2020 Free Software Foundation, Inc.
   2    This file is part of libctf (imported from Gnulib).
   3    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
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 /* If you consider tuning this algorithm, you should consult first:
  20    Engineering a sort function; Jon Bentley and M. Douglas McIlroy;
  21    Software - Practice and Experience; Vol. 23 (11), 1249-1265, 1993.  */
  22
  23 #ifndef _LIBC
  24 # include <config.h>
  25 #endif
  26
  27 #include <limits.h>
  28 #include <stdlib.h>
  29 #include <string.h>
  30 #include "ctf-decls.h"
  31
  32 #ifndef _LIBC
  33 # define _quicksort ctf_qsort_r
  34 # define __compar_d_fn_t compar_d_fn_t
  35 typedef int (*compar_d_fn_t) (const void *, const void *, void *);
  36 #endif
  37
  38 /* Byte-wise swap two items of size SIZE. */
  39 #define SWAP(a, b, size)                                                      \
  40   do                                                                          \
  41     {                                                                         \
  42       size_t __size = (size);                                                 \
  43       char *__a = (a), *__b = (b);                                            \
  44       do                                                                      \
  45         {                                                                     \
  46           char __tmp = *__a;                                                  \
  47           *__a++ = *__b;                                                      \
  48           *__b++ = __tmp;                                                     \
  49         } while (--__size > 0);                                               \
  50     } while (0)
  51
  52 /* Discontinue quicksort algorithm when partition gets below this size.
  53    This particular magic number was chosen to work best on a Sun 4/260. */
  54 #define MAX_THRESH 4
  55
  56 /* Stack node declarations used to store unfulfilled partition obligations. */
  57 typedef struct
  58   {
  59     char *lo;
  60     char *hi;
  61   } stack_node;
  62
  63 /* The next 4 #defines implement a very fast in-line stack abstraction. */
  64 /* The stack needs log (total_elements) entries (we could even subtract
  65    log(MAX_THRESH)).  Since total_elements has type size_t, we get as
  66    upper bound for log (total_elements):
  67    bits per byte (CHAR_BIT) * sizeof(size_t).  */
  68 #define STACK_SIZE      (CHAR_BIT * sizeof(size_t))
  69 #define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
  70 #define POP(low, high)  ((void) (--top, (low = top->lo), (high = top->hi)))
  71 #define STACK_NOT_EMPTY (stack < top)
  72
  73
  74 /* Order size using quicksort.  This implementation incorporates
  75    four optimizations discussed in Sedgewick:
  76
  77    1. Non-recursive, using an explicit stack of pointer that store the
  78       next array partition to sort.  To save time, this maximum amount
  79       of space required to store an array of SIZE_MAX is allocated on the
  80       stack.  Assuming a 32-bit (64 bit) integer for size_t, this needs
  81       only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes).
  82       Pretty cheap, actually.
  83
  84    2. Chose the pivot element using a median-of-three decision tree.
  85       This reduces the probability of selecting a bad pivot value and
  86       eliminates certain extraneous comparisons.
  87
  88    3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
  89       insertion sort to order the MAX_THRESH items within each partition.
  90       This is a big win, since insertion sort is faster for small, mostly
  91       sorted array segments.
  92
  93    4. The larger of the two sub-partitions is always pushed onto the
  94       stack first, with the algorithm then concentrating on the
  95       smaller partition.  This *guarantees* no more than log (total_elems)
  96       stack size is needed (actually O(1) in this case)!  */
  97
  98 void
  99 _quicksort (void *const pbase, size_t total_elems, size_t size,
 100             __compar_d_fn_t cmp, void *arg)
 101 {
 102   char *base_ptr = (char *) pbase;
 103
 104   const size_t max_thresh = MAX_THRESH * size;
 105
 106   if (total_elems == 0)
 107     /* Avoid lossage with unsigned arithmetic below.  */
 108     return;
 109
 110   if (total_elems > MAX_THRESH)
 111     {
 112       char *lo = base_ptr;
 113       char *hi = &lo[size * (total_elems - 1)];
 114       stack_node stack[STACK_SIZE];
 115       stack_node *top = stack;
 116
 117       PUSH (NULL, NULL);
 118
 119       while (STACK_NOT_EMPTY)
 120         {
 121           char *left_ptr;
 122           char *right_ptr;
 123
 124           /* Select median value from among LO, MID, and HI. Rearrange
 125              LO and HI so the three values are sorted. This lowers the
 126              probability of picking a pathological pivot value and
 127              skips a comparison for both the LEFT_PTR and RIGHT_PTR in
 128              the while loops. */
 129
 130           char *mid = lo + size * ((hi - lo) / size >> 1);
 131
 132           if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
 133             SWAP (mid, lo, size);
 134           if ((*cmp) ((void *) hi, (void *) mid, arg) < 0)
 135             SWAP (mid, hi, size);
 136           else
 137             goto jump_over;
 138           if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
 139             SWAP (mid, lo, size);
 140         jump_over:;
 141
 142           left_ptr  = lo + size;
 143           right_ptr = hi - size;
 144
 145           /* Here's the famous ``collapse the walls'' section of quicksort.
 146              Gotta like those tight inner loops!  They are the main reason
 147              that this algorithm runs much faster than others. */
 148           do
 149             {
 150               while ((*cmp) ((void *) left_ptr, (void *) mid, arg) < 0)
 151                 left_ptr += size;
 152
 153               while ((*cmp) ((void *) mid, (void *) right_ptr, arg) < 0)
 154                 right_ptr -= size;
 155
 156               if (left_ptr < right_ptr)
 157                 {
 158                   SWAP (left_ptr, right_ptr, size);
 159                   if (mid == left_ptr)
 160                     mid = right_ptr;
 161                   else if (mid == right_ptr)
 162                     mid = left_ptr;
 163                   left_ptr += size;
 164                   right_ptr -= size;
 165                 }
 166               else if (left_ptr == right_ptr)
 167                 {
 168                   left_ptr += size;
 169                   right_ptr -= size;
 170                   break;
 171                 }
 172             }
 173           while (left_ptr <= right_ptr);
 174
 175           /* Set up pointers for next iteration.  First determine whether
 176              left and right partitions are below the threshold size.  If so,
 177              ignore one or both.  Otherwise, push the larger partition's
 178              bounds on the stack and continue sorting the smaller one. */
 179
 180           if ((size_t) (right_ptr - lo) <= max_thresh)
 181             {
 182               if ((size_t) (hi - left_ptr) <= max_thresh)
 183                 /* Ignore both small partitions. */
 184                 POP (lo, hi);
 185               else
 186                 /* Ignore small left partition. */
 187                 lo = left_ptr;
 188             }
 189           else if ((size_t) (hi - left_ptr) <= max_thresh)
 190             /* Ignore small right partition. */
 191             hi = right_ptr;
 192           else if ((right_ptr - lo) > (hi - left_ptr))
 193             {
 194               /* Push larger left partition indices. */
 195               PUSH (lo, right_ptr);
 196               lo = left_ptr;
 197             }
 198           else
 199             {
 200               /* Push larger right partition indices. */
 201               PUSH (left_ptr, hi);
 202               hi = right_ptr;
 203             }
 204         }
 205     }
 206
 207   /* Once the BASE_PTR array is partially sorted by quicksort the rest
 208      is completely sorted using insertion sort, since this is efficient
 209      for partitions below MAX_THRESH size. BASE_PTR points to the beginning
 210      of the array to sort, and END_PTR points at the very last element in
 211      the array (*not* one beyond it!). */
 212
 213 #define min(x, y) ((x) < (y) ? (x) : (y))
 214
 215   {
 216     char *const end_ptr = &base_ptr[size * (total_elems - 1)];
 217     char *tmp_ptr = base_ptr;
 218     char *thresh = min(end_ptr, base_ptr + max_thresh);
 219     char *run_ptr;
 220
 221     /* Find smallest element in first threshold and place it at the
 222        array's beginning.  This is the smallest array element,
 223        and the operation speeds up insertion sort's inner loop. */
 224
 225     for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
 226       if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
 227         tmp_ptr = run_ptr;
 228
 229     if (tmp_ptr != base_ptr)
 230       SWAP (tmp_ptr, base_ptr, size);
 231
 232     /* Insertion sort, running from left-hand-side up to right-hand-side.  */
 233
 234     run_ptr = base_ptr + size;
 235     while ((run_ptr += size) <= end_ptr)
 236       {
 237         tmp_ptr = run_ptr - size;
 238         while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
 239           tmp_ptr -= size;
 240
 241         tmp_ptr += size;
 242         if (tmp_ptr != run_ptr)
 243           {
 244             char *trav;
 245
 246             trav = run_ptr + size;
 247             while (--trav >= run_ptr)
 248               {
 249                 char c = *trav;
 250                 char *hi, *lo;
 251
 252                 for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
 253                   *hi = *lo;
 254                 *hi = c;
 255               }
 256           }
 257       }
 258   }
 259 }