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1 /* Copyright (C) 1991-2021 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
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
31 #ifndef _LIBC
32 # define _quicksort qsort_r
33 # define __compar_d_fn_t compar_d_fn_t
34 typedef int (*compar_d_fn_t) (const void *, const void *, void *);
35 #endif
36
37 /* Byte-wise swap two items of size SIZE. */
38 #define SWAP(a, b, size) \
39 do \
40 { \
41 size_t __size = (size); \
42 char *__a = (a), *__b = (b); \
43 do \
44 { \
45 char __tmp = *__a; \
46 *__a++ = *__b; \
47 *__b++ = __tmp; \
48 } while (--__size > 0); \
49 } while (0)
50
51 /* Discontinue quicksort algorithm when partition gets below this size.
52 This particular magic number was chosen to work best on a Sun 4/260. */
53 #define MAX_THRESH 4
54
55 /* Stack node declarations used to store unfulfilled partition obligations. */
56 typedef struct
57 {
58 char *lo;
59 char *hi;
60 } stack_node;
61
62 /* The next 4 #defines implement a very fast in-line stack abstraction. */
63 /* The stack needs log (total_elements) entries (we could even subtract
64 log(MAX_THRESH)). Since total_elements has type size_t, we get as
65 upper bound for log (total_elements):
66 bits per byte (CHAR_BIT) * sizeof(size_t). */
67 #define STACK_SIZE (CHAR_BIT * sizeof(size_t))
68 #define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
69 #define POP(low, high) ((void) (--top, (low = top->lo), (high = top->hi)))
70 #define STACK_NOT_EMPTY (stack < top)
71
72
73 /* Order size using quicksort. This implementation incorporates
74 four optimizations discussed in Sedgewick:
75
76 1. Non-recursive, using an explicit stack of pointer that store the
77 next array partition to sort. To save time, this maximum amount
78 of space required to store an array of SIZE_MAX is allocated on the
79 stack. Assuming a 32-bit (64 bit) integer for size_t, this needs
80 only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes).
81 Pretty cheap, actually.
82
83 2. Chose the pivot element using a median-of-three decision tree.
84 This reduces the probability of selecting a bad pivot value and
85 eliminates certain extraneous comparisons.
86
87 3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
88 insertion sort to order the MAX_THRESH items within each partition.
89 This is a big win, since insertion sort is faster for small, mostly
90 sorted array segments.
91
92 4. The larger of the two sub-partitions is always pushed onto the
93 stack first, with the algorithm then concentrating on the
94 smaller partition. This *guarantees* no more than log (total_elems)
95 stack size is needed (actually O(1) in this case)! */
96
97 void
98 _quicksort (void *const pbase, size_t total_elems, size_t size,
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*/
99 __compar_d_fn_t cmp, void *arg)
100 {
101 char *base_ptr = (char *) pbase;
102
103 const size_t max_thresh = MAX_THRESH * size;
104
105 if (total_elems == 0)
106 /* Avoid lossage with unsigned arithmetic below. */
107 return;
108
109 if (total_elems > MAX_THRESH)
110 {
111 char *lo = base_ptr;
112 char *hi = &lo[size * (total_elems - 1)];
113 stack_node stack[STACK_SIZE];
114 stack_node *top = stack;
115
116 PUSH (NULL, NULL);
117
118 while (STACK_NOT_EMPTY)
119 {
120 char *left_ptr;
121 char *right_ptr;
122
123 /* Select median value from among LO, MID, and HI. Rearrange
124 LO and HI so the three values are sorted. This lowers the
125 probability of picking a pathological pivot value and
126 skips a comparison for both the LEFT_PTR and RIGHT_PTR in
127 the while loops. */
128
129 char *mid = lo + size * ((hi - lo) / size >> 1);
130
131 if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
132 SWAP (mid, lo, size);
133 if ((*cmp) ((void *) hi, (void *) mid, arg) < 0)
134 SWAP (mid, hi, size);
135 else
136 goto jump_over;
137 if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
138 SWAP (mid, lo, size);
139 jump_over:;
140
141 left_ptr = lo + size;
142 right_ptr = hi - size;
143
144 /* Here's the famous ``collapse the walls'' section of quicksort.
145 Gotta like those tight inner loops! They are the main reason
146 that this algorithm runs much faster than others. */
147 do
148 {
149 while ((*cmp) ((void *) left_ptr, (void *) mid, arg) < 0)
150 left_ptr += size;
151
152 while ((*cmp) ((void *) mid, (void *) right_ptr, arg) < 0)
153 right_ptr -= size;
154
155 if (left_ptr < right_ptr)
156 {
157 SWAP (left_ptr, right_ptr, size);
158 if (mid == left_ptr)
159 mid = right_ptr;
160 else if (mid == right_ptr)
161 mid = left_ptr;
162 left_ptr += size;
163 right_ptr -= size;
164 }
165 else if (left_ptr == right_ptr)
166 {
167 left_ptr += size;
168 right_ptr -= size;
169 break;
170 }
171 }
172 while (left_ptr <= right_ptr);
173
174 /* Set up pointers for next iteration. First determine whether
175 left and right partitions are below the threshold size. If so,
176 ignore one or both. Otherwise, push the larger partition's
177 bounds on the stack and continue sorting the smaller one. */
178
179 if ((size_t) (right_ptr - lo) <= max_thresh)
180 {
181 if ((size_t) (hi - left_ptr) <= max_thresh)
182 /* Ignore both small partitions. */
183 POP (lo, hi);
184 else
185 /* Ignore small left partition. */
186 lo = left_ptr;
187 }
188 else if ((size_t) (hi - left_ptr) <= max_thresh)
189 /* Ignore small right partition. */
190 hi = right_ptr;
191 else if ((right_ptr - lo) > (hi - left_ptr))
192 {
193 /* Push larger left partition indices. */
194 PUSH (lo, right_ptr);
195 lo = left_ptr;
196 }
197 else
198 {
199 /* Push larger right partition indices. */
200 PUSH (left_ptr, hi);
201 hi = right_ptr;
202 }
203 }
204 }
205
206 /* Once the BASE_PTR array is partially sorted by quicksort the rest
207 is completely sorted using insertion sort, since this is efficient
208 for partitions below MAX_THRESH size. BASE_PTR points to the beginning
209 of the array to sort, and END_PTR points at the very last element in
210 the array (*not* one beyond it!). */
211
212 #define min(x, y) ((x) < (y) ? (x) : (y))
213
214 {
215 char *const end_ptr = &base_ptr[size * (total_elems - 1)];
216 char *tmp_ptr = base_ptr;
217 char *thresh = min(end_ptr, base_ptr + max_thresh);
218 char *run_ptr;
219
220 /* Find smallest element in first threshold and place it at the
221 array's beginning. This is the smallest array element,
222 and the operation speeds up insertion sort's inner loop. */
223
224 for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
225 if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
226 tmp_ptr = run_ptr;
227
228 if (tmp_ptr != base_ptr)
229 SWAP (tmp_ptr, base_ptr, size);
230
231 /* Insertion sort, running from left-hand-side up to right-hand-side. */
232
233 run_ptr = base_ptr + size;
234 while ((run_ptr += size) <= end_ptr)
235 {
236 tmp_ptr = run_ptr - size;
237 while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
238 tmp_ptr -= size;
239
240 tmp_ptr += size;
241 if (tmp_ptr != run_ptr)
242 {
243 char *trav;
244
245 trav = run_ptr + size;
246 while (--trav >= run_ptr)
247 {
248 char c = *trav;
249 char *hi, *lo;
250
251 for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
252 *hi = *lo;
253 *hi = c;
254 }
255 }
256 }
257 }
258 }
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*/