1 /* Copyright (C) 1991-2022 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
_quicksort(void * const pbase,size_t total_elems,size_t size,__compar_d_fn_t cmp,void * arg)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 }
260