1 /* Copyright (C) 1991,1992,1996,1997,1999,2004 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3 Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
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.
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.
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, write to the Free
17 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
20 /* If you consider tuning this algorithm, you should consult first:
21 Engineering a sort function; Jon Bentley and M. Douglas McIlroy;
22 Software - Practice and Experience; Vol. 23 (11), 1249-1265, 1993. */
29 /* Byte-wise swap two items of size SIZE. */
30 #define SWAP(a, b, size) \
33 register size_t __size = (size); \
34 register char *__a = (a), *__b = (b); \
40 } while (--__size > 0); \
43 /* Discontinue quicksort algorithm when partition gets below this size.
44 This particular magic number was chosen to work best on a Sun 4/260. */
47 /* Stack node declarations used to store unfulfilled partition obligations. */
54 /* The next 4 #defines implement a very fast in-line stack abstraction. */
55 /* The stack needs log (total_elements) entries (we could even subtract
56 log(MAX_THRESH)). Since total_elements has type size_t, we get as
57 upper bound for log (total_elements):
58 bits per byte (CHAR_BIT) * sizeof(size_t). */
59 #define STACK_SIZE (CHAR_BIT * sizeof(size_t))
60 #define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
61 #define POP(low, high) ((void) (--top, (low = top->lo), (high = top->hi)))
62 #define STACK_NOT_EMPTY (stack < top)
65 /* Order size using quicksort. This implementation incorporates
66 four optimizations discussed in Sedgewick:
68 1. Non-recursive, using an explicit stack of pointer that store the
69 next array partition to sort. To save time, this maximum amount
70 of space required to store an array of SIZE_MAX is allocated on the
71 stack. Assuming a 32-bit (64 bit) integer for size_t, this needs
72 only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes).
73 Pretty cheap, actually.
75 2. Chose the pivot element using a median-of-three decision tree.
76 This reduces the probability of selecting a bad pivot value and
77 eliminates certain extraneous comparisons.
79 3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
80 insertion sort to order the MAX_THRESH items within each partition.
81 This is a big win, since insertion sort is faster for small, mostly
82 sorted array segments.
84 4. The larger of the two sub-partitions is always pushed onto the
85 stack first, with the algorithm then concentrating on the
86 smaller partition. This *guarantees* no more than log (total_elems)
87 stack size is needed (actually O(1) in this case)! */
90 _quicksort (void *const pbase
, size_t total_elems
, size_t size
,
91 __compar_d_fn_t cmp
, void *arg
)
93 register char *base_ptr
= (char *) pbase
;
95 const size_t max_thresh
= MAX_THRESH
* size
;
98 /* Avoid lossage with unsigned arithmetic below. */
101 if (total_elems
> MAX_THRESH
)
104 char *hi
= &lo
[size
* (total_elems
- 1)];
105 stack_node stack
[STACK_SIZE
];
106 stack_node
*top
= stack
;
110 while (STACK_NOT_EMPTY
)
115 /* Select median value from among LO, MID, and HI. Rearrange
116 LO and HI so the three values are sorted. This lowers the
117 probability of picking a pathological pivot value and
118 skips a comparison for both the LEFT_PTR and RIGHT_PTR in
121 char *mid
= lo
+ size
* ((hi
- lo
) / size
>> 1);
123 if ((*cmp
) ((void *) mid
, (void *) lo
, arg
) < 0)
124 SWAP (mid
, lo
, size
);
125 if ((*cmp
) ((void *) hi
, (void *) mid
, arg
) < 0)
126 SWAP (mid
, hi
, size
);
129 if ((*cmp
) ((void *) mid
, (void *) lo
, arg
) < 0)
130 SWAP (mid
, lo
, size
);
133 left_ptr
= lo
+ size
;
134 right_ptr
= hi
- size
;
136 /* Here's the famous ``collapse the walls'' section of quicksort.
137 Gotta like those tight inner loops! They are the main reason
138 that this algorithm runs much faster than others. */
141 while ((*cmp
) ((void *) left_ptr
, (void *) mid
, arg
) < 0)
144 while ((*cmp
) ((void *) mid
, (void *) right_ptr
, arg
) < 0)
147 if (left_ptr
< right_ptr
)
149 SWAP (left_ptr
, right_ptr
, size
);
152 else if (mid
== right_ptr
)
157 else if (left_ptr
== right_ptr
)
164 while (left_ptr
<= right_ptr
);
166 /* Set up pointers for next iteration. First determine whether
167 left and right partitions are below the threshold size. If so,
168 ignore one or both. Otherwise, push the larger partition's
169 bounds on the stack and continue sorting the smaller one. */
171 if ((size_t) (right_ptr
- lo
) <= max_thresh
)
173 if ((size_t) (hi
- left_ptr
) <= max_thresh
)
174 /* Ignore both small partitions. */
177 /* Ignore small left partition. */
180 else if ((size_t) (hi
- left_ptr
) <= max_thresh
)
181 /* Ignore small right partition. */
183 else if ((right_ptr
- lo
) > (hi
- left_ptr
))
185 /* Push larger left partition indices. */
186 PUSH (lo
, right_ptr
);
191 /* Push larger right partition indices. */
198 /* Once the BASE_PTR array is partially sorted by quicksort the rest
199 is completely sorted using insertion sort, since this is efficient
200 for partitions below MAX_THRESH size. BASE_PTR points to the beginning
201 of the array to sort, and END_PTR points at the very last element in
202 the array (*not* one beyond it!). */
204 #define min(x, y) ((x) < (y) ? (x) : (y))
207 char *const end_ptr
= &base_ptr
[size
* (total_elems
- 1)];
208 char *tmp_ptr
= base_ptr
;
209 char *thresh
= min(end_ptr
, base_ptr
+ max_thresh
);
210 register char *run_ptr
;
212 /* Find smallest element in first threshold and place it at the
213 array's beginning. This is the smallest array element,
214 and the operation speeds up insertion sort's inner loop. */
216 for (run_ptr
= tmp_ptr
+ size
; run_ptr
<= thresh
; run_ptr
+= size
)
217 if ((*cmp
) ((void *) run_ptr
, (void *) tmp_ptr
, arg
) < 0)
220 if (tmp_ptr
!= base_ptr
)
221 SWAP (tmp_ptr
, base_ptr
, size
);
223 /* Insertion sort, running from left-hand-side up to right-hand-side. */
225 run_ptr
= base_ptr
+ size
;
226 while ((run_ptr
+= size
) <= end_ptr
)
228 tmp_ptr
= run_ptr
- size
;
229 while ((*cmp
) ((void *) run_ptr
, (void *) tmp_ptr
, arg
) < 0)
233 if (tmp_ptr
!= run_ptr
)
237 trav
= run_ptr
+ size
;
238 while (--trav
>= run_ptr
)
243 for (hi
= lo
= trav
; (lo
-= size
) >= tmp_ptr
; hi
= lo
)