xref: /csrg-svn/lib/libc/stdlib/qsort.c (revision 42126)
1 /*-
2  * Copyright (c) 1980, 1983 The Regents of the University of California.
3  * All rights reserved.
4  *
5  * %sccs.include.redist.c%
6  */
7 
8 #if defined(LIBC_SCCS) && !defined(lint)
9 static char sccsid[] = "@(#)qsort.c	5.5 (Berkeley) 05/16/90";
10 #endif /* LIBC_SCCS and not lint */
11 
12 /*
13  * qsort.c:
14  * Our own version of the system qsort routine which is faster by an average
15  * of 25%, with lows and highs of 10% and 50%.
16  * The THRESHold below is the insertion sort threshold, and has been adjusted
17  * for records of size 48 bytes.
18  * The MTHREShold is where we stop finding a better median.
19  */
20 
21 #define		THRESH		4		/* threshold for insertion */
22 #define		MTHRESH		6		/* threshold for median */
23 
24 static  int		(*qcmp)();		/* the comparison routine */
25 static  int		qsz;			/* size of each record */
26 static  int		thresh;			/* THRESHold in chars */
27 static  int		mthresh;		/* MTHRESHold in chars */
28 
29 /*
30  * qsort:
31  * First, set up some global parameters for qst to share.  Then, quicksort
32  * with qst(), and then a cleanup insertion sort ourselves.  Sound simple?
33  * It's not...
34  */
35 
36 qsort(base, n, size, compar)
37 	char	*base;
38 	int	n;
39 	int	size;
40 	int	(*compar)();
41 {
42 	register char c, *i, *j, *lo, *hi;
43 	char *min, *max;
44 
45 	if (n <= 1)
46 		return;
47 	qsz = size;
48 	qcmp = compar;
49 	thresh = qsz * THRESH;
50 	mthresh = qsz * MTHRESH;
51 	max = base + n * qsz;
52 	if (n >= THRESH) {
53 		qst(base, max);
54 		hi = base + thresh;
55 	} else {
56 		hi = max;
57 	}
58 	/*
59 	 * First put smallest element, which must be in the first THRESH, in
60 	 * the first position as a sentinel.  This is done just by searching
61 	 * the first THRESH elements (or the first n if n < THRESH), finding
62 	 * the min, and swapping it into the first position.
63 	 */
64 	for (j = lo = base; (lo += qsz) < hi; )
65 		if (qcmp(j, lo) > 0)
66 			j = lo;
67 	if (j != base) {
68 		/* swap j into place */
69 		for (i = base, hi = base + qsz; i < hi; ) {
70 			c = *j;
71 			*j++ = *i;
72 			*i++ = c;
73 		}
74 	}
75 	/*
76 	 * With our sentinel in place, we now run the following hyper-fast
77 	 * insertion sort.  For each remaining element, min, from [1] to [n-1],
78 	 * set hi to the index of the element AFTER which this one goes.
79 	 * Then, do the standard insertion sort shift on a character at a time
80 	 * basis for each element in the frob.
81 	 */
82 	for (min = base; (hi = min += qsz) < max; ) {
83 		while (qcmp(hi -= qsz, min) > 0)
84 			/* void */;
85 		if ((hi += qsz) != min) {
86 			for (lo = min + qsz; --lo >= min; ) {
87 				c = *lo;
88 				for (i = j = lo; (j -= qsz) >= hi; i = j)
89 					*i = *j;
90 				*i = c;
91 			}
92 		}
93 	}
94 }
95 
96 /*
97  * qst:
98  * Do a quicksort
99  * First, find the median element, and put that one in the first place as the
100  * discriminator.  (This "median" is just the median of the first, last and
101  * middle elements).  (Using this median instead of the first element is a big
102  * win).  Then, the usual partitioning/swapping, followed by moving the
103  * discriminator into the right place.  Then, figure out the sizes of the two
104  * partions, do the smaller one recursively and the larger one via a repeat of
105  * this code.  Stopping when there are less than THRESH elements in a partition
106  * and cleaning up with an insertion sort (in our caller) is a huge win.
107  * All data swaps are done in-line, which is space-losing but time-saving.
108  * (And there are only three places where this is done).
109  */
110 
111 static
112 qst(base, max)
113 	char *base, *max;
114 {
115 	register char c, *i, *j, *jj;
116 	register int ii;
117 	char *mid, *tmp;
118 	int lo, hi;
119 
120 	/*
121 	 * At the top here, lo is the number of characters of elements in the
122 	 * current partition.  (Which should be max - base).
123 	 * Find the median of the first, last, and middle element and make
124 	 * that the middle element.  Set j to largest of first and middle.
125 	 * If max is larger than that guy, then it's that guy, else compare
126 	 * max with loser of first and take larger.  Things are set up to
127 	 * prefer the middle, then the first in case of ties.
128 	 */
129 	lo = max - base;		/* number of elements as chars */
130 	do	{
131 		mid = i = base + qsz * ((lo / qsz) >> 1);
132 		if (lo >= mthresh) {
133 			j = (qcmp((jj = base), i) > 0 ? jj : i);
134 			if (qcmp(j, (tmp = max - qsz)) > 0) {
135 				/* switch to first loser */
136 				j = (j == jj ? i : jj);
137 				if (qcmp(j, tmp) < 0)
138 					j = tmp;
139 			}
140 			if (j != i) {
141 				ii = qsz;
142 				do	{
143 					c = *i;
144 					*i++ = *j;
145 					*j++ = c;
146 				} while (--ii);
147 			}
148 		}
149 		/*
150 		 * Semi-standard quicksort partitioning/swapping
151 		 */
152 		for (i = base, j = max - qsz; ; ) {
153 			while (i < mid && qcmp(i, mid) <= 0)
154 				i += qsz;
155 			while (j > mid) {
156 				if (qcmp(mid, j) <= 0) {
157 					j -= qsz;
158 					continue;
159 				}
160 				tmp = i + qsz;	/* value of i after swap */
161 				if (i == mid) {
162 					/* j <-> mid, new mid is j */
163 					mid = jj = j;
164 				} else {
165 					/* i <-> j */
166 					jj = j;
167 					j -= qsz;
168 				}
169 				goto swap;
170 			}
171 			if (i == mid) {
172 				break;
173 			} else {
174 				/* i <-> mid, new mid is i */
175 				jj = mid;
176 				tmp = mid = i;	/* value of i after swap */
177 				j -= qsz;
178 			}
179 		swap:
180 			ii = qsz;
181 			do	{
182 				c = *i;
183 				*i++ = *jj;
184 				*jj++ = c;
185 			} while (--ii);
186 			i = tmp;
187 		}
188 		/*
189 		 * Look at sizes of the two partitions, do the smaller
190 		 * one first by recursion, then do the larger one by
191 		 * making sure lo is its size, base and max are update
192 		 * correctly, and branching back.  But only repeat
193 		 * (recursively or by branching) if the partition is
194 		 * of at least size THRESH.
195 		 */
196 		i = (j = mid) + qsz;
197 		if ((lo = j - base) <= (hi = max - i)) {
198 			if (lo >= thresh)
199 				qst(base, j);
200 			base = i;
201 			lo = hi;
202 		} else {
203 			if (hi >= thresh)
204 				qst(i, max);
205 			max = j;
206 		}
207 	} while (lo >= thresh);
208 }
209