/src/libunistring/lib/array-mergesort.h

Source
/* Stable-sorting of an array using mergesort.
   Copyright (C) 2009-2026 Free Software Foundation, Inc.
   Written by Bruno Haible <bruno@clisp.org>, 2009.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* This file implements stable sorting of an array, using the mergesort
   algorithm.
   Worst-case running time for an array of length N is O(N log N).
   Unlike the mpsort module, the algorithm here attempts to minimize not
   only the number of comparisons, but also the number of copying operations.

   Before including this file, you need to define
     ELEMENT      The type of every array element.
     COMPARE      A two-argument macro that takes two 'const ELEMENT *'
                  pointers and returns a negative, zero, or positive 'int'
                  value if the element pointed to by the first argument is,
                  respectively, less, equal, or greater than the element
                  pointed to by the second argument.
     STATIC       The storage class of the functions being defined.
     STATIC_FROMTO  (Optional.) Overrides STATIC for the 'merge_sort_fromto'
                    function.
   Before including this file, you also need to include:
     #include <stddef.h>
 */

/* Merge the sorted arrays src1[0..n1-1] and src2[0..n2-1] into
   dst[0..n1+n2-1].  In case of ambiguity, put the elements of src1
   before the elements of src2.
   n1 and n2 must be > 0.
   The arrays src1 and src2 must not overlap the dst array, except that
   src1 may be dst[n2..n1+n2-1], or src2 may be dst[n1..n1+n2-1].  */
static void
merge (const ELEMENT *src1, size_t n1,
       const ELEMENT *src2, size_t n2,
       ELEMENT *dst)
{
  for (;;) /* while (n1 > 0 && n2 > 0) */
    {
      if (COMPARE (src1, src2) <= 0)
        {
          *dst++ = *src1++;
          n1--;
          if (n1 == 0)
            break;
        }
      else
        {
          *dst++ = *src2++;
          n2--;
          if (n2 == 0)
            break;
        }
    }
  /* Here n1 == 0 || n2 == 0 but also n1 > 0 || n2 > 0.  */
  if (n1 > 0)
    {
      if (dst != src1)
        do
          {
            *dst++ = *src1++;
            n1--;
          }
        while (n1 > 0);
    }
  else /* n2 > 0 */
    {
      if (dst != src2)
        do
          {
            *dst++ = *src2++;
            n2--;
          }
        while (n2 > 0);
    }
}

/* Sort src[0..n-1] into dst[0..n-1], using tmp[0..n/2-1] as temporary
   (scratch) storage.
   The arrays src, dst, tmp must not overlap.  */
#ifdef STATIC_FROMTO
STATIC_FROMTO
#else
STATIC
#endif
void
merge_sort_fromto (const ELEMENT *src, ELEMENT *dst, size_t n, ELEMENT *tmp)
{
  switch (n)
    {
    case 0:
      return;
    case 1:
      /* Nothing to do.  */
      dst[0] = src[0];
      return;
    case 2:
      /* Trivial case.  */
      if (COMPARE (&src[0], &src[1]) <= 0)
        {
          /* src[0] <= src[1] */
          dst[0] = src[0];
          dst[1] = src[1];
        }
      else
        {
          dst[0] = src[1];
          dst[1] = src[0];
        }
      break;
    case 3:
      /* Simple case.  */
      if (COMPARE (&src[0], &src[1]) <= 0)
        {
          if (COMPARE (&src[1], &src[2]) <= 0)
            {
              /* src[0] <= src[1] <= src[2] */
              dst[0] = src[0];
              dst[1] = src[1];
              dst[2] = src[2];
            }
          else if (COMPARE (&src[0], &src[2]) <= 0)
            {
              /* src[0] <= src[2] < src[1] */
              dst[0] = src[0];
              dst[1] = src[2];
              dst[2] = src[1];
            }
          else
            {
              /* src[2] < src[0] <= src[1] */
              dst[0] = src[2];
              dst[1] = src[0];
              dst[2] = src[1];
            }
        }
      else
        {
          if (COMPARE (&src[0], &src[2]) <= 0)
            {
              /* src[1] < src[0] <= src[2] */
              dst[0] = src[1];
              dst[1] = src[0];
              dst[2] = src[2];
            }
          else if (COMPARE (&src[1], &src[2]) <= 0)
            {
              /* src[1] <= src[2] < src[0] */
              dst[0] = src[1];
              dst[1] = src[2];
              dst[2] = src[0];
            }
          else
            {
              /* src[2] < src[1] < src[0] */
              dst[0] = src[2];
              dst[1] = src[1];
              dst[2] = src[0];
            }
        }
      break;
    default:
      {
        size_t n1 = n / 2;
        size_t n2 = (n + 1) / 2;
        /* Note: n1 + n2 = n, n1 <= n2.  */
        /* Sort src[n1..n-1] into dst[n1..n-1], scratching tmp[0..n2/2-1].  */
        merge_sort_fromto (src + n1, dst + n1, n2, tmp);
        /* Sort src[0..n1-1] into tmp[0..n1-1], scratching dst[0..n1-1].  */
        merge_sort_fromto (src, tmp, n1, dst);
        /* Merge the two half results.  */
        merge (tmp, n1, dst + n1, n2, dst);
      }
      break;
    }
}

/* Sort src[0..n-1], using tmp[0..n-1] as temporary (scratch) storage.
   The arrays src, tmp must not overlap. */
STATIC void
merge_sort_inplace (ELEMENT *src, size_t n, ELEMENT *tmp)
{
  switch (n)
    {
    case 0:
    case 1:
      /* Nothing to do.  */
      return;
    case 2:
      /* Trivial case.  */
      if (COMPARE (&src[0], &src[1]) <= 0)
        {
          /* src[0] <= src[1] */
        }
      else
        {
          ELEMENT t = src[0];
          src[0] = src[1];
          src[1] = t;
        }
      break;
    case 3:
      /* Simple case.  */
      if (COMPARE (&src[0], &src[1]) <= 0)
        {
          if (COMPARE (&src[1], &src[2]) <= 0)
            {
              /* src[0] <= src[1] <= src[2] */
            }
          else if (COMPARE (&src[0], &src[2]) <= 0)
            {
              /* src[0] <= src[2] < src[1] */
              ELEMENT t = src[1];
              src[1] = src[2];
              src[2] = t;
            }
          else
            {
              /* src[2] < src[0] <= src[1] */
              ELEMENT t = src[0];
              src[0] = src[2];
              src[2] = src[1];
              src[1] = t;
            }
        }
      else
        {
          if (COMPARE (&src[0], &src[2]) <= 0)
            {
              /* src[1] < src[0] <= src[2] */
              ELEMENT t = src[0];
              src[0] = src[1];
              src[1] = t;
            }
          else if (COMPARE (&src[1], &src[2]) <= 0)
            {
              /* src[1] <= src[2] < src[0] */
              ELEMENT t = src[0];
              src[0] = src[1];
              src[1] = src[2];
              src[2] = t;
            }
          else
            {
              /* src[2] < src[1] < src[0] */
              ELEMENT t = src[0];
              src[0] = src[2];
              src[2] = t;
            }
        }
      break;
    default:
      {
        size_t n1 = n / 2;
        size_t n2 = (n + 1) / 2;
        /* Note: n1 + n2 = n, n1 <= n2.  */
        /* Sort src[n1..n-1], scratching tmp[0..n2-1].  */
        merge_sort_inplace (src + n1, n2, tmp);
        /* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2*n1-1].  */
        merge_sort_fromto (src, tmp, n1, tmp + n1);
        /* Merge the two half results.  */
        merge (tmp, n1, src + n1, n2, src);
      }
      break;
    }
}

#undef ELEMENT
#undef COMPARE
#undef STATIC

Coverage Report

Created: 2026-05-16 06:52

Line	Count	Source
1		/* Stable-sorting of an array using mergesort.
2		Copyright (C) 2009-2026 Free Software Foundation, Inc.
3		Written by Bruno Haible <bruno@clisp.org>, 2009.
4
5		This file is free software: you can redistribute it and/or modify
6		it under the terms of the GNU Lesser General Public License as
7		published by the Free Software Foundation; either version 2.1 of the
8		License, or (at your option) any later version.
9
10		This file 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
13		GNU Lesser General Public License for more details.
14
15		You should have received a copy of the GNU Lesser General Public License
16		along with this program. If not, see <https://www.gnu.org/licenses/>. */
17
18		/* This file implements stable sorting of an array, using the mergesort
19		algorithm.
20		Worst-case running time for an array of length N is O(N log N).
21		Unlike the mpsort module, the algorithm here attempts to minimize not
22		only the number of comparisons, but also the number of copying operations.
23
24		Before including this file, you need to define
25		ELEMENT The type of every array element.
26		COMPARE A two-argument macro that takes two 'const ELEMENT *'
27		pointers and returns a negative, zero, or positive 'int'
28		value if the element pointed to by the first argument is,
29		respectively, less, equal, or greater than the element
30		pointed to by the second argument.
31		STATIC The storage class of the functions being defined.
32		STATIC_FROMTO (Optional.) Overrides STATIC for the 'merge_sort_fromto'
33		function.
34		Before including this file, you also need to include:
35		#include <stddef.h>
36		*/
37
38		/* Merge the sorted arrays src1[0..n1-1] and src2[0..n2-1] into
39		dst[0..n1+n2-1]. In case of ambiguity, put the elements of src1
40		before the elements of src2.
41		n1 and n2 must be > 0.
42		The arrays src1 and src2 must not overlap the dst array, except that
43		src1 may be dst[n2..n1+n2-1], or src2 may be dst[n1..n1+n2-1]. */
44		static void
45		merge (const ELEMENT *src1, size_t n1,
46		const ELEMENT *src2, size_t n2,
47		ELEMENT *dst)
48	17.0k	{
49	17.0k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	89.1k	{
51	89.1k	if (COMPARE (src1, src2) <= 0)
52	78.1k	{
53	78.1k	dst++ = src1++;
54	78.1k	n1--;
55	78.1k	if (n1 == 0)
56	15.9k	break;
57	78.1k	}
58	10.9k	else
59	10.9k	{
60	10.9k	dst++ = src2++;
61	10.9k	n2--;
62	10.9k	if (n2 == 0)
63	1.06k	break;
64	10.9k	}
65	89.1k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	17.0k	if (n1 > 0)
68	1.06k	{
69	1.06k	if (dst != src1)
70	1.06k	do
71	1.60k	{
72	1.60k	dst++ = src1++;
73	1.60k	n1--;
74	1.60k	}
75	1.60k	while (n1 > 0);
76	1.06k	}
77	15.9k	else /* n2 > 0 */
78	15.9k	{
79	15.9k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	15.9k	}
87	17.0k	}
88
89		/* Sort src[0..n-1] into dst[0..n-1], using tmp[0..n/2-1] as temporary
90		(scratch) storage.
91		The arrays src, dst, tmp must not overlap. */
92		#ifdef STATIC_FROMTO
93		STATIC_FROMTO
94		#else
95		STATIC
96		#endif
97		void
98		merge_sort_fromto (const ELEMENT src, ELEMENT dst, size_t n, ELEMENT *tmp)
99	27.0k	{
100	27.0k	switch (n)
101	27.0k	{
102	0	case 0:
103	0	return;
104	0	case 1:
105		/* Nothing to do. */
106	0	dst[0] = src[0];
107	0	return;
108	13.5k	case 2:
109		/* Trivial case. */
110	13.5k	if (COMPARE (&src[0], &src[1]) <= 0)
111	13.0k	{
112		/* src[0] <= src[1] */
113	13.0k	dst[0] = src[0];
114	13.0k	dst[1] = src[1];
115	13.0k	}
116	561	else
117	561	{
118	561	dst[0] = src[1];
119	561	dst[1] = src[0];
120	561	}
121	13.5k	break;
122	3.48k	case 3:
123		/* Simple case. */
124	3.48k	if (COMPARE (&src[0], &src[1]) <= 0)
125	2.96k	{
126	2.96k	if (COMPARE (&src[1], &src[2]) <= 0)
127	2.48k	{
128		/* src[0] <= src[1] <= src[2] */
129	2.48k	dst[0] = src[0];
130	2.48k	dst[1] = src[1];
131	2.48k	dst[2] = src[2];
132	2.48k	}
133	481	else if (COMPARE (&src[0], &src[2]) <= 0)
134	253	{
135		/* src[0] <= src[2] < src[1] */
136	253	dst[0] = src[0];
137	253	dst[1] = src[2];
138	253	dst[2] = src[1];
139	253	}
140	228	else
141	228	{
142		/* src[2] < src[0] <= src[1] */
143	228	dst[0] = src[2];
144	228	dst[1] = src[0];
145	228	dst[2] = src[1];
146	228	}
147	2.96k	}
148	517	else
149	517	{
150	517	if (COMPARE (&src[0], &src[2]) <= 0)
151	229	{
152		/* src[1] < src[0] <= src[2] */
153	229	dst[0] = src[1];
154	229	dst[1] = src[0];
155	229	dst[2] = src[2];
156	229	}
157	288	else if (COMPARE (&src[1], &src[2]) <= 0)
158	209	{
159		/* src[1] <= src[2] < src[0] */
160	209	dst[0] = src[1];
161	209	dst[1] = src[2];
162	209	dst[2] = src[0];
163	209	}
164	79	else
165	79	{
166		/* src[2] < src[1] < src[0] */
167	79	dst[0] = src[2];
168	79	dst[1] = src[1];
169	79	dst[2] = src[0];
170	79	}
171	517	}
172	3.48k	break;
173	9.98k	default:
174	9.98k	{
175	9.98k	size_t n1 = n / 2;
176	9.98k	size_t n2 = (n + 1) / 2;
177		/* Note: n1 + n2 = n, n1 <= n2. */
178		/* Sort src[n1..n-1] into dst[n1..n-1], scratching tmp[0..n2/2-1]. */
179	9.98k	merge_sort_fromto (src + n1, dst + n1, n2, tmp);
180		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching dst[0..n1-1]. */
181	9.98k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	9.98k	merge (tmp, n1, dst + n1, n2, dst);
184	9.98k	}
185	9.98k	break;
186	27.0k	}
187	27.0k	}
188
189		/* Sort src[0..n-1], using tmp[0..n-1] as temporary (scratch) storage.
190		The arrays src, tmp must not overlap. */
191		STATIC void
192		merge_sort_inplace (ELEMENT src, size_t n, ELEMENT tmp)
193	19.1k	{
194	19.1k	switch (n)
195	19.1k	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	7.18k	case 2:
201		/* Trivial case. */
202	7.18k	if (COMPARE (&src[0], &src[1]) <= 0)
203	6.90k	{
204		/* src[0] <= src[1] */
205	6.90k	}
206	274	else
207	274	{
208	274	ELEMENT t = src[0];
209	274	src[0] = src[1];
210	274	src[1] = t;
211	274	}
212	7.18k	break;
213	4.93k	case 3:
214		/* Simple case. */
215	4.93k	if (COMPARE (&src[0], &src[1]) <= 0)
216	3.54k	{
217	3.54k	if (COMPARE (&src[1], &src[2]) <= 0)
218	3.01k	{
219		/* src[0] <= src[1] <= src[2] */
220	3.01k	}
221	525	else if (COMPARE (&src[0], &src[2]) <= 0)
222	241	{
223		/* src[0] <= src[2] < src[1] */
224	241	ELEMENT t = src[1];
225	241	src[1] = src[2];
226	241	src[2] = t;
227	241	}
228	284	else
229	284	{
230		/* src[2] < src[0] <= src[1] */
231	284	ELEMENT t = src[0];
232	284	src[0] = src[2];
233	284	src[2] = src[1];
234	284	src[1] = t;
235	284	}
236	3.54k	}
237	1.39k	else
238	1.39k	{
239	1.39k	if (COMPARE (&src[0], &src[2]) <= 0)
240	463	{
241		/* src[1] < src[0] <= src[2] */
242	463	ELEMENT t = src[0];
243	463	src[0] = src[1];
244	463	src[1] = t;
245	463	}
246	928	else if (COMPARE (&src[1], &src[2]) <= 0)
247	721	{
248		/* src[1] <= src[2] < src[0] */
249	721	ELEMENT t = src[0];
250	721	src[0] = src[1];
251	721	src[1] = src[2];
252	721	src[2] = t;
253	721	}
254	207	else
255	207	{
256		/* src[2] < src[1] < src[0] */
257	207	ELEMENT t = src[0];
258	207	src[0] = src[2];
259	207	src[2] = t;
260	207	}
261	1.39k	}
262	4.93k	break;
263	7.07k	default:
264	7.07k	{
265	7.07k	size_t n1 = n / 2;
266	7.07k	size_t n2 = (n + 1) / 2;
267		/* Note: n1 + n2 = n, n1 <= n2. */
268		/* Sort src[n1..n-1], scratching tmp[0..n2-1]. */
269	7.07k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	7.07k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	7.07k	merge (tmp, n1, src + n1, n2, src);
274	7.07k	}
275	7.07k	break;
276	19.1k	}
277	19.1k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC