/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: 2026-08-14 07:10

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	16.8k	{
49	16.8k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	86.7k	{
51	86.7k	if (COMPARE (src1, src2) <= 0)
52	76.1k	{
53	76.1k	dst++ = src1++;
54	76.1k	n1--;
55	76.1k	if (n1 == 0)
56	15.9k	break;
57	76.1k	}
58	10.6k	else
59	10.6k	{
60	10.6k	dst++ = src2++;
61	10.6k	n2--;
62	10.6k	if (n2 == 0)
63	920	break;
64	10.6k	}
65	86.7k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	16.8k	if (n1 > 0)
68	920	{
69	920	if (dst != src1)
70	920	do
71	1.43k	{
72	1.43k	dst++ = src1++;
73	1.43k	n1--;
74	1.43k	}
75	1.43k	while (n1 > 0);
76	920	}
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	16.8k	}
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	26.4k	{
100	26.4k	switch (n)
101	26.4k	{
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	13.4k	case 2:
109		/* Trivial case. */
110	13.4k	if (COMPARE (&src[0], &src[1]) <= 0)
111	12.8k	{
112		/* src[0] <= src[1] */
113	12.8k	dst[0] = src[0];
114	12.8k	dst[1] = src[1];
115	12.8k	}
116	563	else
117	563	{
118	563	dst[0] = src[1];
119	563	dst[1] = src[0];
120	563	}
121	13.4k	break;
122	3.42k	case 3:
123		/* Simple case. */
124	3.42k	if (COMPARE (&src[0], &src[1]) <= 0)
125	2.91k	{
126	2.91k	if (COMPARE (&src[1], &src[2]) <= 0)
127	2.44k	{
128		/* src[0] <= src[1] <= src[2] */
129	2.44k	dst[0] = src[0];
130	2.44k	dst[1] = src[1];
131	2.44k	dst[2] = src[2];
132	2.44k	}
133	476	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	223	else
141	223	{
142		/* src[2] < src[0] <= src[1] */
143	223	dst[0] = src[2];
144	223	dst[1] = src[0];
145	223	dst[2] = src[1];
146	223	}
147	2.91k	}
148	508	else
149	508	{
150	508	if (COMPARE (&src[0], &src[2]) <= 0)
151	226	{
152		/* src[1] < src[0] <= src[2] */
153	226	dst[0] = src[1];
154	226	dst[1] = src[0];
155	226	dst[2] = src[2];
156	226	}
157	282	else if (COMPARE (&src[1], &src[2]) <= 0)
158	204	{
159		/* src[1] <= src[2] < src[0] */
160	204	dst[0] = src[1];
161	204	dst[1] = src[2];
162	204	dst[2] = src[0];
163	204	}
164	78	else
165	78	{
166		/* src[2] < src[1] < src[0] */
167	78	dst[0] = src[2];
168	78	dst[1] = src[1];
169	78	dst[2] = src[0];
170	78	}
171	508	}
172	3.42k	break;
173	9.62k	default:
174	9.62k	{
175	9.62k	size_t n1 = n / 2;
176	9.62k	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	9.62k	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	9.62k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	9.62k	merge (tmp, n1, dst + n1, n2, dst);
184	9.62k	}
185	9.62k	break;
186	26.4k	}
187	26.4k	}
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	6.81k	case 2:
201		/* Trivial case. */
202	6.81k	if (COMPARE (&src[0], &src[1]) <= 0)
203	6.52k	{
204		/* src[0] <= src[1] */
205	6.52k	}
206	297	else
207	297	{
208	297	ELEMENT t = src[0];
209	297	src[0] = src[1];
210	297	src[1] = t;
211	297	}
212	6.81k	break;
213	5.13k	case 3:
214		/* Simple case. */
215	5.13k	if (COMPARE (&src[0], &src[1]) <= 0)
216	3.80k	{
217	3.80k	if (COMPARE (&src[1], &src[2]) <= 0)
218	3.25k	{
219		/* src[0] <= src[1] <= src[2] */
220	3.25k	}
221	548	else if (COMPARE (&src[0], &src[2]) <= 0)
222	247	{
223		/* src[0] <= src[2] < src[1] */
224	247	ELEMENT t = src[1];
225	247	src[1] = src[2];
226	247	src[2] = t;
227	247	}
228	301	else
229	301	{
230		/* src[2] < src[0] <= src[1] */
231	301	ELEMENT t = src[0];
232	301	src[0] = src[2];
233	301	src[2] = src[1];
234	301	src[1] = t;
235	301	}
236	3.80k	}
237	1.32k	else
238	1.32k	{
239	1.32k	if (COMPARE (&src[0], &src[2]) <= 0)
240	425	{
241		/* src[1] < src[0] <= src[2] */
242	425	ELEMENT t = src[0];
243	425	src[0] = src[1];
244	425	src[1] = t;
245	425	}
246	904	else if (COMPARE (&src[1], &src[2]) <= 0)
247	694	{
248		/* src[1] <= src[2] < src[0] */
249	694	ELEMENT t = src[0];
250	694	src[0] = src[1];
251	694	src[1] = src[2];
252	694	src[2] = t;
253	694	}
254	210	else
255	210	{
256		/* src[2] < src[1] < src[0] */
257	210	ELEMENT t = src[0];
258	210	src[0] = src[2];
259	210	src[2] = t;
260	210	}
261	1.32k	}
262	5.13k	break;
263	7.22k	default:
264	7.22k	{
265	7.22k	size_t n1 = n / 2;
266	7.22k	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	7.22k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	7.22k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	7.22k	merge (tmp, n1, src + n1, n2, src);
274	7.22k	}
275	7.22k	break;
276	19.1k	}
277	19.1k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC