/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: 2026-09-28 06:47

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	72.9k	{
49	72.9k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	428k	{
51	428k	if (COMPARE (src1, src2) <= 0)
52	368k	{
53	368k	dst++ = src1++;
54	368k	n1--;
55	368k	if (n1 == 0)
56	68.9k	break;
57	368k	}
58	60.3k	else
59	60.3k	{
60	60.3k	dst++ = src2++;
61	60.3k	n2--;
62	60.3k	if (n2 == 0)
63	3.98k	break;
64	60.3k	}
65	428k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	72.9k	if (n1 > 0)
68	3.98k	{
69	3.98k	if (dst != src1)
70	3.98k	do
71	7.19k	{
72	7.19k	dst++ = src1++;
73	7.19k	n1--;
74	7.19k	}
75	7.19k	while (n1 > 0);
76	3.98k	}
77	68.9k	else /* n2 > 0 */
78	68.9k	{
79	68.9k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	68.9k	}
87	72.9k	}
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	125k	{
100	125k	switch (n)
101	125k	{
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	62.3k	case 2:
109		/* Trivial case. */
110	62.3k	if (COMPARE (&src[0], &src[1]) <= 0)
111	58.7k	{
112		/* src[0] <= src[1] */
113	58.7k	dst[0] = src[0];
114	58.7k	dst[1] = src[1];
115	58.7k	}
116	3.63k	else
117	3.63k	{
118	3.63k	dst[0] = src[1];
119	3.63k	dst[1] = src[0];
120	3.63k	}
121	62.3k	break;
122	10.5k	case 3:
123		/* Simple case. */
124	10.5k	if (COMPARE (&src[0], &src[1]) <= 0)
125	7.86k	{
126	7.86k	if (COMPARE (&src[1], &src[2]) <= 0)
127	5.49k	{
128		/* src[0] <= src[1] <= src[2] */
129	5.49k	dst[0] = src[0];
130	5.49k	dst[1] = src[1];
131	5.49k	dst[2] = src[2];
132	5.49k	}
133	2.37k	else if (COMPARE (&src[0], &src[2]) <= 0)
134	1.50k	{
135		/* src[0] <= src[2] < src[1] */
136	1.50k	dst[0] = src[0];
137	1.50k	dst[1] = src[2];
138	1.50k	dst[2] = src[1];
139	1.50k	}
140	868	else
141	868	{
142		/* src[2] < src[0] <= src[1] */
143	868	dst[0] = src[2];
144	868	dst[1] = src[0];
145	868	dst[2] = src[1];
146	868	}
147	7.86k	}
148	2.71k	else
149	2.71k	{
150	2.71k	if (COMPARE (&src[0], &src[2]) <= 0)
151	1.16k	{
152		/* src[1] < src[0] <= src[2] */
153	1.16k	dst[0] = src[1];
154	1.16k	dst[1] = src[0];
155	1.16k	dst[2] = src[2];
156	1.16k	}
157	1.54k	else if (COMPARE (&src[1], &src[2]) <= 0)
158	836	{
159		/* src[1] <= src[2] < src[0] */
160	836	dst[0] = src[1];
161	836	dst[1] = src[2];
162	836	dst[2] = src[0];
163	836	}
164	706	else
165	706	{
166		/* src[2] < src[1] < src[0] */
167	706	dst[0] = src[2];
168	706	dst[1] = src[1];
169	706	dst[2] = src[0];
170	706	}
171	2.71k	}
172	10.5k	break;
173	52.8k	default:
174	52.8k	{
175	52.8k	size_t n1 = n / 2;
176	52.8k	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	52.8k	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	52.8k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	52.8k	merge (tmp, n1, dst + n1, n2, dst);
184	52.8k	}
185	52.8k	break;
186	125k	}
187	125k	}
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	106k	{
194	106k	switch (n)
195	106k	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	89.1k	case 2:
201		/* Trivial case. */
202	89.1k	if (COMPARE (&src[0], &src[1]) <= 0)
203	88.5k	{
204		/* src[0] <= src[1] */
205	88.5k	}
206	612	else
207	612	{
208	612	ELEMENT t = src[0];
209	612	src[0] = src[1];
210	612	src[1] = t;
211	612	}
212	89.1k	break;
213	7.04k	case 3:
214		/* Simple case. */
215	7.04k	if (COMPARE (&src[0], &src[1]) <= 0)
216	5.25k	{
217	5.25k	if (COMPARE (&src[1], &src[2]) <= 0)
218	4.07k	{
219		/* src[0] <= src[1] <= src[2] */
220	4.07k	}
221	1.17k	else if (COMPARE (&src[0], &src[2]) <= 0)
222	583	{
223		/* src[0] <= src[2] < src[1] */
224	583	ELEMENT t = src[1];
225	583	src[1] = src[2];
226	583	src[2] = t;
227	583	}
228	596	else
229	596	{
230		/* src[2] < src[0] <= src[1] */
231	596	ELEMENT t = src[0];
232	596	src[0] = src[2];
233	596	src[2] = src[1];
234	596	src[1] = t;
235	596	}
236	5.25k	}
237	1.79k	else
238	1.79k	{
239	1.79k	if (COMPARE (&src[0], &src[2]) <= 0)
240	704	{
241		/* src[1] < src[0] <= src[2] */
242	704	ELEMENT t = src[0];
243	704	src[0] = src[1];
244	704	src[1] = t;
245	704	}
246	1.09k	else if (COMPARE (&src[1], &src[2]) <= 0)
247	623	{
248		/* src[1] <= src[2] < src[0] */
249	623	ELEMENT t = src[0];
250	623	src[0] = src[1];
251	623	src[1] = src[2];
252	623	src[2] = t;
253	623	}
254	467	else
255	467	{
256		/* src[2] < src[1] < src[0] */
257	467	ELEMENT t = src[0];
258	467	src[0] = src[2];
259	467	src[2] = t;
260	467	}
261	1.79k	}
262	7.04k	break;
263	10.5k	default:
264	10.5k	{
265	10.5k	size_t n1 = n / 2;
266	10.5k	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	10.5k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	10.5k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	10.5k	merge (tmp, n1, src + n1, n2, src);
274	10.5k	}
275	10.5k	break;
276	106k	}
277	106k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC