/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: 2026-10-14 06:53

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	74.8k	{
49	74.8k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	438k	{
51	438k	if (COMPARE (src1, src2) <= 0)
52	381k	{
53	381k	dst++ = src1++;
54	381k	n1--;
55	381k	if (n1 == 0)
56	71.1k	break;
57	381k	}
58	57.2k	else
59	57.2k	{
60	57.2k	dst++ = src2++;
61	57.2k	n2--;
62	57.2k	if (n2 == 0)
63	3.67k	break;
64	57.2k	}
65	438k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	74.8k	if (n1 > 0)
68	3.67k	{
69	3.67k	if (dst != src1)
70	3.67k	do
71	6.39k	{
72	6.39k	dst++ = src1++;
73	6.39k	n1--;
74	6.39k	}
75	6.39k	while (n1 > 0);
76	3.67k	}
77	71.1k	else /* n2 > 0 */
78	71.1k	{
79	71.1k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	71.1k	}
87	74.8k	}
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	129k	{
100	129k	switch (n)
101	129k	{
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	64.6k	case 2:
109		/* Trivial case. */
110	64.6k	if (COMPARE (&src[0], &src[1]) <= 0)
111	61.0k	{
112		/* src[0] <= src[1] */
113	61.0k	dst[0] = src[0];
114	61.0k	dst[1] = src[1];
115	61.0k	}
116	3.57k	else
117	3.57k	{
118	3.57k	dst[0] = src[1];
119	3.57k	dst[1] = src[0];
120	3.57k	}
121	64.6k	break;
122	10.1k	case 3:
123		/* Simple case. */
124	10.1k	if (COMPARE (&src[0], &src[1]) <= 0)
125	7.66k	{
126	7.66k	if (COMPARE (&src[1], &src[2]) <= 0)
127	5.43k	{
128		/* src[0] <= src[1] <= src[2] */
129	5.43k	dst[0] = src[0];
130	5.43k	dst[1] = src[1];
131	5.43k	dst[2] = src[2];
132	5.43k	}
133	2.22k	else if (COMPARE (&src[0], &src[2]) <= 0)
134	1.40k	{
135		/* src[0] <= src[2] < src[1] */
136	1.40k	dst[0] = src[0];
137	1.40k	dst[1] = src[2];
138	1.40k	dst[2] = src[1];
139	1.40k	}
140	825	else
141	825	{
142		/* src[2] < src[0] <= src[1] */
143	825	dst[0] = src[2];
144	825	dst[1] = src[0];
145	825	dst[2] = src[1];
146	825	}
147	7.66k	}
148	2.52k	else
149	2.52k	{
150	2.52k	if (COMPARE (&src[0], &src[2]) <= 0)
151	1.09k	{
152		/* src[1] < src[0] <= src[2] */
153	1.09k	dst[0] = src[1];
154	1.09k	dst[1] = src[0];
155	1.09k	dst[2] = src[2];
156	1.09k	}
157	1.42k	else if (COMPARE (&src[1], &src[2]) <= 0)
158	747	{
159		/* src[1] <= src[2] < src[0] */
160	747	dst[0] = src[1];
161	747	dst[1] = src[2];
162	747	dst[2] = src[0];
163	747	}
164	679	else
165	679	{
166		/* src[2] < src[1] < src[0] */
167	679	dst[0] = src[2];
168	679	dst[1] = src[1];
169	679	dst[2] = src[0];
170	679	}
171	2.52k	}
172	10.1k	break;
173	54.7k	default:
174	54.7k	{
175	54.7k	size_t n1 = n / 2;
176	54.7k	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	54.7k	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	54.7k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	54.7k	merge (tmp, n1, dst + n1, n2, dst);
184	54.7k	}
185	54.7k	break;
186	129k	}
187	129k	}
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	119k	{
194	119k	switch (n)
195	119k	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	101k	case 2:
201		/* Trivial case. */
202	101k	if (COMPARE (&src[0], &src[1]) <= 0)
203	100k	{
204		/* src[0] <= src[1] */
205	100k	}
206	691	else
207	691	{
208	691	ELEMENT t = src[0];
209	691	src[0] = src[1];
210	691	src[1] = t;
211	691	}
212	101k	break;
213	7.40k	case 3:
214		/* Simple case. */
215	7.40k	if (COMPARE (&src[0], &src[1]) <= 0)
216	5.66k	{
217	5.66k	if (COMPARE (&src[1], &src[2]) <= 0)
218	4.49k	{
219		/* src[0] <= src[1] <= src[2] */
220	4.49k	}
221	1.17k	else if (COMPARE (&src[0], &src[2]) <= 0)
222	577	{
223		/* src[0] <= src[2] < src[1] */
224	577	ELEMENT t = src[1];
225	577	src[1] = src[2];
226	577	src[2] = t;
227	577	}
228	598	else
229	598	{
230		/* src[2] < src[0] <= src[1] */
231	598	ELEMENT t = src[0];
232	598	src[0] = src[2];
233	598	src[2] = src[1];
234	598	src[1] = t;
235	598	}
236	5.66k	}
237	1.74k	else
238	1.74k	{
239	1.74k	if (COMPARE (&src[0], &src[2]) <= 0)
240	688	{
241		/* src[1] < src[0] <= src[2] */
242	688	ELEMENT t = src[0];
243	688	src[0] = src[1];
244	688	src[1] = t;
245	688	}
246	1.05k	else if (COMPARE (&src[1], &src[2]) <= 0)
247	595	{
248		/* src[1] <= src[2] < src[0] */
249	595	ELEMENT t = src[0];
250	595	src[0] = src[1];
251	595	src[1] = src[2];
252	595	src[2] = t;
253	595	}
254	460	else
255	460	{
256		/* src[2] < src[1] < src[0] */
257	460	ELEMENT t = src[0];
258	460	src[0] = src[2];
259	460	src[2] = t;
260	460	}
261	1.74k	}
262	7.40k	break;
263	10.7k	default:
264	10.7k	{
265	10.7k	size_t n1 = n / 2;
266	10.7k	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	10.7k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	10.7k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	10.7k	merge (tmp, n1, src + n1, n2, src);
274	10.7k	}
275	10.7k	break;
276	119k	}
277	119k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC