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

Source
/* Stable-sorting of an array using mergesort.
   Copyright (C) 2009-2025 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: 2026-02-14 06:49

Line	Count	Source
1		/* Stable-sorting of an array using mergesort.
2		Copyright (C) 2009-2025 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	16.9k	{
49	16.9k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	88.6k	{
51	88.6k	if (COMPARE (src1, src2) <= 0)
52	77.1k	{
53	77.1k	dst++ = src1++;
54	77.1k	n1--;
55	77.1k	if (n1 == 0)
56	15.9k	break;
57	77.1k	}
58	11.4k	else
59	11.4k	{
60	11.4k	dst++ = src2++;
61	11.4k	n2--;
62	11.4k	if (n2 == 0)
63	956	break;
64	11.4k	}
65	88.6k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	16.9k	if (n1 > 0)
68	956	{
69	956	if (dst != src1)
70	956	do
71	1.72k	{
72	1.72k	dst++ = src1++;
73	1.72k	n1--;
74	1.72k	}
75	1.72k	while (n1 > 0);
76	956	}
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	16.9k	}
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	26.8k	{
100	26.8k	switch (n)
101	26.8k	{
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	13.6k	case 2:
109		/* Trivial case. */
110	13.6k	if (COMPARE (&src[0], &src[1]) <= 0)
111	12.9k	{
112		/* src[0] <= src[1] */
113	12.9k	dst[0] = src[0];
114	12.9k	dst[1] = src[1];
115	12.9k	}
116	627	else
117	627	{
118	627	dst[0] = src[1];
119	627	dst[1] = src[0];
120	627	}
121	13.6k	break;
122	3.31k	case 3:
123		/* Simple case. */
124	3.31k	if (COMPARE (&src[0], &src[1]) <= 0)
125	2.79k	{
126	2.79k	if (COMPARE (&src[1], &src[2]) <= 0)
127	2.31k	{
128		/* src[0] <= src[1] <= src[2] */
129	2.31k	dst[0] = src[0];
130	2.31k	dst[1] = src[1];
131	2.31k	dst[2] = src[2];
132	2.31k	}
133	485	else if (COMPARE (&src[0], &src[2]) <= 0)
134	264	{
135		/* src[0] <= src[2] < src[1] */
136	264	dst[0] = src[0];
137	264	dst[1] = src[2];
138	264	dst[2] = src[1];
139	264	}
140	221	else
141	221	{
142		/* src[2] < src[0] <= src[1] */
143	221	dst[0] = src[2];
144	221	dst[1] = src[0];
145	221	dst[2] = src[1];
146	221	}
147	2.79k	}
148	513	else
149	513	{
150	513	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	284	else if (COMPARE (&src[1], &src[2]) <= 0)
158	207	{
159		/* src[1] <= src[2] < src[0] */
160	207	dst[0] = src[1];
161	207	dst[1] = src[2];
162	207	dst[2] = src[0];
163	207	}
164	77	else
165	77	{
166		/* src[2] < src[1] < src[0] */
167	77	dst[0] = src[2];
168	77	dst[1] = src[1];
169	77	dst[2] = src[0];
170	77	}
171	513	}
172	3.31k	break;
173	9.90k	default:
174	9.90k	{
175	9.90k	size_t n1 = n / 2;
176	9.90k	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	9.90k	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	9.90k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	9.90k	merge (tmp, n1, dst + n1, n2, dst);
184	9.90k	}
185	9.90k	break;
186	26.8k	}
187	26.8k	}
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	18.4k	{
194	18.4k	switch (n)
195	18.4k	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	6.46k	case 2:
201		/* Trivial case. */
202	6.46k	if (COMPARE (&src[0], &src[1]) <= 0)
203	6.19k	{
204		/* src[0] <= src[1] */
205	6.19k	}
206	269	else
207	269	{
208	269	ELEMENT t = src[0];
209	269	src[0] = src[1];
210	269	src[1] = t;
211	269	}
212	6.46k	break;
213	4.94k	case 3:
214		/* Simple case. */
215	4.94k	if (COMPARE (&src[0], &src[1]) <= 0)
216	3.58k	{
217	3.58k	if (COMPARE (&src[1], &src[2]) <= 0)
218	3.04k	{
219		/* src[0] <= src[1] <= src[2] */
220	3.04k	}
221	536	else if (COMPARE (&src[0], &src[2]) <= 0)
222	244	{
223		/* src[0] <= src[2] < src[1] */
224	244	ELEMENT t = src[1];
225	244	src[1] = src[2];
226	244	src[2] = t;
227	244	}
228	292	else
229	292	{
230		/* src[2] < src[0] <= src[1] */
231	292	ELEMENT t = src[0];
232	292	src[0] = src[2];
233	292	src[2] = src[1];
234	292	src[1] = t;
235	292	}
236	3.58k	}
237	1.36k	else
238	1.36k	{
239	1.36k	if (COMPARE (&src[0], &src[2]) <= 0)
240	459	{
241		/* src[1] < src[0] <= src[2] */
242	459	ELEMENT t = src[0];
243	459	src[0] = src[1];
244	459	src[1] = t;
245	459	}
246	905	else if (COMPARE (&src[1], &src[2]) <= 0)
247	703	{
248		/* src[1] <= src[2] < src[0] */
249	703	ELEMENT t = src[0];
250	703	src[0] = src[1];
251	703	src[1] = src[2];
252	703	src[2] = t;
253	703	}
254	202	else
255	202	{
256		/* src[2] < src[1] < src[0] */
257	202	ELEMENT t = src[0];
258	202	src[0] = src[2];
259	202	src[2] = t;
260	202	}
261	1.36k	}
262	4.94k	break;
263	7.01k	default:
264	7.01k	{
265	7.01k	size_t n1 = n / 2;
266	7.01k	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	7.01k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	7.01k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	7.01k	merge (tmp, n1, src + n1, n2, src);
274	7.01k	}
275	7.01k	break;
276	18.4k	}
277	18.4k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC