/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: 2026-11-12 06:28

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	87.2k	{
49	87.2k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	506k	{
51	506k	if (COMPARE (src1, src2) <= 0)
52	439k	{
53	439k	dst++ = src1++;
54	439k	n1--;
55	439k	if (n1 == 0)
56	82.6k	break;
57	439k	}
58	66.5k	else
59	66.5k	{
60	66.5k	dst++ = src2++;
61	66.5k	n2--;
62	66.5k	if (n2 == 0)
63	4.61k	break;
64	66.5k	}
65	506k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	87.2k	if (n1 > 0)
68	4.61k	{
69	4.61k	if (dst != src1)
70	4.61k	do
71	8.66k	{
72	8.66k	dst++ = src1++;
73	8.66k	n1--;
74	8.66k	}
75	8.66k	while (n1 > 0);
76	4.61k	}
77	82.6k	else /* n2 > 0 */
78	82.6k	{
79	82.6k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	82.6k	}
87	87.2k	}
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	149k	{
100	149k	switch (n)
101	149k	{
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	74.5k	case 2:
109		/* Trivial case. */
110	74.5k	if (COMPARE (&src[0], &src[1]) <= 0)
111	70.4k	{
112		/* src[0] <= src[1] */
113	70.4k	dst[0] = src[0];
114	70.4k	dst[1] = src[1];
115	70.4k	}
116	4.04k	else
117	4.04k	{
118	4.04k	dst[0] = src[1];
119	4.04k	dst[1] = src[0];
120	4.04k	}
121	74.5k	break;
122	12.7k	case 3:
123		/* Simple case. */
124	12.7k	if (COMPARE (&src[0], &src[1]) <= 0)
125	9.91k	{
126	9.91k	if (COMPARE (&src[1], &src[2]) <= 0)
127	7.31k	{
128		/* src[0] <= src[1] <= src[2] */
129	7.31k	dst[0] = src[0];
130	7.31k	dst[1] = src[1];
131	7.31k	dst[2] = src[2];
132	7.31k	}
133	2.59k	else if (COMPARE (&src[0], &src[2]) <= 0)
134	1.72k	{
135		/* src[0] <= src[2] < src[1] */
136	1.72k	dst[0] = src[0];
137	1.72k	dst[1] = src[2];
138	1.72k	dst[2] = src[1];
139	1.72k	}
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	9.91k	}
148	2.82k	else
149	2.82k	{
150	2.82k	if (COMPARE (&src[0], &src[2]) <= 0)
151	1.20k	{
152		/* src[1] < src[0] <= src[2] */
153	1.20k	dst[0] = src[1];
154	1.20k	dst[1] = src[0];
155	1.20k	dst[2] = src[2];
156	1.20k	}
157	1.62k	else if (COMPARE (&src[1], &src[2]) <= 0)
158	850	{
159		/* src[1] <= src[2] < src[0] */
160	850	dst[0] = src[1];
161	850	dst[1] = src[2];
162	850	dst[2] = src[0];
163	850	}
164	776	else
165	776	{
166		/* src[2] < src[1] < src[0] */
167	776	dst[0] = src[2];
168	776	dst[1] = src[1];
169	776	dst[2] = src[0];
170	776	}
171	2.82k	}
172	12.7k	break;
173	62.2k	default:
174	62.2k	{
175	62.2k	size_t n1 = n / 2;
176	62.2k	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	62.2k	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	62.2k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	62.2k	merge (tmp, n1, dst + n1, n2, dst);
184	62.2k	}
185	62.2k	break;
186	149k	}
187	149k	}
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	128k	{
194	128k	switch (n)
195	128k	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	102k	case 2:
201		/* Trivial case. */
202	102k	if (COMPARE (&src[0], &src[1]) <= 0)
203	101k	{
204		/* src[0] <= src[1] */
205	101k	}
206	953	else
207	953	{
208	953	ELEMENT t = src[0];
209	953	src[0] = src[1];
210	953	src[1] = t;
211	953	}
212	102k	break;
213	11.0k	case 3:
214		/* Simple case. */
215	11.0k	if (COMPARE (&src[0], &src[1]) <= 0)
216	8.44k	{
217	8.44k	if (COMPARE (&src[1], &src[2]) <= 0)
218	6.97k	{
219		/* src[0] <= src[1] <= src[2] */
220	6.97k	}
221	1.46k	else if (COMPARE (&src[0], &src[2]) <= 0)
222	823	{
223		/* src[0] <= src[2] < src[1] */
224	823	ELEMENT t = src[1];
225	823	src[1] = src[2];
226	823	src[2] = t;
227	823	}
228	640	else
229	640	{
230		/* src[2] < src[0] <= src[1] */
231	640	ELEMENT t = src[0];
232	640	src[0] = src[2];
233	640	src[2] = src[1];
234	640	src[1] = t;
235	640	}
236	8.44k	}
237	2.65k	else
238	2.65k	{
239	2.65k	if (COMPARE (&src[0], &src[2]) <= 0)
240	1.10k	{
241		/* src[1] < src[0] <= src[2] */
242	1.10k	ELEMENT t = src[0];
243	1.10k	src[0] = src[1];
244	1.10k	src[1] = t;
245	1.10k	}
246	1.54k	else if (COMPARE (&src[1], &src[2]) <= 0)
247	1.00k	{
248		/* src[1] <= src[2] < src[0] */
249	1.00k	ELEMENT t = src[0];
250	1.00k	src[0] = src[1];
251	1.00k	src[1] = src[2];
252	1.00k	src[2] = t;
253	1.00k	}
254	541	else
255	541	{
256		/* src[2] < src[1] < src[0] */
257	541	ELEMENT t = src[0];
258	541	src[0] = src[2];
259	541	src[2] = t;
260	541	}
261	2.65k	}
262	11.0k	break;
263	15.3k	default:
264	15.3k	{
265	15.3k	size_t n1 = n / 2;
266	15.3k	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	15.3k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	15.3k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	15.3k	merge (tmp, n1, src + n1, n2, src);
274	15.3k	}
275	15.3k	break;
276	128k	}
277	128k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC