/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: 2026-03-03 07:12

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	712k	{
49	712k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	10.5M	{
51	10.5M	if (COMPARE (src1, src2) <= 0)
52	9.43M	{
53	9.43M	dst++ = src1++;
54	9.43M	n1--;
55	9.43M	if (n1 == 0)
56	706k	break;
57	9.43M	}
58	1.10M	else
59	1.10M	{
60	1.10M	dst++ = src2++;
61	1.10M	n2--;
62	1.10M	if (n2 == 0)
63	5.12k	break;
64	1.10M	}
65	10.5M	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	712k	if (n1 > 0)
68	5.12k	{
69	5.12k	if (dst != src1)
70	5.12k	do
71	16.4k	{
72	16.4k	dst++ = src1++;
73	16.4k	n1--;
74	16.4k	}
75	16.4k	while (n1 > 0);
76	5.12k	}
77	706k	else /* n2 > 0 */
78	706k	{
79	706k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	706k	}
87	712k	}
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	1.40M	{
100	1.40M	switch (n)
101	1.40M	{
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	656k	case 2:
109		/* Trivial case. */
110	656k	if (COMPARE (&src[0], &src[1]) <= 0)
111	653k	{
112		/* src[0] <= src[1] */
113	653k	dst[0] = src[0];
114	653k	dst[1] = src[1];
115	653k	}
116	2.74k	else
117	2.74k	{
118	2.74k	dst[0] = src[1];
119	2.74k	dst[1] = src[0];
120	2.74k	}
121	656k	break;
122	55.5k	case 3:
123		/* Simple case. */
124	55.5k	if (COMPARE (&src[0], &src[1]) <= 0)
125	52.2k	{
126	52.2k	if (COMPARE (&src[1], &src[2]) <= 0)
127	49.5k	{
128		/* src[0] <= src[1] <= src[2] */
129	49.5k	dst[0] = src[0];
130	49.5k	dst[1] = src[1];
131	49.5k	dst[2] = src[2];
132	49.5k	}
133	2.73k	else if (COMPARE (&src[0], &src[2]) <= 0)
134	1.91k	{
135		/* src[0] <= src[2] < src[1] */
136	1.91k	dst[0] = src[0];
137	1.91k	dst[1] = src[2];
138	1.91k	dst[2] = src[1];
139	1.91k	}
140	821	else
141	821	{
142		/* src[2] < src[0] <= src[1] */
143	821	dst[0] = src[2];
144	821	dst[1] = src[0];
145	821	dst[2] = src[1];
146	821	}
147	52.2k	}
148	3.29k	else
149	3.29k	{
150	3.29k	if (COMPARE (&src[0], &src[2]) <= 0)
151	1.49k	{
152		/* src[1] < src[0] <= src[2] */
153	1.49k	dst[0] = src[1];
154	1.49k	dst[1] = src[0];
155	1.49k	dst[2] = src[2];
156	1.49k	}
157	1.80k	else if (COMPARE (&src[1], &src[2]) <= 0)
158	1.04k	{
159		/* src[1] <= src[2] < src[0] */
160	1.04k	dst[0] = src[1];
161	1.04k	dst[1] = src[2];
162	1.04k	dst[2] = src[0];
163	1.04k	}
164	756	else
165	756	{
166		/* src[2] < src[1] < src[0] */
167	756	dst[0] = src[2];
168	756	dst[1] = src[1];
169	756	dst[2] = src[0];
170	756	}
171	3.29k	}
172	55.5k	break;
173	689k	default:
174	689k	{
175	689k	size_t n1 = n / 2;
176	689k	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	689k	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	689k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	689k	merge (tmp, n1, dst + n1, n2, dst);
184	689k	}
185	689k	break;
186	1.40M	}
187	1.40M	}
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	1.42M	{
194	1.42M	switch (n)
195	1.42M	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	1.38M	case 2:
201		/* Trivial case. */
202	1.38M	if (COMPARE (&src[0], &src[1]) <= 0)
203	1.38M	{
204		/* src[0] <= src[1] */
205	1.38M	}
206	1.00k	else
207	1.00k	{
208	1.00k	ELEMENT t = src[0];
209	1.00k	src[0] = src[1];
210	1.00k	src[1] = t;
211	1.00k	}
212	1.38M	break;
213	16.7k	case 3:
214		/* Simple case. */
215	16.7k	if (COMPARE (&src[0], &src[1]) <= 0)
216	13.2k	{
217	13.2k	if (COMPARE (&src[1], &src[2]) <= 0)
218	10.9k	{
219		/* src[0] <= src[1] <= src[2] */
220	10.9k	}
221	2.22k	else if (COMPARE (&src[0], &src[2]) <= 0)
222	1.20k	{
223		/* src[0] <= src[2] < src[1] */
224	1.20k	ELEMENT t = src[1];
225	1.20k	src[1] = src[2];
226	1.20k	src[2] = t;
227	1.20k	}
228	1.02k	else
229	1.02k	{
230		/* src[2] < src[0] <= src[1] */
231	1.02k	ELEMENT t = src[0];
232	1.02k	src[0] = src[2];
233	1.02k	src[2] = src[1];
234	1.02k	src[1] = t;
235	1.02k	}
236	13.2k	}
237	3.56k	else
238	3.56k	{
239	3.56k	if (COMPARE (&src[0], &src[2]) <= 0)
240	1.15k	{
241		/* src[1] < src[0] <= src[2] */
242	1.15k	ELEMENT t = src[0];
243	1.15k	src[0] = src[1];
244	1.15k	src[1] = t;
245	1.15k	}
246	2.40k	else if (COMPARE (&src[1], &src[2]) <= 0)
247	1.35k	{
248		/* src[1] <= src[2] < src[0] */
249	1.35k	ELEMENT t = src[0];
250	1.35k	src[0] = src[1];
251	1.35k	src[1] = src[2];
252	1.35k	src[2] = t;
253	1.35k	}
254	1.05k	else
255	1.05k	{
256		/* src[2] < src[1] < src[0] */
257	1.05k	ELEMENT t = src[0];
258	1.05k	src[0] = src[2];
259	1.05k	src[2] = t;
260	1.05k	}
261	3.56k	}
262	16.7k	break;
263	22.1k	default:
264	22.1k	{
265	22.1k	size_t n1 = n / 2;
266	22.1k	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	22.1k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	22.1k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	22.1k	merge (tmp, n1, src + n1, n2, src);
274	22.1k	}
275	22.1k	break;
276	1.42M	}
277	1.42M	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC