/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: 2026-08-14 06:46

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	70.7k	{
49	70.7k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	411k	{
51	411k	if (COMPARE (src1, src2) <= 0)
52	352k	{
53	352k	dst++ = src1++;
54	352k	n1--;
55	352k	if (n1 == 0)
56	66.7k	break;
57	352k	}
58	59.8k	else
59	59.8k	{
60	59.8k	dst++ = src2++;
61	59.8k	n2--;
62	59.8k	if (n2 == 0)
63	4.05k	break;
64	59.8k	}
65	411k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	70.7k	if (n1 > 0)
68	4.05k	{
69	4.05k	if (dst != src1)
70	4.05k	do
71	7.13k	{
72	7.13k	dst++ = src1++;
73	7.13k	n1--;
74	7.13k	}
75	7.13k	while (n1 > 0);
76	4.05k	}
77	66.7k	else /* n2 > 0 */
78	66.7k	{
79	66.7k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	66.7k	}
87	70.7k	}
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	121k	{
100	121k	switch (n)
101	121k	{
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	60.1k	case 2:
109		/* Trivial case. */
110	60.1k	if (COMPARE (&src[0], &src[1]) <= 0)
111	56.8k	{
112		/* src[0] <= src[1] */
113	56.8k	dst[0] = src[0];
114	56.8k	dst[1] = src[1];
115	56.8k	}
116	3.25k	else
117	3.25k	{
118	3.25k	dst[0] = src[1];
119	3.25k	dst[1] = src[0];
120	3.25k	}
121	60.1k	break;
122	10.6k	case 3:
123		/* Simple case. */
124	10.6k	if (COMPARE (&src[0], &src[1]) <= 0)
125	7.85k	{
126	7.85k	if (COMPARE (&src[1], &src[2]) <= 0)
127	5.40k	{
128		/* src[0] <= src[1] <= src[2] */
129	5.40k	dst[0] = src[0];
130	5.40k	dst[1] = src[1];
131	5.40k	dst[2] = src[2];
132	5.40k	}
133	2.45k	else if (COMPARE (&src[0], &src[2]) <= 0)
134	1.44k	{
135		/* src[0] <= src[2] < src[1] */
136	1.44k	dst[0] = src[0];
137	1.44k	dst[1] = src[2];
138	1.44k	dst[2] = src[1];
139	1.44k	}
140	1.01k	else
141	1.01k	{
142		/* src[2] < src[0] <= src[1] */
143	1.01k	dst[0] = src[2];
144	1.01k	dst[1] = src[0];
145	1.01k	dst[2] = src[1];
146	1.01k	}
147	7.85k	}
148	2.82k	else
149	2.82k	{
150	2.82k	if (COMPARE (&src[0], &src[2]) <= 0)
151	1.22k	{
152		/* src[1] < src[0] <= src[2] */
153	1.22k	dst[0] = src[1];
154	1.22k	dst[1] = src[0];
155	1.22k	dst[2] = src[2];
156	1.22k	}
157	1.60k	else if (COMPARE (&src[1], &src[2]) <= 0)
158	846	{
159		/* src[1] <= src[2] < src[0] */
160	846	dst[0] = src[1];
161	846	dst[1] = src[2];
162	846	dst[2] = src[0];
163	846	}
164	761	else
165	761	{
166		/* src[2] < src[1] < src[0] */
167	761	dst[0] = src[2];
168	761	dst[1] = src[1];
169	761	dst[2] = src[0];
170	761	}
171	2.82k	}
172	10.6k	break;
173	50.5k	default:
174	50.5k	{
175	50.5k	size_t n1 = n / 2;
176	50.5k	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	50.5k	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	50.5k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	50.5k	merge (tmp, n1, dst + n1, n2, dst);
184	50.5k	}
185	50.5k	break;
186	121k	}
187	121k	}
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	102k	{
194	102k	switch (n)
195	102k	{
196	0	case 0:
197	0	case 1:
198		/* Nothing to do. */
199	0	return;
200	85.1k	case 2:
201		/* Trivial case. */
202	85.1k	if (COMPARE (&src[0], &src[1]) <= 0)
203	84.4k	{
204		/* src[0] <= src[1] */
205	84.4k	}
206	679	else
207	679	{
208	679	ELEMENT t = src[0];
209	679	src[0] = src[1];
210	679	src[1] = t;
211	679	}
212	85.1k	break;
213	7.01k	case 3:
214		/* Simple case. */
215	7.01k	if (COMPARE (&src[0], &src[1]) <= 0)
216	5.24k	{
217	5.24k	if (COMPARE (&src[1], &src[2]) <= 0)
218	4.08k	{
219		/* src[0] <= src[1] <= src[2] */
220	4.08k	}
221	1.16k	else if (COMPARE (&src[0], &src[2]) <= 0)
222	640	{
223		/* src[0] <= src[2] < src[1] */
224	640	ELEMENT t = src[1];
225	640	src[1] = src[2];
226	640	src[2] = t;
227	640	}
228	523	else
229	523	{
230		/* src[2] < src[0] <= src[1] */
231	523	ELEMENT t = src[0];
232	523	src[0] = src[2];
233	523	src[2] = src[1];
234	523	src[1] = t;
235	523	}
236	5.24k	}
237	1.76k	else
238	1.76k	{
239	1.76k	if (COMPARE (&src[0], &src[2]) <= 0)
240	695	{
241		/* src[1] < src[0] <= src[2] */
242	695	ELEMENT t = src[0];
243	695	src[0] = src[1];
244	695	src[1] = t;
245	695	}
246	1.07k	else if (COMPARE (&src[1], &src[2]) <= 0)
247	606	{
248		/* src[1] <= src[2] < src[0] */
249	606	ELEMENT t = src[0];
250	606	src[0] = src[1];
251	606	src[1] = src[2];
252	606	src[2] = t;
253	606	}
254	468	else
255	468	{
256		/* src[2] < src[1] < src[0] */
257	468	ELEMENT t = src[0];
258	468	src[0] = src[2];
259	468	src[2] = t;
260	468	}
261	1.76k	}
262	7.01k	break;
263	10.4k	default:
264	10.4k	{
265	10.4k	size_t n1 = n / 2;
266	10.4k	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	10.4k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	10.4k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	10.4k	merge (tmp, n1, src + n1, n2, src);
274	10.4k	}
275	10.4k	break;
276	102k	}
277	102k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC