/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: 2026-02-09 06:47

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	16.8k	{
49	16.8k	for (;;) /* while (n1 > 0 && n2 > 0) */
50	88.2k	{
51	88.2k	if (COMPARE (src1, src2) <= 0)
52	76.8k	{
53	76.8k	dst++ = src1++;
54	76.8k	n1--;
55	76.8k	if (n1 == 0)
56	15.8k	break;
57	76.8k	}
58	11.4k	else
59	11.4k	{
60	11.4k	dst++ = src2++;
61	11.4k	n2--;
62	11.4k	if (n2 == 0)
63	947	break;
64	11.4k	}
65	88.2k	}
66		/* Here n1 == 0 \|\| n2 == 0 but also n1 > 0 \|\| n2 > 0. */
67	16.8k	if (n1 > 0)
68	947	{
69	947	if (dst != src1)
70	947	do
71	1.71k	{
72	1.71k	dst++ = src1++;
73	1.71k	n1--;
74	1.71k	}
75	1.71k	while (n1 > 0);
76	947	}
77	15.8k	else /* n2 > 0 */
78	15.8k	{
79	15.8k	if (dst != src2)
80	0	do
81	0	{
82	0	dst++ = src2++;
83	0	n2--;
84	0	}
85	0	while (n2 > 0);
86	15.8k	}
87	16.8k	}
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	26.6k	{
100	26.6k	switch (n)
101	26.6k	{
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	13.4k	case 2:
109		/* Trivial case. */
110	13.4k	if (COMPARE (&src[0], &src[1]) <= 0)
111	12.8k	{
112		/* src[0] <= src[1] */
113	12.8k	dst[0] = src[0];
114	12.8k	dst[1] = src[1];
115	12.8k	}
116	619	else
117	619	{
118	619	dst[0] = src[1];
119	619	dst[1] = src[0];
120	619	}
121	13.4k	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	477	else if (COMPARE (&src[0], &src[2]) <= 0)
134	256	{
135		/* src[0] <= src[2] < src[1] */
136	256	dst[0] = src[0];
137	256	dst[1] = src[2];
138	256	dst[2] = src[1];
139	256	}
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	520	else
149	520	{
150	520	if (COMPARE (&src[0], &src[2]) <= 0)
151	231	{
152		/* src[1] < src[0] <= src[2] */
153	231	dst[0] = src[1];
154	231	dst[1] = src[0];
155	231	dst[2] = src[2];
156	231	}
157	289	else if (COMPARE (&src[1], &src[2]) <= 0)
158	212	{
159		/* src[1] <= src[2] < src[0] */
160	212	dst[0] = src[1];
161	212	dst[1] = src[2];
162	212	dst[2] = src[0];
163	212	}
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	520	}
172	3.31k	break;
173	9.84k	default:
174	9.84k	{
175	9.84k	size_t n1 = n / 2;
176	9.84k	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	9.84k	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	9.84k	merge_sort_fromto (src, tmp, n1, dst);
182		/* Merge the two half results. */
183	9.84k	merge (tmp, n1, dst + n1, n2, dst);
184	9.84k	}
185	9.84k	break;
186	26.6k	}
187	26.6k	}
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	6.61k	case 2:
201		/* Trivial case. */
202	6.61k	if (COMPARE (&src[0], &src[1]) <= 0)
203	6.33k	{
204		/* src[0] <= src[1] */
205	6.33k	}
206	275	else
207	275	{
208	275	ELEMENT t = src[0];
209	275	src[0] = src[1];
210	275	src[1] = t;
211	275	}
212	6.61k	break;
213	4.88k	case 3:
214		/* Simple case. */
215	4.88k	if (COMPARE (&src[0], &src[1]) <= 0)
216	3.53k	{
217	3.53k	if (COMPARE (&src[1], &src[2]) <= 0)
218	3.00k	{
219		/* src[0] <= src[1] <= src[2] */
220	3.00k	}
221	534	else if (COMPARE (&src[0], &src[2]) <= 0)
222	241	{
223		/* src[0] <= src[2] < src[1] */
224	241	ELEMENT t = src[1];
225	241	src[1] = src[2];
226	241	src[2] = t;
227	241	}
228	293	else
229	293	{
230		/* src[2] < src[0] <= src[1] */
231	293	ELEMENT t = src[0];
232	293	src[0] = src[2];
233	293	src[2] = src[1];
234	293	src[1] = t;
235	293	}
236	3.53k	}
237	1.35k	else
238	1.35k	{
239	1.35k	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	893	else if (COMPARE (&src[1], &src[2]) <= 0)
247	690	{
248		/* src[1] <= src[2] < src[0] */
249	690	ELEMENT t = src[0];
250	690	src[0] = src[1];
251	690	src[1] = src[2];
252	690	src[2] = t;
253	690	}
254	203	else
255	203	{
256		/* src[2] < src[1] < src[0] */
257	203	ELEMENT t = src[0];
258	203	src[0] = src[2];
259	203	src[2] = t;
260	203	}
261	1.35k	}
262	4.88k	break;
263	6.96k	default:
264	6.96k	{
265	6.96k	size_t n1 = n / 2;
266	6.96k	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	6.96k	merge_sort_inplace (src + n1, n2, tmp);
270		/* Sort src[0..n1-1] into tmp[0..n1-1], scratching tmp[n1..2n1-1]. /
271	6.96k	merge_sort_fromto (src, tmp, n1, tmp + n1);
272		/* Merge the two half results. */
273	6.96k	merge (tmp, n1, src + n1, n2, src);
274	6.96k	}
275	6.96k	break;
276	18.4k	}
277	18.4k	}
278
279		#undef ELEMENT
280		#undef COMPARE
281		#undef STATIC