/* 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