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#### 6.27 Sorting utilities —

psb_msort — Sorting by the Merge-sort algorithm

psb_qsort — Sorting by the Quicksort algorithm

psb_hsort — Sorting by the Heapsort algorithm

call psb_msort(x,ix,dir,flag)
call psb_qsort(x,ix,dir,flag)
call psb_hsort(x,ix,dir,flag)

These serial routines sort a sequence X into ascending or descending order. The argument meaning is identical for the three calls; the only difference is the algorithm used to accomplish the task (see Usage Notes below).

Type:
  Asynchronous.
On Entry
x
  The sequence to be sorted.
  Type:required.
  Specified as: an integer, real or complex array of rank 1.
ix
  A vector of indices.
  Type:optional.
  Specified as: an integer array of (at least) the same size as X.
dir
  The desired ordering.
  Type:optional.
  Specified as: an integer value:
  Integer and real data:
    psb_sort_up_, psb_sort_down_, psb_asort_up_, psb_asort_down_; default psb_sort_up_.
  Complex data:
    psb_lsort_up_, psb_lsort_down_, psb_asort_up_, psb_asort_down_; default psb_lsort_up_.
flag
  Whether to keep the original values in IX.
  Type:optional.
  Specified as: an integer value psb_sort_ovw_idx_ or psb_sort_keep_idx_; default psb_sort_ovw_idx_.

On Return
x
  The sequence of values, in the chosen ordering.
  Type:required.
  Specified as: an integer, real or complex array of rank 1.
ix
  A vector of indices.
  Type: Optional
  An integer array of rank 1, whose entries are moved to the same position as the corresponding entries in x.

Notes

1. For integer or real data the sorting can be performed in the up/down direction, on the natural or absolute values;
2. For complex data the sorting can be done in a lexicographic order (i.e.: sort on the real part with ties broken according to the imaginary part) or on the absolute values;
3. The routines return the items in the chosen ordering; the output difference is the handling of ties (i.e. items with an equal value) in the original input. With the merge-sort algorithm ties are preserved in the same relative order as they had in the original sequence, while this is not guaranteed for quicksort or heapsort;
4. If flag = psb_sort_ovw_idx_ then the entries in ix(1 : n) where n is the size of x are initialized to ix(i) ← i; thus, upon return from the subroutine, for each index i we have in ix(i) the position that the item x(i) occupied in the original data sequence;
5. If flag = psb_sort_keep_idx_ the routine will assume that the entries in ix(:) have already been initialized by the user;
6. The three sorting algorithms have a similar O(nlog n) expected running time; in the average case quicksort will be the fastest and merge-sort the slowest. However note that:
   1. The worst case running time for quicksort is O(n2); the algorithm implemented here follows the well-known median-of-three heuristics, but the worst case may still apply;
   2. The worst case running time for merge-sort and heap-sort is O(nlog n) as the average case;
   3. The merge-sort algorithm is implemented to take advantage of subsequences that may be already in the desired ordering prior to the subroutine call; this situation is relatively common when dealing with groups of indices of sparse matrix entries, thus merge-sort is the preferred choice when a sorting is needed by other routines in the library.

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