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shared: implement c_list_sort() as non-recursive merge-sort

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This is still the very same approach (in the way the array is split
and how elements are compared). The only difference is that the
recursive implementation is replaced by a non-recursive one.

It's (still) stable, top-down merge-sort.

The non-recursive implementation better, because it avoids the overhead
of the function call to recurse.
This commit is contained in:
Thomas Haller 2017-12-28 19:53:13 +01:00
parent feeb70ef89
commit 916f53ac24
1 changed files with 36 additions and 10 deletions
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46
shared/nm-utils/c-list-util.c
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@ -111,26 +111,52 @@ _c_list_srt_merge (CList *ls1,
return head.next;
}
typedef struct {
CList *ls1;
CList *ls2;
char ls1_sorted;
} SortStack;
static CList *
_c_list_sort (CList *ls,
CListSortCmp cmp,
const void *user_data)
{
CList *ls1, *ls2;
/* reserve a huge stack-size. We need roughly log2(n) entries, hence this
* is much more we will ever need. We don't guard for stack-overflow either. */
SortStack stack_arr[70];
SortStack *stack_head = stack_arr;
if (!ls->next)
return ls;
ls1 = ls;
stack_arr[0].ls1 = ls;
ls2 = _c_list_srt_split (ls1);
/* A simple top-down, non-recursive, stable merge-sort.
*
* Maybe natural merge-sort would be better, to do better for
* partially sorted lists. */
_split:
stack_head[0].ls2 = _c_list_srt_split (stack_head[0].ls1);
if (stack_head[0].ls2) {
stack_head[0].ls1_sorted = 0;
stack_head[1].ls1 = stack_head[0].ls1;
stack_head++;
goto _split;
}
ls1 = _c_list_sort (ls1, cmp, user_data);
if (!ls2)
return ls1;
_backtrack:
if (stack_head == stack_arr)
return stack_arr[0].ls1;
ls2 = _c_list_sort (ls2, cmp, user_data);
stack_head--;
if (!stack_head[0].ls1_sorted) {
stack_head[0].ls1 = stack_head[1].ls1;
stack_head[0].ls1_sorted = 1;
stack_head[1].ls1 = stack_head[0].ls2;
stack_head++;
goto _split;
}
return _c_list_srt_merge (ls1, ls2, cmp, user_data);
stack_head[0].ls1 = _c_list_srt_merge (stack_head[0].ls1, stack_head[1].ls1, cmp, user_data);
goto _backtrack;
}
/**