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

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

View File

@@ -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;
}
/**