link_t mergesort_list(link_t head)
{
link_t heada,headb;
link_t x,t;
if(head==NULL || head->next==NULL){
return(head);
}
heada=head;
headb=head;
x=head->next;
while(x!=NULL && x->next!=NULL){
headb=headb->next;
x=x->next->next;
}
t=headb;
headb=t->next;
t->next=NULL;
heada=mergesort_list(heada);
headb=mergesort_list(headb);
head=merge_list(heada,headb);
return(head);
}
/* Sorts a linear list, given a comparison function.
Note: This uses a variant of mergesort that is *not* a stable sorting
algorithm. */
Keyword_List *
mergesort_list (Keyword_List *list, Keyword_Comparison less)
{
if (list == NULL || list->rest() == NULL)
/* List of length 0 or 1. Nothing to do. */
return list;
else
{
/* Determine a list node in the middle. */
Keyword_List *middle = list;
for (Keyword_List *temp = list->rest();;)
{
temp = temp->rest();
if (temp == NULL)
break;
temp = temp->rest();
middle = middle->rest();
if (temp == NULL)
break;
}
/* Cut the list into two halves.
If the list has n elements, the left half has ceiling(n/2) elements
and the right half has floor(n/2) elements. */
Keyword_List *right_half = middle->rest();
middle->rest() = NULL;
/* Sort the two halves, then merge them. */
return merge (mergesort_list (list, less),
mergesort_list (right_half, less),
less);
}
}
/*
** The implementation of top-down list mergesort that carries the
** list length as a parameter to the recursive procedure and uses
** it to split the lists.
*/
link_t mergesort_list(link_t head,int length)
{
link_t heada,headb;
link_t t;
int middle=(length+1)/2;
int i;
if(head==NULL || head->next==NULL){
return(head);
}
heada=head;
headb=head;
for(i=1;i<middle;i++){
headb=headb->next;
}
t=headb;
headb=t->next;
t->next=NULL;
heada=mergesort_list(heada,middle);
headb=mergesort_list(headb,length-middle);
head=merge_list(heada,headb);
return(head);
}
KeywordExt_List *
mergesort_list (KeywordExt_List *list,
bool (*less) (KeywordExt *keyword1, KeywordExt *keyword2))
{
return
static_cast<KeywordExt_List *>
(mergesort_list (static_cast<Keyword_List *> (list),
reinterpret_cast<Keyword_Comparison> (less)));
}
int main(int argc,char *argv[])
{
link_t head,t;
int N=atoi(argv[1]);
int i;
srand((unsigned)time(NULL));
head=new_node(rand()%N);
for(i=1;i<=N-1;i++){
t=new_node(rand()%N);
insert_next(head,t);
}
print_list(head);
head=mergesort_list(head,N);
print_list(head);
return(0);
}
