Merge sort variant: using link array - c++

Question

I was looking for the variants of Merge Sort. So my textbook says,
A variant of function Merge in which No records need to be moved at all, can be implemented by use of an auxiliary array links.
Firstly, I would like to state the code.
Algo MergeSort(low,high)
{
// a is the array to be sorted using auxiliary array link.
if(high-low<15)
return InsertionSort1(a,link,low,high);
else
{
mid = (low+high)/2;
q=MergeSort(low,mid);
r=MergeSort(mid+1,high);
return Merge1(q,r);
}
}
// The function Merge1 is as defined:
function Merge1(q,r)
{
// q and r are pointers to list contained in the global array link[0:n], the lists pointed at by q //and r are merged and a pointer to the beginning of the merged list is returned.
i=q;
j=r;
k=0;
while(i!=0 && j!=0)
{
if(a[i]<=a[j])
{
link[k]=i;
k=i;
i=link[i];
}
else
{
link[k]=j;
k=j;
j=link[j];
}
} // end of while
if(i=0)
link[k]=j;
else
link[k]=i;
return link[0];
}
Okay so what I understood of the algorithm is:
If the number of elements are less than 15, apply insertion sort and sort those elements.
This way, we will get many lists that will be sorted by themselves but the entire array will not be sorted as such.
To sort the entire array, the function Merge is used.
My question is,
How is the function Merge combining the different sorted lists to one sorted list? I dont have any idea of the concept of the link array.
I am sorry but I tried very hard to understand but I dont get how the output array is "sorted" ?
Any kind of example will be of utmost help.
Thank You.

Answer 1

I cleaned up the code, and I also added a bottom up version that uses an array of starting indexes (see below). I changed high in MergeSort() to end, so the call is now MergeSort(0, SIZE). i = MergeSort() returns the index of the smallest value in a[], then i = link[i] is the 2nd element, i = link[i] is the 3rd element, until i = -1. Instead of using insertion sort, MergeSort() directly sorts groups of size==1 or size==2 and initializes link[].
MergeLists() uses head for the start of a list (the old code uses link[0]), and -1 for the end of a list (the old code uses 0). This allows sorting of a[0] to a[n-1] (the old code was sorting a[1] to a[n], with a[0] unused).
If a[] ={5,4,8,7}, then MergeSort() returns a 1, and link[] = {3,0,-1,2}, link[1] = 0, link[0] = 3, link[3] = 2, link[2] = -1, so the order is a[1], a[0], a[3], a[2].
#define SIZE 4
static unsigned int a[SIZE] = {5,8,4,7};
static size_t link[SIZE]; /* index to next element */
size_t MergeLists(size_t i, size_t j)
{
size_t head;
size_t *pprev = &head; /* ptr: head or link[] */
while((i != -1) && (j != -1)){ /* while not end lists */
if(a[i] <= a[j]){ /* if i < j */
*pprev = i; /* link to i */
pprev = &link[i]; /* advance pprev */
i=*pprev; /* advance i */
} else { /* else */
*pprev = j; /* link to j */
pprev = &link[j]; /* advance pprev */
j=*pprev; /* advance j */
}
}
if(i == -1) /* if end of i list */
*pprev=j; /* link to rest of j */
else /* else */
*pprev=i; /* link to rest of i */
return head;
}
size_t MergeSort(size_t low, size_t end)
{
size_t mid, i, j;
if((end - low) == 0){ /* if size == 0 */
return low; /* (only on first call) */
}
if((end - low) == 1){ /* if size == 1 */
link[low] = -1; /* initialize link[] */
return low; /* return index */
}
if((end - low) == 2){ /* if size == 2 */
if(a[low] <= a[end-1]){ /* if in order */
link[low] = end-1; /* initialize link[] */
link[end-1] = -1;
return low; /* return index */
} else { /* else */
link[end-1] = low; /* initialize link[] */
link[low] = -1;
return end-1; /* return index */
}
}
mid = (low+end)/2; /* size > 2, recursively */
i = MergeSort(low, mid); /* split lists until */
j = MergeSort(mid, end); /* size <= 2 */
return MergeLists(i, j); /* merge a pair of lists */
}
int main(void)
{
size_t i;
i = MergeSort(0, SIZE);
do{
printf("%3d", a[i]);
i = link[i];
}while(i != -1);
return 0;
}
This is an example that is non-recursive. It uses an array of starting indexes S[]. N[] is the same a link[] above, and MergeLists() is the same as before. S[0] points to lists of size 1, S[1] points to lists of size 2, S[2] points to lists of size 4, ... S[i] points to lists of size 2^i (2 to the power i). S[31] points to a list of unlimited size. Elements are merged into the array one at a time, then the array lists are merged to form a single list.
#define NUMIDX (32) // number of indexes in array
// A[] is array to be sorted
// N[] is array of indexes to next index
// l is index of N[] to left list
// r is index of N[] to right list
// returns starting index (l or r) for merged list
size_t MergeLists(int A[], size_t N[], size_t l, size_t r)
{
size_t head;
size_t *pprev = &head; // ptr: head or N[]
while((l != -1) && (r != -1)){ // while not end lists
if(A[l] <= A[r]){ // if l <= r
*pprev = l; // link to l
pprev = &N[l]; // advance pprev
l=*pprev; // advance l
} else { // else
*pprev = r; // link to r
pprev = &N[r]; // advance pprev
r=*pprev; // advance r
}
}
if(l == -1) // if end of l list
*pprev=r; // link to rest of r
else // else
*pprev=l; // link to rest of l
return head;
}
// A[] is array to be sorted
// N[] is set to array of indexes to next index (-1 = end list)
// low is starting index of A[]
// end is ending index of A[] (1 past last)
// returns starting index of N[] for merged list
// S[] is array of starting indexes in N[]
// S[i] is starting index of list of size pow(2,i)
size_t MergeSort(int A[], size_t N[], size_t low, size_t end)
{
size_t S[NUMIDX]; // array of starting indexes
size_t i,j;
if((end - low) == 0){ // if size == 0
return low; // (only on first call)
}
for(i = 0; i < (end-low); i++) // init N[]
N[i] = -1;
for(i = 0; i < NUMIDX; i++) // init S[]
S[i] = -1;
for(j = low; j < end; j++){ // merge index lists into S[], N[]
low = j;
for(i = 0; (i < NUMIDX) && (S[i] != -1); i++){
low = MergeLists(A, N, S[i], low);
S[i] = -1;
}
if(i == NUMIDX)
i--;
S[i] = low;
}
low = -1; // merge S[] lists to one list in N[]
for(i = 0; i < NUMIDX; i++)
low = MergeLists(A, N, S[i], low);
return low;
}