// A standard function to heapify at given idx
// This function also updates position of nodes when they are swapped.
// Position is needed for decreaseKey()
void minHeapify(struct MinHeap* minHeap, int idx)
{
int smallest, left, right;
smallest = idx;
left = 2 * idx + 1;
right = 2 * idx + 2;
if (left < minHeap->size &&
minHeap->array[left]->dist < minHeap->array[smallest]->dist )
smallest = left;
if (right < minHeap->size &&
minHeap->array[right]->dist < minHeap->array[smallest]->dist )
smallest = right;
if (smallest != idx)
{
// The nodes to be swapped in min heap
MinHeapNode *smallestNode = minHeap->array[smallest];
MinHeapNode *idxNode = minHeap->array[idx];
// Swap positions
minHeap->pos[smallestNode->v] = idx;
minHeap->pos[idxNode->v] = smallest;
// Swap nodes
swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
minHeapify(minHeap, smallest);
}
}
void minHeapify (MinHeap* minHeap, int idx) //for the first call, idx is always 0
{
int smallest, left, right;
smallest = idx; //reset
left = 2 * idx + 1; //left node index has this relation
right = 2 * idx + 2; //right node index has this relation
if (left < minHeap->size && minHeap->array[left]->freq < minHeap->array[smallest]->freq)
smallest=left; //this is done, as it was not following the rule otherwise
if (right < minHeap->size && minHeap->array[right]->freq < minHeap->array[smallest]->freq)
smallest=right; //this is done, as it was not following the rule otherwise
//the rule: children nodes have values larger than parent node
if (smallest != idx) //meaning nothing needs to be changed!
{
swapMinHeapNode ( &minHeap->array[smallest], &minHeap->array[idx] ); //sending addresses of nodes
minHeapify (minHeap, smallest); //recursive call of the function, till smallest = idx
}
}
// Function to decreasy dist value of a given vertex v. This function
// uses pos[] of min heap to get the current index of node in min heap
void decreaseKey(struct MinHeap* minHeap, int v, int dist)
{
// Get the index of v in heap array
int i = minHeap->pos[v];
// Get the node and update its dist value
minHeap->array[i]->dist = dist;
// Travel up while the complete tree is not hepified.
// This is a O(Logn) loop
while (i && minHeap->array[i]->dist < minHeap->array[(i - 1) / 2]->dist)
{
// Swap this node with its parent
minHeap->pos[minHeap->array[i]->v] = (i-1)/2;
minHeap->pos[minHeap->array[(i-1)/2]->v] = i;
swapMinHeapNode(&minHeap->array[i], &minHeap->array[(i - 1) / 2]);
// move to parent index
i = (i - 1) / 2;
}
}
void minHeapify(struct MinHeap* minHeap, int idx)
{
int smallest = idx;
int left = 2 * idx + 1;
int right = 2 * idx + 2;
if (left < minHeap->size && minHeap->array[left]->freq < minHeap->array[smallest]->freq)
smallest = left;
if (right < minHeap->size && minHeap->array[right]->freq < minHeap->array[smallest]->freq)
smallest = right;
if (smallest != idx)
{
swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
minHeapify(minHeap, smallest);
}
}