What is the Invariant for iterative mergesort?

[MelaOS]

Uhm sorry, newbie question here, but i'm supposed to write an invariant for the below code:

void Mergesort::iterMerge(int array[],int temp[],int lt,int md,int rt)
{
    int i = lt;
    int j = md+1;
    int k = lt;       
   
   resetResult();
   while(( i <= md )&&( j <= rt ))
	{
		comparison++;
		if( array[ i ] <= array[ j ] )  
      {
	      temp[ k++ ]= array[ i++ ];
	      assignment++;
	   }
      else
      {
	      temp[ k++ ]= array[ j++ ];
	      assignment++;
	   }
	}
   while( i <= md )
   {
    	temp[ k++ ]= array[ i++ ];
    	assignment++;
   }

   while( j <= rt )
   {
     	temp[ k++ ]= array[ j++ ];
     	assignment++;
   }
}

void Mergesort::mergePass( int array[], int temp[], int current, 
									int arraySize )
{
   int count = 0;

   while ( count <= arraySize - 2 * current ) 
   {
   		iterMerge( array, temp, count, count+current-1, count+2*current-1 );
      	count = count + 2*current;
   }
   
   if ( count + current < arraySize )
      	iterMerge( array, temp, count, count+current-1, arraySize-1);
   else
   		for ( int i = count; i <= arraySize-1; i++ )
      	{
      		temp[ i ] = array[ i ];   
      		assignment++;
   		}
}

void Mergesort::iterativeMergesort( DataType* array, int arraySize )
{
   int current = 1;
   int *temp;
   temp = new int [ arraySize ];

   while( current < arraySize )
   {  
   	  mergePass ( array, temp, current, arraySize );
      current += current;                
      mergePass ( temp, array, current, arraySize );
      current += current;                
   } 
}

Can anyone please lend me a hint? I'm not quite sure where to start... Thanks in advance