What is the Invariant for iterative mergesort?

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

Code:

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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;               
  }
}

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