Tower of Hanoi: Recursive Algorithm Uses Mysterious Hardcoded Constant 6

Question

This is a follow up question from Tower of Hanoi: Recursive Algorithm

where the basic principle of this algorithm is well explained.

However I have a slightly different implementation of this algorithm here which I don't fully understand (code is from a university lecture, so I can't link to any source):

// k = nr. of discs, a = start peg, b = destination peg 
public static void hanoi( int k, int a, int b) 
{
    if( k > 0)
    {
       hanoi( k - 1, a, 6 - a - b);  // 1. move (k-1) discs to temporary peg
       System.out.println("" + k + ": " + a + " => " + b); // 2. move k. disc from a to b
       hanoi( k - 1, 6 - a - b, b);  // 3. move (k-1) discs from temporary peg to peg b
    }
}

The principle is the same as in the other topic (as far as I understand), but I don't understand where the '6' in '6 - a -b" is coming from, could someone explain ?

Answer 1

6 - a - b comes from (1 + 2 + 3) = 6 and gives you index of "the other" peg:

a = 1, b = 3 

than

6 - a - b = 2