VampireNumber Java task, code explanation with the number 1827:

1827 (Number that we are checking)
0123 (Indexes of digits, in code they are named: first, second, third & fourth)

The first and second number that are multiplied are illustrated combined as one (the first half of the calculated numbers is the first multiple, and the second half - the second)
For example, the number 1278 is actually 12 and 78 and they are multiplied to check for the result
This is done like that to better illustrate how the numbers change

Cycle:  Swap 1 with 3 (indexes):
1827    1728
1782    1287
1278    1872

2178	2871
2817	2718
2781	2187 NOTE: 2187 is the answer, so the code would stop here, but this example is continued to the end

7281	7182
7128	7821
7812	7218

8712	8217
8271	8172
8127	8721

1827 (Here the cycles end)
								
Cycle: it rotates the last 3 digits, for example if we have 123,the 3 gets in the front and the others move back, i.e. 312

       1 2 3 => 3 1 2
     ^-----'     

Swap 1 with 3: as the name suggests, it makes a second version of the number by swapping the 1st (index) and 2nd (index) digit 

     0 1 2 3 => 0 3 2 1
       ^---^
 
	       -the swap is there because with the cycle we miss out on half of the possible numbers