I'm wanting to work out the algorithm what gives these results
It's always a $6$ digit code that is turned into a $4$ digit code
$000000$ is $1558$
$000001$ is $9617$
$000002$ is $8633$
$000010$ is $2882$
I have a program that can give me the $4$ digit code for any $6$ digit code but I would like to work out the maths behind it if possible.
Where should I start trying to figure it out? Are there any 'common' things I could do/read up on to help me figure this out?