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[Vincent Diepeveen]

A first simple attempt would be :

for given 'sq' and 'piece' :

x1 = lookupSQ[sq]; // random number
x2 = lookupPIECE[piece]; // random number

r1 = lookupR1[piece]; // list of small prime numbers
r2 = lookupR2[sq]; // list of small prime numbers

Now we want to combine this into unique number

randomresult = rotateleft(x1,r1) + rotateleft(x2,r2);

You can try get it cheaper by replacing lookupSQ or lookupPIECE to constants and just add in the 'sq' there.

I have no time to verify the result of the above. Might work.