Luhn algorithm
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The Luhn algorithm created by Hans Peter Luhn. It is specified in ISO/IEC 7812-1. It is used by e.g. some credit card.
function LuhnAlgorithmCheckDigit(const s:string):integer;
var
l, i, sum, deo : integer;
begin
l := length( s );
sum := 0;
for i := l downto 1 do
begin
if odd(i) then sum := sum + StrToInt(s[i])
else
begin
deo := StrToInt(s[i]) * 2 ;
if deo > 9 then deo := deo mod 10 + 1;
sum := sum + deo;
end;
end;
result := 10 - sum mod 10;
end;
function isLuhnValid( const s : string ): boolean;
var
i, l :integer;
begin
l := length( s );
i := LuhnAlgorithmCheckDigit( copy( s, 1, l-1) );
result := i = StrToInt( s[ l ] );
end;