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var
a: array[0 .. 3] of shortint;
function largest_digit(n: smallint): shortint;
var
m: shortint;
begin
m := n div 1000;
if m < n mod 1000 div 100 then
m := n mod 1000 div 100;
if m < n mod 100 div 10 then
m := n mod 100 div 10;
if m < n mod 10 then
exit(n mod 10)
else
exit(m)
end;
procedure sort_digits(n: smallint);
var
i, j: shortint;
begin
a[0] := n div 1000;
a[1] := n mod 1000 div 100;
a[2] := n mod 100 div 10;
a[3] := n mod 10;
for i := 0 to 2 do
for j := i + 1 to 3 do
if a[i] < a[j] then
begin
inc(a[i], a[j]);
a[j] := a[i] - a[j];
dec(a[i], a[j])
end
end;
function desc_digits(n: smallint): smallint;
begin
sort_digits(n);
exit(a[0] * 1000 + a[1] * 100 + a[2] * 10 + a[3])
end;
function kaprekar(n: smallint): qword;
var
i: shortint = 0;
begin
while n <> 6174 do
begin
inc(i);
sort_digits(n);
n := (a[0] - a[3]) * 999 + (a[1] - a[2]) * 90
end;
exit(i)
end;
begin
writeln('largest_digit(1234) -> ', largest_digit(1234));
writeln('largest_digit(3253) -> ', largest_digit(3253));
writeln('largest_digit(9800) -> ', largest_digit(9800));
writeln('largest_digit(3333) -> ', largest_digit(3333));
writeln('largest_digit(120) -> ', largest_digit(120));
writeln;
writeln('desc_digits(1234) -> ', desc_digits(1234));
writeln('desc_digits(3253) -> ', desc_digits(3253));
writeln('desc_digits(9800) -> ', desc_digits(9800));
writeln('desc_digits(3333) -> ', desc_digits(3333));
writeln('desc_digits(120) -> ', desc_digits(120));
writeln;
writeln('kaprekar(6589) -> ', kaprekar(6589));
writeln('kaprekar(5455) -> ', kaprekar(5455));
writeln('kaprekar(6174) -> ', kaprekar(6174))
end.
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