program BinaryDivisibleBy7;
{$mode objfpc} { enables modern Pascal features }
{ Check if a string contains only '0' and '1' }
function IsBinary(const s: string): Boolean;
var
i: Integer;
begin
if s = '' then
Exit(False);
for i := 1 to Length(s) do
if not (s[i] in ['0', '1']) then
Exit(False);
Result := True;
end;
{
In decimal: reading "123" means
remainder = remainder * 10 + digit
In binary: reading "1011" means
remainder = remainder * 2 + bit
}
{
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
1 (0*2 + 1) % 7 1
1 (1*2 + 1) % 7 3
1 (3*2 + 1) % 7 0
0 (0*2 + 0) % 7 0
0 (0*2 + 0) % 7 0
}
{ Compute binary string modulo 7 without overflow }
function DivisibleBy7(const s: string): Boolean;
var
i: Integer;
remainder: Integer;
begin
remainder := 0;
for i := 1 to Length(s) do
begin
remainder := (remainder * 2 + (Ord(s[i]) - Ord('0'))) mod 7;
end;
Result := (remainder = 0);
end;
var
s: string;
begin
s := '00011100'; { 28 }
if not IsBinary(s) then
begin
Writeln('Not a binary string');
Halt;
end;
if DivisibleBy7(s) then
Writeln('Divisible by 7')
else
Writeln('Not divisible by 7');
end.
(*
run:
Divisible by 7
*)
