56 lines
1.1 KiB
ObjectPascal
56 lines
1.1 KiB
ObjectPascal
function IsPrime(N: int64): boolean;
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{Optimised prime test - about 40% faster than the naive approach}
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var I,Stop: integer;
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begin
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if (N = 2) or (N=3) then Result:=true
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else if (n <= 1) or ((n mod 2) = 0) or ((n mod 3) = 0) then Result:= false
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else
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begin
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I:=5;
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Stop:=Trunc(sqrt(N*1.0));
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Result:=False;
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while I<=Stop do
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begin
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if ((N mod I) = 0) or ((N mod (i + 2)) = 0) then exit;
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Inc(I,6);
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end;
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Result:=True;
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end;
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end;
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function Factorial(N: Word): int64;
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var I: integer;
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begin
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Result:= 1;
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for I := 2 to N do Result:=Result * I;
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end;
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procedure ShowFactorialPrimes(Memo: TMemo);
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{Show factorials where F+1 or F-1 are prime}
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var I,Cnt: integer;
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var F: int64;
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procedure DisplayItem(Minus: boolean);
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var S: string;
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var Sign: char;
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var F1: int64;
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begin
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Inc(Cnt);
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if Minus then F1:=F-1 else F1:=F+1;
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if Minus then Sign:='-' else Sign:='+';
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S:=Format('%2d: %3d! %s 1 = %d',[Cnt,I,Sign,F1]);
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Memo.Lines.Add(S);
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end;
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begin
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Cnt:=0;
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for I:=1 to High(Integer) do
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begin
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F:=Factorial(I);
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if IsPrime(F+1) then DisplayItem(False);
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if IsPrime(F-1) then DisplayItem(True);
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if Cnt>=10 then break;
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end;
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end;
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