26 lines
1.7 KiB
Forth
26 lines
1.7 KiB
Forth
( comments refer to the Pascal implementation )
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8190 CONSTANT SIZE ( const size = 8190; )
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VARIABLE FLAGS SIZE ALLOT ( flagType = array[ 0..size ] of boolean; var flags : flagType )
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: SIEVE ( program sieve{ OUTPUT }; )
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10 0 DO ( for iter := 1 to 10 do begin )
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FLAGS SIZE -1 FILL ( for i := 0 to size do flags[ i ] := true; )
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0 ( count := 0; -- push it on the stack )
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SIZE 0 DO ( for i := 0 to size do begin )
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I FLAGS + C@ IF ( if flags[ i ] then begin -- do loop index is i, c@ reads byte from flags element i )
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I I + 3 + ( prime := i + i + 3; )
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DUP I + ( k := i + prime; -- DUP duplicates the top of stack )
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BEGIN DUP SIZE < WHILE ( while k <= size do begin )
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DUP FLAGS + 0 SWAP C! ( flags[ k ] := false; -- c! writes to address with value both on stack )
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OVER + ( k := k + prime; -- OVER copies second item [prime] to the top )
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REPEAT
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DROP DROP ( remove k and prime from stack )
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1 + ( count := count + 1; -- count is on the top of the stack )
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THEN ( end the if block )
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LOOP
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LOOP
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." count of primes: " . ( writeln{ 'count of primes: ', count }; )
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; ( end. )
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