%!PS

% A quickie number spiral (of the primes) as per http://numberspiral.com/
% 2005-11-28

/paperx 8.5 72 mul def % letter size
/papery 11 72 mul def
paperx 2 div papery 2 div translate % start in the center

/max 100000 def % How high shall we count? Be careful of overflows.

/mark? { % n mark? boolean
    % Your filter here. This tests for primes (the dumb way).
    % Try, e.g., { 3 mod 0 eq } for some neat patterns.
    /n exch def
    n 1 eq { false } {
        n 2 mod 0 eq { n 2 eq } {
            /divide? false def % variables are poor style
            3 2 n sqrt ceiling {
                n exch mod 0 eq
                { /divide? true def exit }  if
            } for divide? not
        } ifelse
    } ifelse
} bind def

0 1 max {
    dup mark? {
        dup sqrt dup 360 mul dup rotate
        % Pretty dragon pattern (not quite as cool with thin markings
        % like the primes, of course; try it with every integer):
        dup floor sub 1.0 1.0 sethsbcolor
        0 1 0 360 arc fill
        sqrt 360 mul neg rotate % Got a better idea?
    } if
} bind for

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