%
% quicksort by Don Woods at Sun Microsystems, Inc.
%
systemdict begin
/quicksort { % array proc => array (sorted, reuses same storage)
10 dict begin
    /Bigger? exch cvx def               % a b bigger? => t if a<b
    dup quickrecur                      % start recursion
end
} def % quicksort

/quickrecur { % array => --  sorts array in place, using Bigger? for comparisons
    dup length dup 2 gt {               % A N
	% the next lines (until but not incl /Key...) subsort three elements
	% so we can use the median as the partitioning element; this improves
	% performance for the case where the array is initially nearly sorted,
	% but is not strictly necessary for the algorithm to work (it does
	% seem to improve average runtime by about 10%)
	2 copy 1 sub 2 copy 2 idiv 1 index 0    % A N A N-1 A (N-1)/2 A 0
	6 copy get 5 1 roll get 3 1 roll get    % above & A[N-1] A[(N-1)/2] A[0]
	2 copy Bigger? {exch} if                % subsort for three elements
	3 1 roll 2 copy Bigger? {exch} if       %   ... (call them min mid max)
	3 -1 roll 2 copy Bigger? {exch} if      %   ... subsort finished
	9 index % A N A N-1 A (N-1)/2 A 0 min mid max N
	3 eq {
	    5 2 roll put 4 1 roll put put       % store min/mid/max back
	    pop pop                             % pop A & N
	} { % else store mid at 0, max at N-1, min at (N-1)/2, then partition
	    3 -1 roll 5 2 roll put exch 4 1 roll put put        % A N
	    /Key 2 index 0 get def              % partitioning value
	    0                                   % A N 0, also known as A j i
	    {   % main partitioning loop
		% incr i until i=j or A[i]>=A[0]; note A[j] is rangecheck
		{   1 add 2 copy gt {           % i++; A j i j>i?
			dup 3 index exch get    % A j i A[i]
			Key exch Bigger? not {exit} if
		    } {exit} ifelse
		} loop
		% decr j until A[j]<=A[0]; happens at j=i-1 if not sooner
		exch {                          % A i j
		    1 sub dup 3 index exch get  % A i j A[j]
		    Key Bigger? not {exit} if
		} loop
		2 copy gt {exit} if             % if i>=j, finished partition
		% swap A[j] & A[i]; stack has: A i j
		2 index 4 copy exch get         % A i j A A i A[j]
		4 1 roll get                    % A i j A[j] A A[i]
		3 index exch put                % A i j A[j]
		4 copy exch pop put pop exch    % A j i
	    } loop
	    % finish partition by exchanging A[j] with A[0]; stack has: A i j
	    exch pop 2 copy 4 copy get          % A j A j A j A[j]
	    exch pop 0 exch put Key put         % A j
	    % now recur on A[0..j-1] and A[j+1..N-1]
	    2 copy 1 add 1 index length 1 index sub     % A j A j+1 N-1
	    getinterval 3 1 roll 0 exch getinterval     % A[j+1..N-1] A[0..j-1]
	    2 copy length exch length gt {exch} if      % put smaller on top
	    quickrecur quickrecur       % tail recursion avoids deep stack
	} ifelse % =3 or >3 elements
    } { % handle 1- and 2-element cases specially for efficiency
	2 eq {
	    dup aload pop Bigger? {aload 3 1 roll exch 3 -1 roll astore} if
	    } if
	pop     % pop the array
    } ifelse
} def % quickrecur
end % systemdict