/frames 8 def
/NN 420 def /rows 2 def /figflag 2 def 
/SNAP NN NN 8 div sub def
/Ndots NN def

%include /u/rs/book3/pslib/tools.ps
%include /u/rs/book3/pslib/dots.ps

/swap
 {/ii exch def /jj exch def
  A ii get /t exch def
    A ii  A jj get put A jj t put 
   } def
/partition
 { 
   /r exch def /l exch def  
   /v A r get def
   /j r def /i l 1 sub def
   {
     { /j j 1 sub def A j get v le { exit } if } loop
     { /i i 1 add def A i get v ge { exit } if } loop
     j i lt { exit } if
     j i swap
j i sub SNAP lt {ARRAY /SNAP SNAP NN 8 div sub def} if
   } loop
   r i swap 
   i
 } def

newRow
NN randomperm /A exch def
0 NN 1 sub partition
pop
ARRAY
---------------------------------------
T 3.1pc Quicksort partitioning

The partitioning process divides a file into two subfiles that
can be sorted independently.  None of the elements to the left of the
left scan pointer are larger, so there are no dots above and to
its left; and none of the elements to the right of the right scan pointer are
smaller, so there are no dots below and to its right.  
Thus, when we plot the process for a randomly ordered array,
partitioning divides a square of randomly arranged
dots into two smaller squares, with one element (the partitioning
element) ending up on the diagonal.