Paste: q

:: primes ( n -- pi(n) )
    [let | 
      xs [ n [ 1 ] B{ } replicate-as B{ 0 0 } prepend ]
      primes [ H{ } clone ]
      ppairs [ H{ } clone ]
    |
      xs length sqrt ! >integer
      [ dup xs nth 1 =
        [| i | 
          i 2 * n 1 - i <range> [ 0 swap xs set-nth ] each i 
        ] when drop 
      ] each
      
      xs [ 1 = [ 1 swap primes set-at ] [ drop ] if ] each-index
     
      primes [ drop 99999 > ] assoc-filter
      
    ] ;