gcc/ada/g-hesorg.adb

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT RUNTIME COMPONENTS                          --
   4 --                                                                          --
   5 --                     G N A T . H E A P _ S O R T _ G                      --
   6 --                                                                          --
   7 --                                 B o d y                                  --
   8 --                                                                          --
   9 --                            $Revision: 1.1 $
  10 --                                                                          --
  11 --           Copyright (C) 1995-1999 Ada Core Technologies, Inc.            --
  12 --                                                                          --
  13 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  14 -- terms of the  GNU General Public License as published  by the Free Soft- --
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  16 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  17 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  18 -- or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License --
  19 -- for  more details.  You should have  received  a copy of the GNU General --
  20 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  21 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  22 -- MA 02111-1307, USA.                                                      --
  23 --                                                                          --
  24 -- As a special exception,  if other files  instantiate  generics from this --
  25 -- unit, or you link  this unit with other files  to produce an executable, --
  26 -- this  unit  does not  by itself cause  the resulting  executable  to  be --
  27 -- covered  by the  GNU  General  Public  License.  This exception does not --
  28 -- however invalidate  any other reasons why  the executable file  might be --
  29 -- covered by the  GNU Public License.                                      --
  30 --                                                                          --
  31 -- GNAT is maintained by Ada Core Technologies Inc (http://www.gnat.com).   --
  32 --                                                                          --
  33 ------------------------------------------------------------------------------
  34
  35 package body GNAT.Heap_Sort_G is
  36
  37    ----------
  38    -- Sort --
  39    ----------
  40
  41    --  We are using the classical heapsort algorithm (i.e. Floyd's Treesort3)
  42    --  as described by Knuth ("The Art of Programming", Volume III, first
  43    --  edition, section 5.2.3, p. 145-147) with the modification that is
  44    --  mentioned in exercise 18. For more details on this algorithm, see
  45    --  Robert B. K. Dewar PhD thesis "The use of Computers in the X-ray
  46    --  Phase Problem". University of Chicago, 1968, which was the first
  47    --  publication of the modification, which reduces the number of compares
  48    --  from 2NlogN to NlogN.
  49
  50    procedure Sort (N : Natural) is
  51
  52       Max : Natural := N;
  53       --  Current Max index in tree being sifted
  54
  55       procedure Sift (S : Positive);
  56       --  This procedure sifts up node S, i.e. converts the subtree rooted
  57       --  at node S into a heap, given the precondition that any sons of
  58       --  S are already heaps. On entry, the contents of node S is found
  59       --  in the temporary (index 0), the actual contents of node S on
  60       --  entry are irrelevant. This is just a minor optimization to avoid
  61       --  what would otherwise be two junk moves in phase two of the sort.
  62
  63       procedure Sift (S : Positive) is
  64          C      : Positive := S;
  65          Son    : Positive;
  66          Father : Positive;
  67
  68       begin
  69          --  This is where the optimization is done, normally we would do a
  70          --  comparison at each stage between the current node and the larger
  71          --  of the two sons, and continue the sift only if the current node
  72          --  was less than this maximum. In this modified optimized version,
  73          --  we assume that the current node will be less than the larger
  74          --  son, and unconditionally sift up. Then when we get to the bottom
  75          --  of the tree, we check parents to make sure that we did not make
  76          --  a mistake. This roughly cuts the number of comparisions in half,
  77          --  since it is almost always the case that our assumption is correct.
  78
  79          --  Loop to pull up larger sons
  80
  81          loop
  82             Son := 2 * C;
  83             exit when Son > Max;
  84
  85             if Son < Max and then Lt (Son, Son + 1) then
  86                Son := Son + 1;
  87             end if;
  88
  89             Move (Son, C);
  90             C := Son;
  91          end loop;
  92
  93          --  Loop to check fathers
  94
  95          while C /= S loop
  96             Father := C / 2;
  97
  98             if Lt (Father, 0) then
  99                Move (Father, C);
 100                C := Father;
 101             else
 102                exit;
 103             end if;
 104          end loop;
 105
 106          --  Last step is to pop the sifted node into place
 107
 108          Move (0, C);
 109       end Sift;
 110
 111    --  Start of processing for Sort
 112
 113    begin
 114       --  Phase one of heapsort is to build the heap. This is done by
 115       --  sifting nodes N/2 .. 1 in sequence.
 116
 117       for J in reverse 1 .. N / 2 loop
 118          Move (J, 0);
 119          Sift (J);
 120       end loop;
 121
 122       --  In phase 2, the largest node is moved to end, reducing the size
 123       --  of the tree by one, and the displaced node is sifted down from
 124       --  the top, so that the largest node is again at the top.
 125
 126       while Max > 1 loop
 127          Move (Max, 0);
 128          Move (1, Max);
 129          Max := Max - 1;
 130          Sift (1);
 131       end loop;
 132
 133    end Sort;
 134
 135 end GNAT.Heap_Sort_G;