gcc/ada/g-hesora.adb

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