fixing pr42337
[official-gcc.git] / gcc / ada / g-hesora.adb
blob60f307bba8abcf33b514f3a1309bde3886c39a03
1 ------------------------------------------------------------------------------
2 -- --
3 -- GNAT RUN-TIME 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) 1995-2008, AdaCore --
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- --
13 -- ware Foundation; either version 2, or (at your option) any later ver- --
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, 51 Franklin Street, Fifth Floor, --
20 -- Boston, MA 02110-1301, 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 was originally developed by the GNAT team at New York University. --
30 -- Extensive contributions were provided by Ada Core Technologies Inc. --
31 -- --
32 ------------------------------------------------------------------------------
34 pragma Compiler_Unit;
36 package body GNAT.Heap_Sort_A is
38 ----------
39 -- Sort --
40 ----------
42 -- We are using the classical heapsort algorithm (i.e. Floyd's Treesort3)
43 -- as described by Knuth ("The Art of Programming", Volume III, first
44 -- edition, section 5.2.3, p. 145-147) with the modification that is
45 -- mentioned in exercise 18. For more details on this algorithm, see
46 -- Robert B. K. Dewar PhD thesis "The use of Computers in the X-ray
47 -- Phase Problem". University of Chicago, 1968, which was the first
48 -- publication of the modification, which reduces the number of compares
49 -- from 2NlogN to NlogN.
51 procedure Sort (N : Natural; Move : Move_Procedure; Lt : Lt_Function) is
53 Max : Natural := N;
54 -- Current Max index in tree being sifted
56 procedure Sift (S : Positive);
57 -- This procedure sifts up node S, i.e. converts the subtree rooted
58 -- at node S into a heap, given the precondition that any sons of
59 -- S are already heaps. On entry, the contents of node S is found
60 -- in the temporary (index 0), the actual contents of node S on
61 -- entry are irrelevant. This is just a minor optimization to avoid
62 -- what would otherwise be two junk moves in phase two of the sort.
64 procedure Sift (S : Positive) is
65 C : Positive := S;
66 Son : Positive;
67 Father : Positive;
69 begin
70 -- This is where the optimization is done, normally we would do a
71 -- comparison at each stage between the current node and the larger
72 -- of the two sons, and continue the sift only if the current node
73 -- was less than this maximum. In this modified optimized version,
74 -- we assume that the current node will be less than the larger
75 -- son, and unconditionally sift up. Then when we get to the bottom
76 -- of the tree, we check parents to make sure that we did not make
77 -- a mistake. This roughly cuts the number of comparisons in half,
78 -- since it is almost always the case that our assumption is correct.
80 -- Loop to pull up larger sons
82 loop
83 Son := 2 * C;
84 exit when Son > Max;
86 if Son < Max and then Lt (Son, Son + 1) then
87 Son := Son + 1;
88 end if;
90 Move (Son, C);
91 C := Son;
92 end loop;
94 -- Loop to check fathers
96 while C /= S loop
97 Father := C / 2;
99 if Lt (Father, 0) then
100 Move (Father, C);
101 C := Father;
102 else
103 exit;
104 end if;
105 end loop;
107 -- Last step is to pop the sifted node into place
109 Move (0, C);
110 end Sift;
112 -- Start of processing for Sort
114 begin
115 -- Phase one of heapsort is to build the heap. This is done by
116 -- sifting nodes N/2 .. 1 in sequence.
118 for J in reverse 1 .. N / 2 loop
119 Move (J, 0);
120 Sift (J);
121 end loop;
123 -- In phase 2, the largest node is moved to end, reducing the size
124 -- of the tree by one, and the displaced node is sifted down from
125 -- the top, so that the largest node is again at the top.
127 while Max > 1 loop
128 Move (Max, 0);
129 Move (1, Max);
130 Max := Max - 1;
131 Sift (1);
132 end loop;
134 end Sort;
136 end GNAT.Heap_Sort_A;