1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
|
------------------------------------------------------------------------------
-- --
-- GNAT RUNTIME COMPONENTS --
-- --
-- G N A T . H E A P _ S O R T _ A --
-- --
-- B o d y --
-- --
-- Copyright (C) 1995-1999 Ada Core Technologies, Inc. --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 2, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
-- for more details. You should have received a copy of the GNU General --
-- Public License distributed with GNAT; see file COPYING. If not, write --
-- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, --
-- MA 02111-1307, USA. --
-- --
-- As a special exception, if other files instantiate generics from this --
-- unit, or you link this unit with other files to produce an executable, --
-- this unit does not by itself cause the resulting executable to be --
-- covered by the GNU General Public License. This exception does not --
-- however invalidate any other reasons why the executable file might be --
-- covered by the GNU Public License. --
-- --
-- GNAT is maintained by Ada Core Technologies Inc (http://www.gnat.com). --
-- --
------------------------------------------------------------------------------
package body GNAT.Heap_Sort_A is
----------
-- Sort --
----------
-- We are using the classical heapsort algorithm (i.e. Floyd's Treesort3)
-- as described by Knuth ("The Art of Programming", Volume III, first
-- edition, section 5.2.3, p. 145-147) with the modification that is
-- mentioned in exercise 18. For more details on this algorithm, see
-- Robert B. K. Dewar PhD thesis "The use of Computers in the X-ray
-- Phase Problem". University of Chicago, 1968, which was the first
-- publication of the modification, which reduces the number of compares
-- from 2NlogN to NlogN.
procedure Sort (N : Natural; Move : Move_Procedure; Lt : Lt_Function) is
Max : Natural := N;
-- Current Max index in tree being sifted
procedure Sift (S : Positive);
-- This procedure sifts up node S, i.e. converts the subtree rooted
-- at node S into a heap, given the precondition that any sons of
-- S are already heaps. On entry, the contents of node S is found
-- in the temporary (index 0), the actual contents of node S on
-- entry are irrelevant. This is just a minor optimization to avoid
-- what would otherwise be two junk moves in phase two of the sort.
procedure Sift (S : Positive) is
C : Positive := S;
Son : Positive;
Father : Positive;
begin
-- This is where the optimization is done, normally we would do a
-- comparison at each stage between the current node and the larger
-- of the two sons, and continue the sift only if the current node
-- was less than this maximum. In this modified optimized version,
-- we assume that the current node will be less than the larger
-- son, and unconditionally sift up. Then when we get to the bottom
-- of the tree, we check parents to make sure that we did not make
-- a mistake. This roughly cuts the number of comparisions in half,
-- since it is almost always the case that our assumption is correct.
-- Loop to pull up larger sons
loop
Son := 2 * C;
exit when Son > Max;
if Son < Max and then Lt (Son, Son + 1) then
Son := Son + 1;
end if;
Move (Son, C);
C := Son;
end loop;
-- Loop to check fathers
while C /= S loop
Father := C / 2;
if Lt (Father, 0) then
Move (Father, C);
C := Father;
else
exit;
end if;
end loop;
-- Last step is to pop the sifted node into place
Move (0, C);
end Sift;
-- Start of processing for Sort
begin
-- Phase one of heapsort is to build the heap. This is done by
-- sifting nodes N/2 .. 1 in sequence.
for J in reverse 1 .. N / 2 loop
Move (J, 0);
Sift (J);
end loop;
-- In phase 2, the largest node is moved to end, reducing the size
-- of the tree by one, and the displaced node is sifted down from
-- the top, so that the largest node is again at the top.
while Max > 1 loop
Move (Max, 0);
Move (1, Max);
Max := Max - 1;
Sift (1);
end loop;
end Sort;
end GNAT.Heap_Sort_A;
|
