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Diffstat (limited to 'gcc/ada/g-heasor.adb')
| -rw-r--r-- | gcc/ada/g-heasor.adb | 132 |
1 files changed, 132 insertions, 0 deletions
diff --git a/gcc/ada/g-heasor.adb b/gcc/ada/g-heasor.adb new file mode 100644 index 00000000000..bd406a8fabb --- /dev/null +++ b/gcc/ada/g-heasor.adb @@ -0,0 +1,132 @@ +------------------------------------------------------------------------------ +-- -- +-- GNAT RUNTIME COMPONENTS -- +-- -- +-- G N A T . H E A P _ S O R T -- +-- -- +-- B o d y -- +-- -- +-- Copyright (C) 1995-2002 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 was originally developed by the GNAT team at New York University. -- +-- Extensive contributions were provided by Ada Core Technologies Inc. -- +-- -- +------------------------------------------------------------------------------ + +package body GNAT.Heap_Sort 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; Xchg : Xchg_Procedure; Lt : Lt_Function) is + Max : Natural := N; + -- Current Max index in tree being sifted. Note that we make Max + -- Natural rather than Positive so that the case of sorting zero + -- elements is correctly handled (i.e. does nothing at all). + + 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. + + ---------- + -- Sift -- + ---------- + + 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 := C + C; + + if Son < Max then + if Lt (Son, Son + 1) then + Son := Son + 1; + end if; + elsif Son > Max then + exit; + end if; + + Xchg (Son, C); + C := Son; + end loop; + + -- Loop to check fathers + + while C /= S loop + Father := C / 2; + + if Lt (Father, C) then + Xchg (Father, C); + C := Father; + else + exit; + end if; + end loop; + 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 + 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 + Xchg (1, Max); + Max := Max - 1; + Sift (1); + end loop; + end Sort; + +end GNAT.Heap_Sort; |